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John Kohl
Theodore Ts'o
Tom Yu
Sam Hartman
Ken Raeburn
Jeffrey Altman
September 9,
November 1, 2002
Expires 9 March, 1 May, 2003
The Kerberos Network Authentication Service (V5)
draft-ietf-krb-wg-kerberos-clarifications-01.txt
draft-ietf-krb-wg-kerberos-clarifications-02
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet-Drafts are working documents
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The distribution of this memo is unlimited. It is filed as
draft-ietf-krb-wg-kerberos-clarifications-01.txt,
draft-ietf-krb-wg-kerberos-clarifications-02.txt, and expires 9 March 1 May 2003.
Please send comments to: ietf-krb-wg@anl.gov
<mailto:ietf-krb-wg@anl.gov>
ABSTRACT
This document provides an overview and specification of Version 5 of the
Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol
and its intended use that require more detailed or clearer explanation than
was provided in RFC1510. This document is intended to provide a detailed
description of the protocol, suitable for implementation, together with
descriptions of the appropriate use of protocol messages and fields within
those messages.
This document contains a subset of the changes considered and discussed in
the Kerberos working group and is intended as an interim description of
Kerberos. Additional changes to the Kerberos protocol have been proposed and
will appear in a subsequent extensions document.
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This document is not intended to describe Kerberos to the end user, system
administrator, or application developer. Higher level papers describing
Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88],
are available elsewhere.
OVERVIEW
This INTERNET-DRAFT describes the concepts and model upon which the Kerberos
network authentication system is based. It also specifies Version 5 of the
Kerberos protocol.
The motivations, goals, assumptions, and rationale behind most design
decisions are treated cursorily; they are more fully described in a paper
available in IEEE communications [NT94] and earlier in the Kerberos portion
of the Athena Technical Plan [MNSS87]. The protocols have been a proposed
standard and are being considered for advancement for draft standard through
the IETF standard process. Comments are encouraged on the presentation, but
only minor refinements to the protocol as implemented or extensions that fit
within current protocol framework will be considered at this time.
Requests for addition to an electronic mailing list for discussion of
Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU
<mailto:kerberos-request@MIT.EDU>. kerberos-request@MIT.EDU.
This mailing list is gatewayed onto the Usenet as the group
comp.protocols.kerberos. Requests for further information, including
documents and code availability, may be sent to
info-kerberos@MIT.EDU <mailto:info-kerberos@MIT.EDU>. info-kerberos@MIT.EDU.
BACKGROUND
The Kerberos model is based in part on Needham and Schroeder's trusted
third-party authentication protocol [NS78] and on modifications suggested by
Denning and Sacco [DS81]. The original design and implementation of Kerberos
Versions 1 through 4 was the work of two former Project Athena staff
members, Steve Miller of Digital Equipment Corporation and Clifford Neuman
(now at the Information Sciences Institute of the University of Southern
California), along with Jerome Saltzer, Technical Director of Project
Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members
of Project Athena have also contributed to the work on Kerberos.
Version 5 of the Kerberos protocol (described in this document) has evolved
from Version 4 based on new requirements and desires for features not
available in Version 4. The design of Version 5 of the Kerberos protocol was
led by Clifford Neuman and John Kohl with much input from the community. The
development of the MIT reference implementation was led at MIT by John Kohl
and Theodore T'so, with help and contributed code from many others. Since
RFC1510 was issued, extensions and revisions to the protocol have been
proposed by many individuals. Some of these proposals are reflected in this
document. Where such changes involved significant effort, the document cites
the contribution of the proposer.
Reference implementations of both version 4 and version 5 of Kerberos are
publicly available and commercial implementations have been developed and
are widely used. Details on the differences between Kerberos Versions 4 and
5 can be found in [KNT92].
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Table of Contents
1. Introduction
1.1. Cross-realm operation
1.2. Choosing a principal with which to communicate
1.3. Authorization
1.5. Environmental assumptions
2. Ticket flag uses and requests
2.1. Initial, pre-authenticated, and hardware authenticated tickets
2.2. Invalid tickets
2.3. Renewable tickets
2.4. Postdated tickets
2.5. Proxiable and proxy tickets
2.6. Forwardable tickets
2.8. Other KDC options
3. Message Exchanges
3.1. The Authentication Service Exchange
3.1.1. Generation of KRB_AS_REQ message
3.1.2. Receipt of KRB_AS_REQ message
3.1.3. Generation of KRB_AS_REP message
3.1.4. Generation of KRB_ERROR message
3.1.5. Receipt of KRB_AS_REP message
3.1.6. Receipt of KRB_ERROR message
3.2. The Client/Server Authentication Exchange
3.2.1. The KRB_AP_REQ message
3.2.2. Generation of a KRB_AP_REQ message
3.2.3. Receipt of KRB_AP_REQ message
3.2.4. Generation of a KRB_AP_REP message
3.2.5. Receipt of KRB_AP_REP message
3.2.6. Using the encryption key
3.3. The Ticket-Granting Service (TGS) Exchange
3.3.1. Generation of KRB_TGS_REQ message
3.3.2. Receipt of KRB_TGS_REQ message
3.3.3. Generation of KRB_TGS_REP message
3.3.3.1. Checking for revoked tickets
3.3.3.2. Encoding the transited field
3.3.4. Receipt of KRB_TGS_REP message
3.4. The KRB_SAFE Exchange
3.4.1. Generation of a KRB_SAFE message
3.4.2. Receipt of KRB_SAFE message
3.5. The KRB_PRIV Exchange
3.5.1. Generation of a KRB_PRIV message
3.5.2. Receipt of KRB_PRIV message
3.6. The KRB_CRED Exchange
3.6.1. Generation of a KRB_CRED message
3.6.2. Receipt of KRB_CRED message
3.7. User to User Authentication Exchanges
4. Encryption and Checksum Specifications
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5. Message Specifications
5.1. Specific Compatibility Notes on ASN.1
5.1.1. ASN.1 Distinguished Encoding Rules
5.1.2. Optional Integer Fields
5.1.3. Empty SEQUENCE OF Types
5.1.4. Unrecognized Tag Numbers
5.1.5. Tag Numbers Greater Than 30
5.2. Basic Kerberos Types
5.2.1. KerberosString
5.2.2. Realm and PrincipalName
5.2.3. KerberosTime
5.2.4. Constrained Integer types
5.2.5. HostAddress and HostAddresses
5.2.6. AuthorizationData
5.2.6.1. IF-RELEVANT
5.2.6.4. KDCIssued
5.2.6.5. AND-OR
5.2.6.8. MANDATORY-FOR-KDC
5.2.7. PA-DATA
5.2.7.1. PA-TGS-REQ
5.2.7.2. Encrypted Timestamp Pre-authentication
5.2.7.5. PA-ETYPE-INFO2
5.2.8. KerberosFlags
5.2.9. Cryptosystem-related Types
5.3. Tickets
5.4. Specifications for the AS and TGS exchanges
5.4.1. KRB_KDC_REQ definition
5.4.2. KRB_KDC_REP definition
5.5. Client/Server (CS) message specifications
5.5.1. KRB_AP_REQ definition
5.5.2. KRB_AP_REP definition
5.5.3. Error message reply
5.6. KRB_SAFE message specification
5.7. KRB_PRIV message specification
5.8. KRB_CRED message specification
5.9. Error message specification
5.10. Application Tag Numbers
6. Naming Constraints
6.1. Realm Names
6.2. Principal Names
6.2.1. Name of server principals
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7. Constants and other defined values
7.1. Host address types
7.2. KDC messaging
7.2.1 IP Transports
7.2.1.1. UDP/IP transport
7.2.1.2. TCP/IP transport
7.2.1.3 KDC Discovery on IP Networks
7.2.1.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names
7.2.1.3.2. Specifying KDC Location information with DNS SRV records
7.2.1.3.3. KDC Discovery for Domain Style Realm Names on IP Networks
7.3. Name of the TGS
7.4. OID arc for KerberosV5
7.5. Protocol constants and associated values
7.5.1. Key usage numbers
7.5.2. PreAuthentication Data Types
7.5.3. Address Types
7.5.4. Authorization Data Types
7.5.5. Transited Encoding Types
7.5.6. Protocol Version Number
7.5.7. Kerberos Message Types
7.5.8. Name Types
7.5.9. Error Codes
8. Interoperability requirements
8.1. Specification 2
8.2. Recommended KDC values
9. IANA considerations
10. Security Considerations
11. Acknowledgement and References
A. ASN.1 module
B. Changes since RFC-1510
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1. Introduction
Kerberos provides a means of verifying the identities of principals, (e.g. a
workstation user or a network server) on an open (unprotected) network. This
is accomplished without relying on assertions by the host operating system,
without basing trust on host addresses, without requiring physical security
of all the hosts on the network, and under the assumption that packets
traveling along the network can be read, modified, and inserted at will[1.1] <#fn1.1>.
will[1.1]. Kerberos performs authentication under these conditions as a
trusted third-party authentication service by using conventional (shared
secret key [1.2]
<#fn1.2>) [1.2]) cryptography. Kerberos extensions (outside the scope of
this document) can provide for the use of public key cryptography during
certain phases of the authentication protocol [@RFCE: if PKINIT advances
concurrently include reference to the RFC here]. Such extensions support
Kerberos authentication for users registered with public key certification
authorities and provide certain benefits of public key cryptography in
situations where they are needed.
The basic Kerberos authentication process proceeds as follows: A client
sends a request to the authentication server (AS) requesting "credentials"
for a given server. The AS responds with these credentials, encrypted in the
client's key. The credentials consist of a "ticket" for the server and a
temporary encryption key (often called a "session key"). The client
transmits the ticket (which contains the client's identity and a copy of the
session key, all encrypted in the server's key) to the server. The session
key (now shared by the client and server) is used to authenticate the
client, and may optionally be used to authenticate the server. It may also
be used to encrypt further communication between the two parties or to
exchange a separate sub-session key to be used to encrypt further
communication.
Implementation of the basic protocol consists of one or more authentication
servers running on physically secure hosts. The authentication servers
maintain a database of principals (i.e., users and servers) and their secret
keys. Code libraries provide encryption and implement the Kerberos protocol.
In order to add authentication to its transactions, a typical network
application adds one or two calls to the Kerberos library directly or
through the Generic Security Services Application Programming Interface,
GSSAPI, described in separate document [ref to GSSAPI RFC]. These calls
result in the transmission of the necessary messages to achieve
authentication.
The Kerberos protocol consists of several sub-protocols (or exchanges).
There are two basic methods by which a client can ask a Kerberos server for
credentials. In the first approach, the client sends a cleartext request for
a ticket for the desired server to the AS. The reply is sent encrypted in
the client's secret key. Usually this request is for a ticket-granting
ticket (TGT) which can later be used with the ticket-granting server (TGS).
In the second method, the client sends a request to the TGS. The client uses
the TGT to authenticate itself to the TGS in the same manner as if it were
contacting any other application server that requires Kerberos
authentication. The reply is encrypted in the session key from the TGT.
Though the protocol specification describes the AS and the TGS as separate
servers, they are implemented in practice as different protocol entry points
within a single Kerberos server.
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Once obtained, credentials may be used to verify the identity of the
principals in a transaction, to ensure the integrity of messages exchanged
between them, or to preserve privacy of the messages. The application is
free to choose whatever protection may be necessary.
To verify the identities of the principals in a transaction, the client
transmits the ticket to the application server. Since the ticket is sent "in
the clear" (parts of it are encrypted, but this encryption doesn't thwart
replay) and might be intercepted and reused by an attacker, additional
information is sent to prove that the message originated with the principal
to whom the ticket was issued. This information (called the authenticator)
is encrypted in the session key, and includes a timestamp. The timestamp
proves that the message was recently generated and is not a replay.
Encrypting the authenticator in the session key proves that it was generated
by a party possessing the session key. Since no one except the requesting
principal and the server know the session key (it is never sent over the
network in the clear) this guarantees the identity of the client.
The integrity of the messages exchanged between principals can also be
guaranteed using the session key (passed in the ticket and contained in the
credentials). This approach provides detection of both replay attacks and
message stream modification attacks. It is accomplished by generating and
transmitting a collision-proof checksum (elsewhere called a hash or digest
function) of the client's message, keyed with the session key. Privacy and
integrity of the messages exchanged between principals can be secured by
encrypting the data to be passed using the session key contained in the
ticket or the sub-session key found in the authenticator.
The authentication exchanges mentioned above require read-only access to the
Kerberos database. Sometimes, however, the entries in the database must be
modified, such as when adding new principals or changing a principal's key.
This is done using a protocol between a client and a third Kerberos server,
the Kerberos Administration Server (KADM). There is also a protocol for
maintaining multiple copies of the Kerberos database. Neither of these
protocols are described in this document.
1.1. Cross-realm operation
The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a server in
another. Each organization wishing to run a Kerberos server establishes its
own "realm". The name of the realm in which a client is registered is part
of the client's name, and can be used by the end-service to decide whether
to honor a request.
By establishing "inter-realm" keys, the administrators of two realms can
allow a client authenticated in the local realm to prove its identity to
servers in other realms[1.3] <#fn1.3>. realms[1.3]. The exchange of inter-realm keys (a separate
key may be used for each direction) registers the ticket-granting service of
each realm as a principal in the other realm. A client is then able to
obtain a ticket-granting ticket for the remote realm's ticket-granting
service from its local realm. When that ticket-granting ticket is used, the
remote ticket-granting service uses the inter-realm key (which usually
differs from its own normal TGS key) to decrypt the ticket-granting ticket,
and is thus certain that it was issued by the client's own TGS. Tickets
issued by the remote ticket-granting service will indicate to the
end-service that the client was authenticated from another realm.
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A realm is said to communicate with another realm if the two realms share an
inter-realm key, or if the local realm shares an inter-realm key with an
intermediate realm that communicates with the remote realm. An
authentication path is the sequence of intermediate realms that are
transited in communicating from one realm to another.
Realms are typically may be organized hierarchically. Each realm shares a key with its
parent and a different key with each child. If an inter-realm key is not
directly shared by two realms, the hierarchical organization allows an
authentication path to be easily constructed. If a hierarchical organization
is not used, it may be necessary to consult a database in order to construct
an authentication path between realms.
Although realms are typically hierarchical, intermediate realms may be
bypassed to achieve cross-realm authentication through alternate
authentication paths (these might be established to make communication
between two realms more efficient). It is important for the end-service to
know which realms were transited when deciding how much faith to place in
the authentication process. To facilitate this decision, a field in each
ticket contains the names of the realms that were involved in authenticating
the client.
The application server is ultimately responsible for accepting or rejecting
authentication and should check the transited field. The application server
may choose to rely on the KDC for the application server's realm to check
the transited field. The application server's KDC will set the
TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate
realms may also check the transited field as they issue
ticket-granting-tickets for other realms, but they are encouraged not to do
so. A client may request that the KDC's not check the transited field by
setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not
required to honor this flag.
1.2. Choosing a principal with which to communicate
The Kerberos protocol provides the means for verifying (subject to the
assumptions in 1.4) 1.5) that the entity with which one communicates is the same
entity that was registered with the KDC using the claimed identity
(principal name). It is still necessary to determine whether that identity
corresponds to the entity with which one intends to communicate.
When appropriate data has been exchanged in advance, this determination may
be performed syntactically by the application based on the application
protocol specification, information provided by the user, and configuration
files. For example, the server principal name (including realm) for a telnet
server might be derived from the user specified host name (from the telnet
command line), the "host/" prefix specified in the application protocol
specification, and a mapping to a Kerberos realm derived syntactically from
the domain part of the specified hostname and information from the local
Kerberos realms database.
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One can also rely on trusted third parties to make this determination, but
only when the data obtained from the third party is suitably integrity
protected wile resident on the third party server and when transmitted.
Thus, for example, one should not rely on an unprotected domain name system
record to map a host alias to the primary name of a server, accepting the
primary name as the party one intends to contact, since an attacker can
modify the mapping and impersonate the party with which one intended to
communicate.
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1.3. Authorization
As an authentication service, Kerberos provides a means
Implementations of verifying Kerberos, GSSAPI, and SASL [RFC 2222] MUST NOT use DNS to
canonicalize the
identity host components of principals on service principal names. Application
authors MAY wish to append a statically configured domain name to
unqualified hosts before passing the name to security mechanisms.
Implementation note: Many current implementations do some degree of
canonicalization of the provided service name, often using DNS even though
it creates security problems. However there is no consistency among
implementations about whether the service name is case folded to lower case
or wether reverse resolution is used. To maximize interoperability and
security, applications SHOULD provide security mechanisms with names which
result from folding the user-entered name to lower case, without performing
any other modifications or canonicalization.
1.3. Authorization
As an authentication service, Kerberos provides a means of verifying the
identity of principals on a network. Authentication is usually useful
primarily as a first step in the process of authorization, determining
whether a client may use a service, which objects the client is allowed to
access, and the type of access allowed for each. Kerberos does not, by
itself, provide authorization. Possession of a client ticket for a service
provides only for authentication of the client to that service, and in the
absence of a separate authorization procedure, it should not be considered
by an application as authorizing the use of that service.
Such separate authorization methods may be implemented as application
specific access control functions and may utilize files on the application
server, or on separately issued authorization credentials such as those
based on proxies [Neu93], or on other authorization services. Separately
authenticated authorization credentials may be embedded in a tickets
authorization data when encapsulated by the kdc-issued authorization data
element.
Applications should not accept the mere issuance of a service ticket by the
Kerberos server (even by a modified Kerberos server) as granting authority
to use the service, since such applications may become vulnerable to the
bypass of this authorization check in an environment if they interoperate
with other KDCs or where other options for application authentication (e.g.
the PKTAPP proposal) are provided.
[Section 1.4 is replaced in the extensions document <krbext1.4-1.html>]
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1.4. Extending Kerberos Without Breaking Interoperability
As the deployed base of Kerberos implementations grows, extending Kerberos
becomes more important. Unfortunately some extensions to the existing
Kerberos protocol create interoperability issues because of uncertainty
regarding the treatment of certain extensibility options by some
implementations. This section includes guidelines that will enable future
implementations to maintain interoperability.
Kerberos provides a general mechanism for protocol extensibility. Some
protocol messages contain typed holes -- sub-messages that contain an
octet-string along with an integer that defines how to interpret the
octet-string. The integer types are registered centrally, but can be used
both for vendor extensions and for extensions standardized through the IETF.
1.4.1. Compatibility with RFC 1510
It is important to note that existing Kerberos message formats can not be
readily extended by adding fields to the ASN.1 types. Sending additional
fields often results in the entire message being discarded without an error
indication. Future versions of this specification will provide guidelines to
ensure that ASN.1 fields can be added without creating an interoperability
problem.
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In the meantime, all new or modified implementations of Kerberos that
receive an unknown message extension should preserve the encoding of the
extension but otherwise ignore the presence of the extension. Recipients
MUST NOT decline a request simply because an extension is present.
There is one exception to this rule. If an unknown authorization data
element type is received by a server other than the ticket granting service
either in an AP-REQ or in a ticket contained in an AP-REQ, then
authentication SHOULD fail. One of the primary uses of authorization data is
to restrict the use of the ticket. If the service cannot determine whether
the restriction applies to that service then a security weakness may result
if the ticket can be used for that service. Authorization elements that are
optional should be enclosed in the AD-IF-RELEVANT element.
The ticket granting service must ignore but propagate to derivative tickets
any unknown authorization data types, unless those data types are embedded
in a MANDATORY-FOR-KDC element, in which case the request will be rejected.
This behavior is appropriate because requiring that the ticket granting
service understand unknown authorization data types would require that KDC
software be upgraded to understand new application-level restrictions before
applications used these restrictions, decreasing the utility of
authorization data as a mechanism for restricting the use of tickets. No
security problem is created because services to which the tickets are issued
will verify the authorization data.
Implementation note: Many RFC 1510 implementations ignore unknown
authorization data elements. Depending on these implementations to honor
authorization data restrictions may create a security weakness.
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1.4.2. Sending Extensible Messages
Care must be taken to ensure that old implementations can understand
messages sent to them even if they do not understand an extension that is
used. Unless the sender knows an extension is supported, the extension
cannot change the semantics of the core message or previously defined
extensions.
For example, an extension including key information necessary to decrypt the
encrypted part of a KDC-REP could only be used in situations where the
recipient was known to support the extension. Thus when designing such
extensions it is important to provide a way for the recipient to notify the
sender of support for the extension. For example in the case of an extension
that changes the KDC-REP reply key, the client could indicate support for
the extension by including a padata element in the AS-REQ sequence. The KDC
should only use the extension if this padata element is present in the
AS-REQ. Even if policy requires the use of the extension, it is better to
return an error indicating that the extension is required than to use the
extension when the recipient may not support it; debugging why
implementations do not interoperate is easier when errors are returned.
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1.6.
1.5. Environmental assumptions
Kerberos imposes a few assumptions on the environment in which it can
properly function:
* "Denial of service" attacks are not solved with Kerberos. There are
places in the protocols where an intruder can prevent an application
from participating in the proper authentication steps. Detection and
solution of such attacks (some of which can appear to be not-uncommon
"normal" failure modes for the system) is usually best left to the
human administrators and users.
* Principals must keep their secret keys secret. If an intruder somehow
steals a principal's key, it will be able to masquerade as that
principal or impersonate any server to the legitimate principal.
* "Password guessing" attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to successfully
mount an offline dictionary attack by repeatedly attempting to decrypt,
with successive entries from a dictionary, messages obtained which are
encrypted under a key derived from the user's password.
* Each host on the network must have a clock which is "loosely
synchronized" to the time of the other hosts; this synchronization is
used to reduce the bookkeeping needs of application servers when they
do replay detection. The degree of "looseness" can be configured on a
per-server basis, but is typically on the order of 5 minutes. If the
clocks are synchronized over the network, the clock synchronization
protocol must itself be secured from network attackers.
* Principal identifiers are not recycled on a short-term basis. A typical
mode of access control will use access control lists (ACLs) to grant
permissions to particular principals. If a stale ACL entry remains for
a deleted principal and the principal identifier is reused, the new
principal will inherit rights specified in the stale ACL entry. By not
re-using principal identifiers, the danger of inadvertent access is
removed.
1.7.
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1.6. Glossary of terms
Below is a list of terms used throughout this document.
Authentication
Verifying the claimed identity of a principal.
Authentication header
A record containing a Ticket and an Authenticator to be presented to a
server as part of the authentication process.
Authentication path
A sequence of intermediate realms transited in the authentication
process when communicating from one realm to another.
Authenticator
A record containing information that can be shown to have been recently
generated using the session key known only by the client and server.
Authorization
The process of determining whether a client may use a service, which
objects the client is allowed to access, and the type of access allowed
for each.
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Capability
A token that grants the bearer permission to access an object or
service. In Kerberos, this might be a ticket whose use is restricted by
the contents of the authorization data field, but which lists no
network addresses, together with the session key necessary to use the
ticket.
Ciphertext
The output of an encryption function. Encryption transforms plaintext
into ciphertext.
Client
A process that makes use of a network service on behalf of a user. Note
that in some cases a Server may itself be a client of some other server
(e.g. a print server may be a client of a file server).
Credentials
A ticket plus the secret session key necessary to successfully use that
ticket in an authentication exchange.
KDC
Key Distribution Center, a network service that supplies tickets and
temporary session keys; or an instance of that service or the host on
which it runs. The KDC services both initial ticket and ticket-granting
ticket requests. The initial ticket portion is sometimes referred to as
the Authentication Server (or service). The ticket-granting ticket
portion is sometimes referred to as the ticket-granting server (or
service).
Kerberos
Aside from the 3-headed dog guarding Hades, the name given to Project
Athena's authentication service, the protocol used by that service, or
the code used to implement the authentication service.
Plaintext
The input to an encryption function or the output of a decryption
function. Decryption transforms ciphertext into plaintext.
Principal
A named client or server entity that participates in a network
communication, with one name that is considered canonical.
Principal identifier
The canonical name used to uniquely identify each different principal.
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Seal
To encipher a record containing several fields in such a way that the
fields cannot be individually replaced without either knowledge of the
encryption key or leaving evidence of tampering.
Secret key
An encryption key shared by a principal and the KDC, distributed
outside the bounds of the system, with a long lifetime. In the case of
a human user's principal, the secret key may be derived from a
password.
Server
A particular Principal which provides a resource to network clients.
The server is sometimes referred to as the Application Server.
Service
A resource provided to network clients; often provided by more than one
server (for example, remote file service).
Session key
A temporary encryption key used between two principals, with a lifetime
limited to the duration of a single login "session".
Sub-session key
A temporary encryption key used between two principals, selected and
exchanged by the principals using the session key, and with a lifetime
limited to the duration of a single association.
Ticket
A record that helps a client authenticate itself to a server; it
contains the client's identity, a session key, a timestamp, and other
information, all sealed using the server's secret key. It only serves
to authenticate a client when presented along with a fresh
Authenticator.
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2. Ticket flag uses and requests
Each Kerberos ticket contains a set of flags which are used to indicate
attributes of that ticket. Most flags may be requested by a client when the
ticket is obtained; some are automatically turned on and off by a Kerberos
server as required. The following sections explain what the various flags
mean and give examples of reasons to use them. With the exception of the ANONYMOUS and
INVALID flags flag clients MUST ignore ticket flags that are not recognized. KDCs
MUST ignore KDC options that are not recognized. Some implementations of RFC
1510 are known to reject unknown KDC options, so clients may need to resend
a request without KDC new options absent if the request was rejected when
sent with option added since RFC 1510. Since new KDCs will ignore unknown
options, clients MUST confirm that the ticket returned by the KDC meets
their needs. For example, as
discussed in section 2.8, a client requiring anonymous communication
needs to make sure that the ticket is actually anonymous. A KDC that
prohibits issuing of anonymous tickets or that does not understand the
anonymous option would not return an anonymous ticket.
Note that it is not in general possible to determine whether an option was
not honored because it was not understood or because it was rejected either
through configuration or policy. When adding a new option to the Kerberos
protocol, designers should consider whether the distinction is important for
their option. In cases where it is, a mechanism for the KDC to return an
indication that the option was understood but rejected needs to be provided
in the specification of the option. Often in such cases, the mechanism needs
to be broad enough to permit an error or reason to be returned.
2.1. Initial, pre-authenticated, and hardware authenticated tickets
The INITIAL flag indicates that a ticket was issued using the AS protocol
and not issued based on a ticket-granting ticket. Application servers that
want to require the demonstrated knowledge of a client's secret key (e.g. a
password-changing program) can insist that this flag be set in any tickets
they accept, and thus be assured that the client's key was recently
presented to the application client.
The PRE-AUTHENT and HW-AUTHENT flags provide additional information about
the initial authentication, regardless of whether the current ticket was
issued directly (in which case INITIAL will also be set) or issued on the
basis of a ticket-granting ticket (in which case the INITIAL flag is clear,
but the PRE-AUTHENT and HW-AUTHENT flags are carried forward from the
ticket-granting ticket).
2.2. Invalid tickets
The INVALID flag indicates that a ticket is invalid. Application servers
must reject tickets which have this flag set. A postdated ticket will
usually be issued in this form. Invalid tickets must be validated by the KDC
before use, by presenting them to the KDC in a TGS request with the VALIDATE
option specified. The KDC will only validate tickets after their starttime
has passed. The validation is required so that postdated tickets which have
been stolen before their starttime can be rendered permanently invalid
(through a hot-list mechanism) (see section 3.3.3.1).
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2.3. Renewable tickets
Applications may desire to hold tickets which can be valid for long periods
of time. However, this can expose their credentials to potential theft for
equally long periods, and those stolen credentials would be valid until the
expiration time of the ticket(s). Simply using short-lived tickets and
obtaining new ones periodically would require the client to have long-term
access to its secret key, an even greater risk. Renewable tickets can be
used to mitigate the consequences of theft. Renewable tickets have two
"expiration times": the first is when the current instance of the ticket
expires, and the second is the latest permissible value for an individual
expiration time. An application client must periodically (i.e. before it
expires) present a renewable ticket to the KDC, with the RENEW option set in
the KDC request. The KDC will issue a new ticket with a new session key and
a later expiration time. All other fields of the ticket are left unmodified
by the renewal process. When the latest permissible expiration time arrives,
the ticket expires permanently. At each renewal, the KDC may consult a
hot-list to determine if the ticket had been reported stolen since its last
renewal; it will refuse to renew such stolen tickets, and thus the usable
lifetime of stolen tickets is reduced.
The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can usually be
ignored by application servers. However, some particularly careful
application servers may wish to disallow renewable tickets.
If a renewable ticket is not renewed by its expiration time, the KDC will
not renew the ticket. The RENEWABLE flag is reset by default, but a client
may request it be set by setting the RENEWABLE option in the KRB_AS_REQ
message. If it is set, then the renew-till field in the ticket contains the
time after which the ticket may not be renewed.
2.4. Postdated tickets
Applications may occasionally need to obtain tickets for use much later,
e.g. a batch submission system would need tickets to be valid at the time
the batch job is serviced. However, it is dangerous to hold valid tickets in
a batch queue, since they will be on-line longer and more prone to theft.
Postdated tickets provide a way to obtain these tickets from the KDC at job
submission time, but to leave them "dormant" until they are activated and
validated by a further request of the KDC. If a ticket theft were reported
in the interim, the KDC would refuse to validate the ticket, and the thief
would be foiled.
The MAY-POSTDATE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. This flag
must be set in a ticket-granting ticket in order to issue a postdated ticket
based on the presented ticket. It is reset by default; it may be requested
by a client by setting the ALLOW-POSTDATE option in the KRB_AS_REQ message.
This flag does not allow a client to obtain a postdated ticket-granting
ticket; postdated ticket-granting tickets can only by obtained by requesting
the postdating in the KRB_AS_REQ message. The life (endtime-starttime) of a
postdated ticket will be the remaining life of the ticket-granting ticket at
the time of the request, unless the RENEWABLE option is also set, in which
case it can be the full life (endtime-starttime) of the ticket-granting
ticket. The KDC may limit how far in the future a ticket may be postdated.
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The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to see when
the original authentication occurred. Some services may choose to reject
postdated tickets, or they may only accept them within a certain period
after the original authentication. When the KDC issues a POSTDATED ticket,
it will also be marked as INVALID, so that the application client must
present the ticket to the KDC to be validated before use.
2.5. Proxiable and proxy tickets
At times it may be necessary for a principal to allow a service to perform
an operation on its behalf. The service must be able to take on the identity
of the client, but only for a particular purpose. A principal can allow a
service to take on the principal's identity for a particular purpose by
granting it a proxy.
The process of granting a proxy using the proxy and proxiable flags is used
to provide credentials for use with specific services. Though conceptually
also a proxy, user's wishing to delegate their identity for ANY purpose must
use the ticket forwarding mechanism described in the next section to forward
a ticket granting ticket.
The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. When set,
this flag tells the ticket-granting server that it is OK to issue a new
ticket (but not a ticket-granting ticket) with a different network address
based on this ticket. This flag is set if requested by the client on initial
authentication. By default, the client will request that it be set when
requesting a ticket granting ticket, and reset when requesting any other
ticket.
This flag allows a client to pass a proxy to a server to perform a remote
request on its behalf, e.g. a print service client can give the print server
a proxy to access the client's files on a particular file server in order to
satisfy a print request.
In order to complicate the use of stolen credentials, Kerberos tickets are
usually valid from only those network addresses specifically included in the ticket[2.1] <#fn2.1>.
ticket[2.1]. When granting a proxy, the client must specify the new network
address from which the proxy is to be used, or indicate that the proxy is to
be issued for use from any address.
The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket.
Application servers may check this flag and at their option they may require
additional authentication from the agent presenting the proxy in order to
provide an audit trail.
2.6. Forwardable tickets
Authentication forwarding is an instance of a proxy where the service
granted is complete use of the client's identity. An example where it might
be used is when a user logs in to a remote system and wants authentication
to work from that system as if the login were local.
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The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. The
FORWARDABLE flag has an interpretation similar to that of the PROXIABLE
flag, except ticket-granting tickets may also be issued with different
network addresses. This flag is reset by default, but users may request that
it be set by setting the FORWARDABLE option in the AS request when they
request their initial ticket-granting ticket.
This flag allows for authentication forwarding without requiring the user to
enter a password again. If the flag is not set, then authentication
forwarding is not permitted, but the same result can still be achieved if
the user engages in the AS exchange specifying the requested network
addresses and supplies a password.
The FORWARDED flag is set by the TGS when a client presents a ticket with
the FORWARDABLE flag set and requests a forwarded ticket by specifying the
FORWARDED KDC option and supplying a set of addresses for the new ticket. It
is also set in all tickets issued based on tickets with the FORWARDED flag
set. Application servers may choose to process FORWARDED tickets differently
than non-FORWARDED tickets.
If addressless tickets are forwarded from one system to another, clients
SHOULD still use this option to obtain a new TGT in order to have different
session keys on the different systems.
2.7 Transited Policy Checking
In Kerberos, the application server is ultimately responsible for accepting
or rejecting authentication and should check that only suitably trusted KDCs
are relied upon to authenticate a principal. The transited field in the
ticket identifies which KDCs were involved in the authentication process and
an application server would normally check this field. If any of these are
untrusted to authenticate the indicated client principal (probably
determined by a realm-based policy), the authentication attempt must be
rejected. The presence of trusted KDCs in this list does not provide any
guarantee; an untrusted KDC may have fabricated the list.
While the end server ultimately decides whether authentication is valid, the
KDC for the end server's realm may apply a realm specific policy for
validating the transited field and accepting credentials for cross-realm
authentication. When the KDC applies such checks and accepts such
cross-realm authentication it will set the TRANSITED-POLICY-CHECKED flag in
the service tickets it issues based on the cross-realm TGT. A client may
request that the KDCs not check the transited field by setting the
DISABLE-TRANSITED-CHECK flag. KDCs are encouraged but not required to honor
this flag.
2.8 Anonymous Tickets
When policy allows, a KDC may issue anonymous tickets for the purpose of
enabling encrypted communication between a client and server without
identifying the client to the server. Such anonymous tickets are issued
with a generic principal name configured on the KDC (e.g. "anonymous@")
and will have the ANONYMOUS flag set. A server accepting such a ticket
may assume that subsequent requests using the same ticket and session
key originate from the same user. Requests with the same username but
different tickets are likely to originate from different users. Users
request anonymous ticket by setting the REQUEST-ANONYMOUS option in an
AS or TGS request.
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If a client requires anonymous communication then the client should
check to make sure that the resulting ticket is actually anonymous. A
KDC that does not understand the anonymous-requested flag will not
return an error, but will instead return a normal ticket.
2.9.
2.8. Other KDC options
There are three additional options which may be set in a client's request of
the KDC.
2.9.1
2.8.1 Renewable-OK
The RENEWABLE-OK option indicates that the client will accept a renewable
ticket if a ticket with the requested life cannot otherwise be provided. If
a ticket with the requested life cannot be provided, then the KDC may issue
a renewable ticket with a renew-till equal to the the requested endtime. The
value of the renew-till field may still be adjusted by site-determined
limits or limits imposed by the individual principal or server.
2.9.2
2.8.2 ENC-TKT-IN-SKEY
In its basic form the Kerberos protocol supports authentication in a client
server setting and is not well suited to authentication in a peer-to-peer
environment because the long term key of the user does not remain on the
workstation after initial login. Authentication of such peers may be
supported by Kerberos in its user-to-user variant. The ENC-TKT-IN-SKEY
option supports user-to-user authentication by allowing the KDC to issue a
service ticket encrypted using the session key from another ticket granting
ticket issued to another user. The ENC-TKT-IN-SKEY option is honored only by
the ticket-granting service. It indicates that the ticket to be issued for
the end server is to be encrypted in the session key from the additional
second ticket-granting ticket provided with the request. See section 3.3.3
for specific details.
2.9.4
2.8.3 Passwordless Hardware Authentication
The OPT-HARDWARE-AUTH option indicates that the client wishes to use some
form of hardware authentication instead of or in addition to the client's
password or other long-lived encryption key. OPT-HARDWARE-AUTH is honored
only by the authentication service. If supported and allowed by policy, the
KDC will return an errorcode KDC_ERR_PREAUTH_REQUIRED and include the
required METHOD-DATA to perform such authentication.
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3. Message Exchanges
The following sections describe the interactions between network clients and
servers and the messages involved in those exchanges.
[Insert paragraph for extensions <krbext3-1.html>]
3.1. The Authentication Service Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_AS_REQ 5.4.1
2. Kerberos to client KRB_AS_REP or 5.4.2
KRB_ERROR 5.9.1
The Authentication Service (AS) Exchange between the client and the Kerberos
Authentication Server is initiated by a client when it wishes to obtain
authentication credentials for a given server but currently holds no
credentials. In its basic form, the client's secret key is used for
encryption and decryption. This exchange is typically used at the initiation
of a login session to obtain credentials for a Ticket-Granting Server which
will subsequently be used to obtain credentials for other servers (see
section 3.3) without requiring further use of the client's secret key. This
exchange is also used to request credentials for services which must not be
mediated through the Ticket-Granting Service, but rather require a
principal's secret key, such as the password-changing service[3.1] <#fn3.1>. service[3.1]. This
exchange does not by itself provide any assurance of the the identity of the user[3.2]
<#fn3.2>.
user[3.2].
The exchange consists of two messages: KRB_AS_REQ from the client to
Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own identity and the
identity of the server for which it is requesting credentials. credentials, other
information about the credentials it is requesting, and a randomly generated
nonce which can be used to detect replays, and to associate replies with the
matching requests. This nonce must be generated randomly by the client and
remembered for checking against the nonce in the expected reply. The
response, KRB_AS_REP, contains a ticket for the client to present to the
server, and a session key that will be shared by the client and the server.
The session key and additional information are encrypted in the client's
secret key. The encrypted part of the KRB_AS_REP message also contains information the
nonce which
can must be used to detect replays, and to associate it matched with the message to
which it replies. [Insert sentence for extensions <krbext3.1-1.html>] nonce from the KRB_AS_REQ message.
Without pre-authentication, the authentication server does not know whether
the client is actually the principal named in the request. It simply sends a
reply without knowing or caring whether they are the same. This is
acceptable because nobody but the principal whose identity was given in the
request will be able to use the reply. Its critical information is encrypted
in that principal's key. However, an attacker can send a KRB_AS_REQ message
to get known plaintext in order to attack the principal's key. Especially if
the key is based on a password, this may create a security exposure. So, the
initial request supports an optional field that can be used to pass
additional information that might be needed for the initial exchange. This
field should be used for pre-authentication as described in section 3.1.1.
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Various errors can occur; these are indicated by an error response
(KRB_ERROR) instead of the KRB_AS_REP response. The error message is not
encrypted. The KRB_ERROR message contains information which can be used to
associate it with the message to which it replies. [This rest of this
paragraph is replaced in revisions <krbext3.1b-1.html>] The contents of the
KRB_ERROR message are not integrity-protected. As such, the client cannot
detect replays, fabrications or modifications. A solution to this problem
will be included in a future version of the protocol.
3.1.1. Generation of KRB_AS_REQ message
The client may specify a number of options in the initial request. Among
these options are whether pre-authentication is to be performed; whether the
requested ticket is to be renewable, proxiable, or forwardable; whether it
should be postdated or allow postdating of derivative tickets; whether the
client requests an anonymous ticket; and whether a renewable ticket will be
accepted in lieu of a non-renewable ticket if the requested ticket
expiration date cannot be satisfied by a non-renewable ticket (due to
configuration constraints).
The client prepares the KRB_AS_REQ message and sends it to the KDC.
[Insert the linked text in extensions <krbext3.1.1-1.html>]
3.1.2. Receipt of KRB_AS_REQ message
If all goes well, processing the KRB_AS_REQ message will result in the
creation of a ticket for the client to present to the server. The format for
the ticket is described in section 5.3.1. 5.3. The contents of the ticket are
determined as follows.
Because Kerberos can run over unreliable transports such as UDP, the KDC
MUST be prepared to retransmit responses in case they are lost. If a KDC
receives a request identical to one it has recently successfully processed,
the KDC MUST respond with an a KRB_AS_REP message rather than a replay error.
In order to reduce ciphertext given to a potential attacker, KDCs MAY wish
to send an the same response generated when the request was first handled. KDCs
MUST obey this replay behavior even if the actual transport in use is
reliable.
3.1.3. Generation of KRB_AS_REP message
The authentication server looks up the client and server principals named in
the KRB_AS_REQ in its database, extracting their respective keys. If the
requested client principal named in the request is not known because it
doesn't exist in the KDC's principal database, then an error message with a
KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.
If required, the server pre-authenticates the request, and if the
pre-authentication check fails, an error message with the code
KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is required, but
was not present in the request, an error message with the code
KDC_ERR_PREAUTH_REQUIRED is returned and the PA-ETYPE-INFO
pre-authentication field will be included in the KRB-ERROR message. If the
server cannot accommodate an encryption type requested by the client, an
error message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the KDC
generates a 'random' session key[3.3] <#fn3.3>.
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When responding to an AS request, if there are multiple encryption keys
registered for a client in the Kerberos database , then the etype field from
the AS request is used by the KDC to select the encryption method to be used
to protect the encrypted part of the KRB_AS_REP message which is sent to the
client. If there is more than one supported strong encryption type in the
etype list, the first valid strong etype for which an encryption key is
available is used. The encryption method used to
protect the encrypted part of the KRB_TGS_REP message is the keytype of
the session key found in the ticket granting ticket presented in the
KRB_TGS_REQ.
When the user's key is generated from a password or pass phrase, the
string-to-key function for the particular encryption key type is used, as
specified in [KCRYPTO]. The salt value and additional parameters for the
string-to-key function have default values (specified by section 6 and by
the encryption mechanism specification, respectively) that may be overridden
by preauthentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO,
PA-S2K-PARAMS, etc). Since the KDC is presumed to store a copy of the
resulting key only, these values should not be changed for password-based
keys except when changing the principal's key.
It is not possible to reliably generate a user's key given a pass phrase
without contacting the KDC, since it will not be known whether alternate
salt or parameter values are required.
When the etype field is present in a KDC request, whether an AS or TGS
request, the KDC will attempt to assign the type of the random session key
from the list of methods in the etype field. The KDC will select the
appropriate type using the list of methods provided together with
information from the Kerberos database indicating acceptable encryption
methods for the application server. The KDC will not issue tickets with a
weak session key encryption type.
If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise
the requested start time is checked against the policy of the local realm
(the administrator might decide to prohibit certain types or ranges of
postdated tickets), and if acceptable, the ticket's start time is set as
requested and the INVALID flag is set in the new ticket. The postdated
ticket must be validated before use by presenting it to the KDC after the
start time has been reached.
The expiration time of the ticket will be set to the earlier of the
requested endtime and a time determined by local policy, possibly determined
using realm or principal specific factors. For example, the expiration time
may be set to the minimum of the following:
* The expiration time (endtime) requested in the KRB_AS_REQ message.
* The ticket's start time plus the maximum allowable lifetime associated
with the client principal from the authentication server's database.
* The ticket's start time plus the maximum allowable lifetime associated
with the server principal.
* The ticket's start time plus the maximum lifetime set by the policy of
the local realm.
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If the requested expiration time minus the start time (as determined above)
is less than a site-determined minimum lifetime, an error message with code
KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the
ticket exceeds what was determined as above, and if the 'RENEWABLE-OK'
option was requested, then the 'RENEWABLE' flag is set in the new ticket,
and the renew-till value is set as if the 'RENEWABLE' option were requested
(the field and option names are described fully in section 5.4.1).
If the RENEWABLE option has been requested or if the RENEWABLE-OK option has
been set and a renewable ticket is to be issued, then the renew-till field
is set to the minimum of:
* Its requested value.
* The start time of the ticket plus the minimum of the two maximum
renewable lifetimes associated with the principals' database entries.
* The start time of the ticket plus the maximum renewable lifetime set by
the policy of the local realm.
The flags field of the new ticket will have the following options set if
they have been requested and if the policy of the local realm allows:
FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE, ANONYMOUS. If
the new ticket is post-dated (the start time is in the future), its INVALID
flag will also be set.
If all of the above succeed, the server will encrypt ciphertext part of the
ticket using the encryption key extracted from the server principal's record
in the Kerberos database using the encryption type associated with the
server principal's key (this choice is NOT affected by the etype field in
the request). It then formats a KRB_AS_REP message (see section 5.4.2),
copying the addresses in the request into the caddr of the response, placing
any required pre-authentication data into the padata of the response, and
encrypts the ciphertext part in the client's key using an acceptable
encryption method requested in the etype field of the request, or in some
key specified by pre-authentication mechanisms being used.
3.1.4. Generation of KRB_ERROR message
Several errors can occur, and the Authentication Server responds by
returning an error message, KRB_ERROR, to the client, with the
error-code, e-text, error-code
and optional e-cksum e-text fields set to appropriate values. The error message contents and
details are described in Section 5.9.1.
3.1.5. Receipt of KRB_AS_REP message
If the reply message type is KRB_AS_REP, then the client verifies that the
cname and crealm fields in the cleartext portion of the reply match what it
requested. If any padata fields are present, they may be used to derive the
proper secret key to decrypt the message. The client decrypts the encrypted
part of the response using its secret key, verifies that the nonce in the
encrypted part matches the nonce it supplied in its request (to detect
replays). It also verifies that the sname and srealm in the response match
those in the request (or are otherwise expected values), and that the host
address field is also correct. It then stores the ticket, session key, start
and expiration times, and other information for later use. The
key-expiration field from the encrypted part of the response may be checked
to notify the user of impending key expiration (the client program could
then suggest remedial action, such as a password change).
draft-ietf-krb-wg-kerberos-clarifications-01
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
Proper decryption
Upon validation of the KRB_AS_REP message is not sufficient for (by checking the
host to verify returned nonce
against that sent in the KRB_AS_REQ message) the client knows that the
current time on the KDC is that read from the authtime field of the
encrypted part of the reply. The client can optionally use this value for
clock synchronization in subsequent messages by recording with the ticket
the difference (offset) between the authtime value and the local clock. This
offset can then be used by the same user to adjust the time read from the
system clock when generating messages [cite Davis and Geer].
This technique must be used when adjusting for clock skew instead of
directly changing the system clock because the KDC reply is only
authenticated to the user whose secret key was used, but not to the system
or workstation. If the clock were adjusted, an attacker colluding with a
user logging into a workstation could agree on a password, resulting in a
KDC reply that would be correctly validated even though it did not originate
from a KDC trusted by the workstation.
Proper decryption of the KRB_AS_REP message is not sufficient for the host
to verify the identity of the user; the user and an attacker could cooperate
to generate a KRB_AS_REP format message which decrypts properly but is not
from the proper KDC. If the host wishes to verify the identity of the user,
it must require the user to present application credentials which can be
verified using a securely-stored secret key for the host. If those
credentials can be verified, then the identity of the user can be assured.
3.1.6. Receipt of KRB_ERROR message
If the reply message type is KRB_ERROR, then the client interprets it as an
error and performs whatever application-specific tasks are necessary to
recover.
3.2. The Client/Server Authentication Exchange
Summary
Message direction Message type Section
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.9.1
The client/server authentication (CS) exchange is used by network
applications to authenticate the client to the server and vice versa. The
client must have already acquired credentials for the server using the AS or
TGS exchange.
3.2.1. The KRB_AP_REQ message
The KRB_AP_REQ contains authentication information which should be part of
the first message in an authenticated transaction. It contains a ticket, an
authenticator, and some additional bookkeeping information (see section
5.5.1 for the exact format). The ticket by itself is insufficient to
authenticate a client, since tickets are passed across the network in cleartext[3.4] <#fn3.4>,
cleartext[3.4], so the authenticator is used to prevent invalid replay of
tickets by proving to the server that the client knows the session key of
the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message is
referred to elsewhere as the 'authentication header.'
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
3.2.2. Generation of a KRB_AP_REQ message
When a client wishes to initiate authentication to a server, it obtains
(either through a credentials cache, the AS exchange, or the TGS exchange) a
ticket and session key for the desired service. The client may re-use any
tickets it holds until they expire. To use a ticket the client constructs a
new Authenticator from the the system time, its name, and optionally an
application specific checksum, an initial sequence number to be used in
KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in
negotiations for a session key unique to this particular session.
Authenticators may not be re-used and will be rejected if replayed to a server[3.5] <#fn3.5>.
server[3.5]. If a sequence number is to be included, it should be randomly
chosen so that even after many messages have been exchanged it is not likely
to collide with other sequence numbers in use.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
The client may indicate a requirement of mutual authentication or the use of
a session-key based ticket (for user to user authentciation - see section
3.7) by setting the appropriate flag(s) in the ap-options field of the
message.
The Authenticator is encrypted in the session key and combined with the
ticket to form the KRB_AP_REQ message which is then sent to the end server
along with any additional application-specific information.
[Insert paragraph for extensions <krbext3.2.2-1.html>]
3.2.3. Receipt of KRB_AP_REQ message
Authentication is based on the server's current time of day (clocks must be
loosely synchronized), the authenticator, and the ticket. Several errors are
possible. If an error occurs, the server is expected to reply to the client
with a KRB_ERROR message. This message may be encapsulated in the
application protocol if its 'raw' form is not acceptable to the protocol.
The format of error messages is described in section 5.9.1.
The algorithm for verifying authentication information is as follows. If the
message type is not KRB_AP_REQ, the server returns the KRB_AP_ERR_MSG_TYPE
error. If the key version indicated by the Ticket in the KRB_AP_REQ is not
one the server can use (e.g., it indicates an old key, and the server no
longer possesses a copy of the old key), the KRB_AP_ERR_BADKEYVER error is
returned. If the USE-SESSION-KEY flag is set in the ap-options field, it
indicates to the server that user-to-user authentication is in use, and that
the ticket is encrypted in the session key from the server's ticket-granting
ticket rather than its in the server's secret key [3.6] <#fn3.6>. key. See section 3.7 for a more
complete description of the affect of user to user authentication on all
messages in the Kerberos protocol.
Since it is possible for the server to be registered in multiple realms,
with different keys in each, the srealm field in the unencrypted portion of
the ticket in the KRB_AP_REQ is used to specify which secret key the server
should use to decrypt that ticket. The KRB_AP_ERR_NOKEY error code is
returned if the server doesn't have the proper key to decipher the ticket.
The ticket is decrypted using the version of the server's key specified by
the ticket. If the decryption routines detect a modification of the ticket
(each encryption system must provide safeguards to detect modified
ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned
(chances are good that different keys were used to encrypt and decrypt).
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
The authenticator is decrypted using the session key extracted from the
decrypted ticket. If decryption shows it to have been modified, the
KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the client
from the ticket are compared against the same fields in the authenticator.
If they don't match, the KRB_AP_ERR_BADMATCH error is returned; this
normally is caused by a client error or attempted attack. The addresses in
the ticket (if any) are then searched for an address matching the
operating-system reported address of the client. If no match is found or the
server insists on ticket addresses but none are present in the ticket, the
KRB_AP_ERR_BADADDR error is returned. If the local (server) time and the
client time in the authenticator differ by more than the allowable clock
skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is returned.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Unless the application server provides its own suitable means to protect
against replay (for example, a challenge-response sequence initiated by the
server after authentication, or use of a server-generated encryption
subkey), the server must utilize a replay cache to remember any
authenticator presented within the allowable clock skew. Careful analysis of
the application protocol and implementation is recommended before
eliminating this cache. The replay cache will store the server name, along
with the client name, time and microsecond fields from the recently-seen
authenticators and if a matching tuple is found, the KRB_AP_ERR_REPEAT error
is returned [3.7] <#fn3.7>. [3.7]. If a server loses track of authenticators presented
within the allowable clock skew, it must reject all requests until the clock
skew interval has passed, providing assurance that any lost or re-played
authenticators will fall outside the allowable clock skew and can no longer
be successfully
replayed[3.8] <#fn3.8>. replayed[3.8].
Implementation note: If a sequence number is provided in client generates multiple requests to the authenticator, KDC with
the server saves
it same timestamp, including the microsecond field, all but the first of
the requests received will be rejected as replays. This might happen, for later use in processing KRB_SAFE and/or KRB_PRIV messages. If a
subkey
example, if the resolution of the client's clock is present, too coarse.
Implementations should ensure that the server either saves it for later use or uses it
to help generate its own choice for a subkey to be returned timestamps are not reused, possibly
by incrementing the microseconds field in a
KRB_AP_REP message. the time stamp when the clock
returns the same time for multiple requests.
If multiple servers (for example, different services on one machine, or a
single service implemented on multiple machines) share a service principal
(a practice we do not recommend in general, but acknowledge will be used in
some cases), they should also share this replay cache, or the application
protocol should be designed so as to eliminate the need for it. Note that
this applies to all of the services, if any of the application protocols
does not have replay protection built in; an authenticator used with such a
service could later be replayed to a different service with the same service
principal but no replay protection, if the former doesn't record the
authenticator information in the common replay cache.
If a sequence number is provided in the authenticator, the server saves it
for later use in processing KRB_SAFE and/or KRB_PRIV messages. If a subkey
is present, the server either saves it for later use or uses it to help
generate its own choice for a subkey to be returned in a KRB_AP_REP message.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
The server computes the age of the ticket: local (server) time minus the
start time inside the Ticket. If the start time is later than the current
time by more than the allowable clock skew or if the INVALID flag is set in
the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the
current time is later than end time by more than the allowable clock skew,
the KRB_AP_ERR_TKT_EXPIRED error is returned.
If all these checks succeed without an error, the server is assured that the
client possesses the credentials of the principal named in the ticket and
thus, the client has been authenticated to the server.
Passing these checks provides only authentication of the named principal; it
does not imply authorization to use the named service. Applications must
make a separate authorization decisions based upon the authenticated name of
the user, the requested operation, local access control information such as
that contained in a .k5login or .k5users file, and possibly a separate
distributed authorization service.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
3.2.4. Generation of a KRB_AP_REP message
Typically, a client's request will include both the authentication
information and its initial request in the same message, and the server need
not explicitly reply to the KRB_AP_REQ. However, if mutual authentication
(not only authenticating the client to the server, but also the server to
the client) is being performed, the KRB_AP_REQ message will have
MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message is
required in response. As with the error message, this message may be
encapsulated in the application protocol if its "raw" form is not acceptable
to the application's protocol. The timestamp and microsecond field used in
the reply must be the client's timestamp and microsecond field (as provided
in the authenticator)[3.9]
<#fn3.9>. authenticator)[3.9]. If a sequence number is to be included, it
should be randomly chosen as described above for the authenticator. A subkey
may be included if the server desires to negotiate a different subkey. The
KRB_AP_REP message is encrypted in the session key extracted from the
ticket.
[Insert paragraph for extensions <krbext3.2.4-1.html>]
3.2.5. Receipt of KRB_AP_REP message
If a KRB_AP_REP message is returned, the client uses the session key from
the credentials obtained for the server[3.10] <#fn3.10> to decrypt the message, and
verifies that the timestamp and microsecond fields match those in the
Authenticator it sent to the server. If they match, then the client is
assured that the server is genuine. The sequence number and subkey (if
present) are retained for later use.
3.2.6. Using the encryption key
[This seems inconsistent with crypto-architecture; we should look at
before publication.]
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server
share an encryption key which can be used by the application. In some cases,
the use of this session key will be implicit in the protocol; in others the
method of use must be chosen from several alternatives. The 'true session
key' to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
may be chosen by the application based on the session key from the ticket
and subkeys in the KRB_AP_REP message and the authenticator[3.11] <#fn3.11>. authenticator[3.11]. To
mitigate the effect of failures in random number generation on the client it
is strongly encouraged that any key derived by an application for subsequent
use include the full key entropy derived from the KDC generated session key
carried in the ticket. We leave the protocol negotiations of how to use the
key (e.g. selecting an encryption or checksum type) to the application
programmer; the Kerberos protocol does not constrain the implementation
options, but an example of how this might be done follows.
One way that an application may choose to negotiate a key to be used for
subsequent integrity and privacy protection is for the client to propose a
key in the subkey field of the authenticator. The server can then choose a
key using the proposed key from the client as input, returning the new
subkey in the subkey field of the application reply. This key could then be
used for subsequent communication.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
To make this example more concrete, if the communication patterns of an
application dictates the use of encryption modes of operation incompatible
with the encryption system used for the authenticator, then a key compatible
with the required encryption system may be generated by either the client,
the server, or collaboratively by both and exchanged using the subkey field.
This generation might involve the use of a random number as a pre-key,
initially generated by either party, which could then be encrypted using the
session key from the ticket, and the result exchanged and used for
subsequent encryption. By encrypting the pre-key with the session key from
the ticket, randomness from the KDC generated key is assured of being
present in the negotiated key. Application developers must be careful
however, to use a means of introducing this entropy that does not allow an
attacker to learn the session key from the ticket if it learns the key
generated and used for subsequent communication. The reader should note that
this is only an example, and that an analysis of the particular cryptosystem
to be used, must be made before deciding how to generate values for the
subkey fields, and the key to be used for subsequent communication.
With both the one-way and mutual authentication exchanges, the peers should
take care not to send sensitive information to each other without proper
assurances. In particular, applications that require privacy or integrity
should use the KRB_AP_REP response from the server to client to assure both
client and server of their peer's identity. If an application protocol
requires privacy of its messages, it can use the KRB_PRIV message (section
3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos Ticket-Granting Server is
initiated by a client when it wishes to obtain authentication credentials
for a given server (which might be registered in a remote realm), when it
wishes to renew or validate an existing ticket, or when it wishes to obtain
a proxy ticket. In the first case, the client must already have acquired a
ticket for the Ticket-Granting Service using the AS exchange (the
ticket-granting ticket is usually obtained when a client initially
authenticates to the system, such as when a user logs in). The message
format for the TGS exchange is almost identical to that for the AS exchange.
The primary difference is that encryption and decryption in the TGS exchange
does not take place under the client's key. Instead, the session key from
the ticket-granting ticket or renewable ticket, or sub-session key from an
Authenticator is used. As is the case for all application servers, expired
tickets are not accepted by the TGS, so once a renewable or ticket-granting
ticket expires, the client must use a separate exchange to obtain valid
tickets.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the
client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or
KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the
client plus a request for credentials. The authentication information
consists of the authentication header (KRB_AP_REQ) which includes the
client's previously obtained ticket-granting, renewable, or invalid ticket.
In the ticket-granting ticket and proxy cases, the request may include one
or more of: a list of network addresses, a collection of typed authorization
data to be sealed in the ticket for authorization use by the application
server, or additional tickets (the use of which are described later). The
TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted in the
session key from the ticket-granting ticket or renewable ticket, or if
present, in the sub-session key from the Authenticator (part of the
authentication header). The KRB_ERROR message contains an error code and
text explaining what went wrong. The KRB_ERROR message is not encrypted. The
KRB_TGS_REP message contains information which can be used to detect
replays, and to associate it with the message to which it replies. The
KRB_ERROR message also contains information which can be used to associate
it with the message to which it replies. The same comments about integrity
protection of KRB_ERROR messages mentioned in section 3.1 apply to the TGS
exchange.
3.3.1. Generation of KRB_TGS_REQ message
Before sending a request to the ticket-granting service, the client must
determine in which realm the application server is believed to be
registered[3.12] <#fn3.12>.
registered[3.12]. If the client knows the service principal name and realm
and it does not already possess a ticket-granting ticket for the appropriate
realm, then one must be obtained. This is first attempted by requesting a
ticket-granting ticket for the destination realm from a Kerberos server for
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
which the client possesses a ticket-granting ticket (using the KRB_TGS_REQ
message recursively). The Kerberos server may return a TGT for the desired
realm in which case one can proceed. Alternatively, the Kerberos server may
return a TGT for a realm which is 'closer' to the desired realm (further
along the standard hierarchical path between the client's realm and the
requested realm server's realm).
Once the client obtains a ticket-granting ticket for It should be noted in this case that
misconfiguration of the appropriate
realm, it determines which Kerberos servers serve that realm, and
contacts one. The list might be obtained through a configuration file or
network service or it may be generated from cause loops in the name of resulting
authentication path, which the realm; as
long as client should be careful to detect and avoid.
If the secret keys exchanged by realms are kept secret, only denial
of service results from using a false Kerberos server.
As in server returns a TGT for a 'closer' realm other than the AS exchange,
desired realm, the client may specify wish to use local policy configuration to
verify that the authentication path used is an acceptable one.
Alternatively, a client may wish to choose its own authentication path,
rather than relying on the Kerberos server to select one. In either case,
any policy or configuration information used to choose or validate
authentication paths, whether by the Kerberos server or client, must be
obtained from a trusted source.
When a client obtains a ticket-granting ticket that is 'closer' to the
destination realm, the client may cache this ticket and reuse it in future
KRB-TGS exchanges with services in the 'closer' realm. However, if the
client were to obtain a ticket-granting ticket for the 'closer' realm by
starting at the initial KDC rather than as part of obtaining another ticket,
then a shorter path to the 'closer' realm might be used. This shorter path
may be desirable because fewer intermediate KDCs would know the ssesion key
of the ticket involved. For this reason, clients should evaluate whether
they trust the realms transited in obtaining the 'closer' ticket when making
a decision to use the ticket in future.
Once the client obtains a ticket-granting ticket for the appropriate realm,
it determines which Kerberos servers serve that realm, and contacts one. The
list might be obtained through a configuration file or network service or it
may be generated from the name of the realm; as long as the secret keys
exchanged by realms are kept secret, only denial of service results from
using a false Kerberos server.
(This paragraph changed) As in the AS exchange, the client may specify a
number of options in the KRB_TGS_REQ message. One of these options is the
ENC-TKT-IN-SKEY option used for user to user authentication. An overview of
user to user authentication can be found in section 3.7. When generating the
KRB_TGS_REQ message, this option indicates that the client is including a
ticket granting ticket obtained from the application server in the
additional tickets field of the request and that the KDC should encrypt the
ticket for the application server using the session key from this additional
ticket, instead of using a server key from the principal database.
The client prepares the KRB_TGS_REQ message, providing an authentication
header as an element of the padata field, and including the same fields as
used in the KRB_AS_REQ message along with several optional fields: the enc-authorization-data
enc-authorizatfion-data field for application server use and additional
tickets required by some options.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
In preparing the authentication header, the client can select a sub-session
key under which the response from the Kerberos server will be encrypted[3.13] <#fn3.13>.
encrypted[3.13]. If the sub-session key is not specified, the session key
from the ticket-granting ticket will be used. If the enc-authorization-data
is present, it must be encrypted in the sub-session key, if present, from
the authenticator portion of the authentication header, or if not present,
using the session key from the ticket-granting ticket.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Once prepared, the message is sent to a Kerberos server for the destination
realm.
3.3.2. Receipt of KRB_TGS_REQ message
The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ
message, but there are many additional checks to be performed. First, the
Kerberos server must determine which server the accompanying ticket is for
and it must select the appropriate key to decrypt it. For a normal
KRB_TGS_REQ message, it will be for the ticket granting service, and the
TGS's key will be used. If the TGT was issued by another realm, then the
appropriate inter-realm key must be used. If the accompanying ticket is not
a ticket granting ticket for the current realm, but is for an application
server in the current realm, the RENEW, VALIDATE, or PROXY options are
specified in the request, and the server for which a ticket is requested is
the server named in the accompanying ticket, then the KDC will decrypt the
ticket in the authentication header using the key of the server for which it
was issued. If no ticket can be found in the padata field, the
KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
Once the accompanying ticket has been decrypted, the user-supplied checksum
in the Authenticator must be verified against the contents of the request,
and the message rejected if the checksums do not match (with an error code
of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not
collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the
checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is
returned. If the authorization-data are present, they are decrypted using
the sub-session key from the Authenticator.
If any of the decryptions indicate failed integrity checks, the
KRB_AP_ERR_BAD_INTEGRITY error is returned.
As discussed in section 3.1.2, the KDC MUST send a valid KRB_TGS_REP message
if it receives a KRB_TGS_REQ message identical to one it has recently
processed. However, if the authenticator is a replay, but the rest of the
request is not identical, then the KDC SHOULD return KRB_AP_ERR_REPEAT.
3.3.3. Generation of KRB_TGS_REP message
The KRB_TGS_REP message shares its format with the KRB_AS_REP (KRB_KDC_REP),
but with its type field set to KRB_TGS_REP. The detailed specification is in
section 5.4.2.
The response will include a ticket for the requested server or for a ticket
granting server of an intermediate KDC to be contacted to obtain the
requested ticket. The Kerberos database is queried to retrieve the record
for the appropriate server (including the key with which the ticket will be
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
encrypted). If the request is for a ticket granting ticket for a remote
realm, and if no key is shared with the requested realm, then the Kerberos
server will select the realm 'closest' to the requested realm with which it
does share a key, and use that realm instead. If the requested server cannot
be found in the TGS database, then a TGT for another trusted realm may be
returned instead of a ticket for the service. This TGT is a referral
mechanism to cause the client to retry the request to the realm of the TGT.
These are the only cases where the response for the KDC will be for a
different server than that requested by the client.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
By default, the address field, the client's name and realm, the list of
transited realms, the time of initial authentication, the expiration time,
and the authorization data of the newly-issued ticket will be copied from
the ticket-granting ticket (TGT) or renewable ticket. If the transited field
needs to be updated, but the transited type is not supported, the
KDC_ERR_TRTYPE_NOSUPP error is returned.
If the request specifies an endtime, then the endtime of the new ticket is
set to the minimum of (a) that request, (b) the endtime from the TGT, and
(c) the starttime of the TGT plus the minimum of the maximum life for the
application server and the maximum life for the local realm (the maximum
life for the requesting principal was already applied when the TGT was
issued). If the new ticket is to be a renewal, then the endtime above is
replaced by the minimum of (a) the value of the renew_till field of the
ticket and (b) the starttime for the new ticket plus the life
(endtime-starttime) of the old ticket.
If the FORWARDED option has been requested, then the resulting ticket will
contain the addresses specified by the client. This option will only be
honored if the FORWARDABLE flag is set in the TGT. The PROXY option is
similar; the resulting ticket will contain the addresses specified by the
client. It will be honored only if the PROXIABLE flag in the TGT is set. The
PROXY option will not be honored on requests for additional ticket-granting
tickets.
If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified or the MAY-POSTDATE flag is not set in the TGT, then the
error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-granting
ticket has the MAY-POSTDATE flag set, then the resulting ticket will be
postdated and the requested starttime is checked against the policy of the
local realm. If acceptable, the ticket's start time is set as requested, and
the INVALID flag is set. The postdated ticket must be validated before use
by presenting it to the KDC after the starttime has been reached. However,
in no case may the starttime, endtime, or renew-till time of a newly-issued
postdated ticket extend beyond the renew-till time of the ticket-granting
ticket.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
If the ENC-TKT-IN-SKEY option has been specified and an additional ticket
has been included in the request, it indicates that the client is using user
to user authentication to prove its identity to a server that does not have
access to a persistent key. Section 3.7 describes the affect of this option
on the entire Kerberos protocol. When generating the KRB_TGS_REP message,
this option in the KRB_TGS_REQ message tells the KDC will to decrypt the
additional ticket using the key for the server to which the additional
ticket was issued and verify that it is a ticket-granting ticket. If the
name of the requested server is missing from the request, the name of the
client in the additional ticket will be used. Otherwise the name of the
requested server will be compared to the name of the client in the
additional ticket and if different, the request will be rejected. If the
request succeeds, the session key from the additional ticket will be used to
encrypt the new ticket that is issued instead of using the key of the server
for which the new ticket will be used.
If the name of the server in the ticket that is presented to the KDC as part
of the authentication header is not that of the ticket-granting server
itself, the server is registered in the realm of the KDC, and the RENEW
option is requested, then the KDC will verify that the RENEWABLE flag is set
in the ticket, that the INVALID flag is not set in the ticket, and that the
renew_till time is still in the future. If the
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003 VALIDATE option is requested,
the KDC will check that the starttime has passed and the INVALID flag is
set. If the PROXY option is requested, then the KDC will check that the
PROXIABLE flag is set in the ticket. If the tests succeed, and the ticket
passes the hotlist check described in the next section, the KDC will issue
the appropriate new ticket.
The ciphertext part of the response in the KRB_TGS_REP message is encrypted
in the sub-session key from the Authenticator, if present, or the session
key key from the ticket-granting ticket. It is not encrypted using the
client's secret key. Furthermore, the client's key's expiration date and the
key version number fields are left out since these values are stored along
with the client's database record, and that record is not needed to satisfy
a request based on a ticket-granting ticket.
3.3.3.1. Checking for revoked tickets
Whenever a request is made to the ticket-granting server, the presented
ticket(s) is(are) checked against a hot-list of tickets which have been
canceled. This hot-list might be implemented by storing a range of issue
timestamps for 'suspect tickets'; if a presented ticket had an authtime in
that range, it would be rejected. In this way, a stolen ticket-granting
ticket or renewable ticket cannot be used to gain additional tickets
(renewals or otherwise) once the theft has been reported to the KDC for the
realm in which the server resides. Any normal ticket obtained before it was
reported stolen will still be valid (because they require no interaction
with the KDC), but only until their normal expiration time. If TGT's have
been issued for cross-realm authentication, use of the cross-realm TGT will
not be affected unless the hot-list is propagated to the KDC's for the
realms for which such cross-realm tickets were issued.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
3.3.3.2. Encoding the transited field
If the identity of the server in the TGT that is presented to the KDC as
part of the authentication header is that of the ticket-granting service,
but the TGT was issued from another realm, the KDC will look up the
inter-realm key shared with that realm and use that key to decrypt the
ticket. If the ticket is valid, then the KDC will honor the request, subject
to the constraints outlined above in the section describing the AS exchange.
The realm part of the client's identity will be taken from the
ticket-granting ticket. The name of the realm that issued the
ticket-granting ticket, if it is not the realm of the client principal, will
be added to the transited field of the ticket to be issued. This is
accomplished by reading the transited field from the ticket-granting ticket
(which is treated as an unordered set of realm names), adding the new realm
to the set, then constructing and writing out its encoded (shorthand) form
(this may involve a rearrangement of the existing encoding).
Note that the ticket-granting service does not add the name of its own
realm. Instead, its responsibility is to add the name of the previous realm.
This prevents a malicious Kerberos server from intentionally leaving out its
own name (it could, however, omit other realms' names).
The names of neither the local realm nor the principal's realm are to be
included in the transited field. They appear elsewhere in the ticket and
both are known to have taken part in authenticating the principal. Since the
endpoints are not included, both local and single-hop inter-realm
authentication result in a transited field that is empty.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Because the name of each realm transited is added to this field, it might
potentially be very long. To decrease the length of this field, its contents
are encoded. The initially supported encoding is optimized for the normal
case of inter-realm communication: a hierarchical arrangement of realms
using either domain or X.500 style realm names. This encoding (called
DOMAIN-X500-COMPRESS) is now described.
Realm names in the transited field are separated by a ",". The ",", "\",
trailing "."s, and leading spaces (" ") are special characters, and if they
are part of a realm name, they must be quoted in the transited field by
preceding them with a "\".
A realm name ending with a "." is interpreted as being prepended to the
previous realm. For example, we can encode traversal of EDU, MIT.EDU,
ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that they
would not be included in this field, and we would have:
"EDU,MIT.,WASHINGTON.EDU"
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
A realm name beginning with a "/" is interpreted as being appended to the
previous realm[18]. If it is to stand by itself, then it should be preceded
by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO,
/COM/HP, /COM, and /COM/DEC as:
"/COM,/HP,/APOLLO, /COM/DEC".
Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they
they would not be included in this field, and we would have:
"/COM,/HP"
A null subfield preceding or following a "," indicates that all realms
between the previous realm and the next realm have been traversed[19]. Thus,
"," means that all realms along the path between the client and the server
have been traversed. ",EDU, /COM," means that that all realms from the
client's realm up to EDU (in a domain style hierarchy) have been traversed,
and that everything from /COM down to the server's realm in an X.500 style
has also been traversed. This could occur if the EDU realm in one hierarchy
shares an inter-realm key directly with the /COM realm in another hierarchy.
3.3.4. Receipt of KRB_TGS_REP message
When the KRB_TGS_REP is received by the client, it is processed in the same
manner as the KRB_AS_REP processing described above. The primary difference
is that the ciphertext part of the response must be decrypted using the
sub-session key from the Authenticator, if it was specified in the request,
or the session key key from the ticket-granting ticket ticket, rather than the
client's secret key. The server name returned in the reply is the true
principal name of the service.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
3.4. The KRB_SAFE Exchange
The KRB_SAFE message may be used by clients requiring the ability to detect
modifications of messages they exchange. It achieves this by including a
keyed collision-proof checksum of the user data and some control
information. The checksum is keyed with an encryption key (usually the last
key negotiated via subkeys, or the session key if no negotiation has
occurred).
3.4.1. Generation of a KRB_SAFE message
When an application wishes to send a KRB_SAFE message, it collects its data
and the appropriate control information and computes a checksum over them.
The checksum algorithm should be the keyed checksum mandated to be
implemented along with the crypto system used for the sub-session or session
key. The checksum is generated using the sub-session key if present, or the
session key. Some implementations use a different checksum algorithm for
KRB_SAFE messages but doing so in a interoperable manner is impossible.
Implementations should accept any checksum algorithm they implement that
both has adequate security and that has keys compatible with the sub-session
or session key. Unkeyed or non-collision-proof checksums are not suitable
for this use.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
The control information for the KRB_SAFE message includes both a timestamp
and a sequence number. The designer of an application using the KRB_SAFE
message must choose at least one of the two mechanisms. This choice should
be based on the needs of the application protocol.
Sequence numbers are useful when all messages sent will be received by one's
peer. Connection state is presently required to maintain the session key, so
maintaining the next sequence number should not present an additional
problem.
If the application protocol is expected to tolerate lost messages without
them being resent, the use of the timestamp is the appropriate replay
detection mechanism. Using timestamps is also the appropriate mechanism for
multi-cast protocols where all of one's peers share a common sub-session
key, but some messages will be sent to a subset of one's peers.
After computing the checksum, the client then transmits the information and
checksum to the recipient in the message format specified in section 5.6.1.
3.4.2. Receipt of KRB_SAFE message
When an application receives a KRB_SAFE message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.
The message is first checked by verifying that the protocol version and type
fields match the current version and KRB_SAFE, respectively. A mismatch
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application verifies that the checksum used is a collision-proof keyed
checksum that uses keys compatible with the sub-session or session key as
appropriate, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated.
The sender's address MUST be included in the control information; the
recipient verifies that the operating system's report
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003 of the sender's
address matches the sender's address in the message, and (if a recipient
address is specified or the recipient requires an address) that one of the
recipient's addresses appears as the recipient's address in the message. To
work with network address translation, senders MAY wish to use the
directional address type specified in section 8.1 for the sender address and
not include recipient addresses. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the sequence
number fields are checked. If timestamp and usec are expected and not
present, or they are present but not current, the KRB_AP_ERR_SKEW error is
generated. If the server name, along with the client name, time and
microsecond fields from the Authenticator match any recently-seen (sent or
received[20] ) such tuples, the KRB_AP_ERR_REPEAT error is generated. If an
incorrect sequence number is included, or a sequence number is expected but
not present, the KRB_AP_ERR_BADORDER error is generated. If neither a
time-stamp and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED
error is generated. Finally, the checksum is computed over the data and
control information, and if it doesn't match the received checksum, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application is assured that the message was
generated by its peer and was not modified in transit.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
3.5. The KRB_PRIV Exchange
The KRB_PRIV message may be used by clients requiring confidentiality and
the ability to detect modifications of exchanged messages. It achieves this
by encrypting the messages and adding control information.
3.5.1. Generation of a KRB_PRIV message
When an application wishes to send a KRB_PRIV message, it collects its data
and the appropriate control information (specified in section 5.7.1) and
encrypts them under an encryption key (usually the last key negotiated via
subkeys, or the session key if no negotiation has occurred). As part of the
control information, the client must choose to use either a timestamp or a
sequence number (or both); see the discussion in section 3.4.1 for
guidelines on which to use. After the user data and control information are
encrypted, the client transmits the ciphertext and some 'envelope'
information to the recipient.
3.5.2. Receipt of KRB_PRIV message
When an application receives a KRB_PRIV message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.
The message is first checked by verifying that the protocol version and type
fields match the current version and KRB_PRIV, respectively. A mismatch
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application then decrypts the ciphertext and processes the resultant
plaintext. If decryption shows the data to have been modified, a
KRB_AP_ERR_BAD_INTEGRITY error is generated. The sender's address MUST be
included in the control information; the recipient verifies that the
operating system's report of the sender's address matches the sender's
address in the message, and (if a recipient address is specified or the
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
recipient requires an address) that one of the recipient's addresses appears
as the recipient's address in the message. A failed match for either case
generates a KRB_AP_ERR_BADADDR error. To work with network address
translation, implementations MAY wish to use the directional address type
defined in section 8.1 7.1 for the sender address and include no recipient
address. Then the timestamp and usec and/or the sequence number fields are
checked. If timestamp and usec are expected and not present, or they are
present but not current, the KRB_AP_ERR_SKEW error is generated. If the
server name, along with the client name, time and microsecond fields from
the Authenticator match any recently-seen such tuples, the KRB_AP_ERR_REPEAT
error is generated. If an incorrect sequence number is included, or a
sequence number is expected but not present, the KRB_AP_ERR_BADORDER error
is generated. If neither a time-stamp and usec or a sequence number is
present, a KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application can assume the message was
generated by its peer, and was securely transmitted (without intruders able
to see the unencrypted contents).
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
3.6. The KRB_CRED Exchange
The KRB_CRED message may be used by clients requiring the ability to send
Kerberos credentials from one host to another. It achieves this by sending
the tickets together with encrypted data containing the session keys and
other information associated with the tickets.
3.6.1. Generation of a KRB_CRED message
When an application wishes to send a KRB_CRED message it first (using the
KRB_TGS exchange) obtains credentials to be sent to the remote host. It then
constructs a KRB_CRED message using the ticket or tickets so obtained,
placing the session key needed to use each ticket in the key field of the
corresponding KrbCredInfo sequence of the encrypted part of the the KRB_CRED
message.
Other information associated with each ticket and obtained during the
KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in
the encrypted part of the KRB_CRED message. The current time and, if
specifically required by the application the nonce, s-address, and r-address
fields, are placed in the encrypted part of the KRB_CRED message which is
then encrypted under an encryption key previously exchanged in the KRB_AP
exchange (usually the last key negotiated via subkeys, or the session key if
no negotiation has occurred).
Implementation note: When constructing a KRB_CRED message for inclusion in a
GSSAPI initial context token, the MIT implementation of Kerberos will not
encrypt the KRB_CRED message if the session key is a DES or tripple DES key.
For interoperability with MIT, the Microsoft implementation will not encrypt
the KRB_CRED in a GSSAPI token if it is using a DES session key. Starting at
version 1.2.5, MIT Kerberos can receive and decode either encrypted or
unencrypted KRB_CRED tokens in the GSSAPI exchange. The Heimdal
implementation of Kerberos can also accept either encrypted or unencrypted
KRB_CRED messages. Since the KRB_CRED message in a GSSAPI token is encrypted
in the authenticator, the MIT behavior does not present a security problem,
although it is a violation of the Kerberos specification.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
3.6.2. Receipt of KRB_CRED message
When an application receives a KRB_CRED message, it verifies it. If any
error occurs, an error code is reported for use by the application. The
message is verified by checking that the protocol version and type fields
match the current version and KRB_CRED, respectively. A mismatch generates a
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then
decrypts the ciphertext and processes the resultant plaintext. If decryption
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
generated. v
If present or required, the recipient MAY verify that the operating system's
report of the sender's address matches the sender's address in the message,
and that one of the recipient's addresses appears as the recipient's address
in the message. The address check does not provide any added security, since
the address if present has already been checked in the KRB_AP_REQ message
and there is not any benefit to be gained by an attacker in reflecting a
KRB_CRED message back to its originator. Thus, the recipient MAY wish to
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
ignore the address even if present in order to work better in NAT
environments. A failed match for either case generates a KRB_AP_ERR_BADADDR
error. Recipients MAY wish to skip the address check as the KRB_CRED message
cannot generally be reflected back toThe timestamp and usec fields (and the
nonce field if required) are checked next. If the timestamp and usec are not
present, or they are present but not current, the KRB_AP_ERR_SKEW error is
generated.
If all the checks succeed, the application stores each of the new tickets in
its ticket cache together with the session key and other information in the
corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED
message.
4. SECTION HAS BEEN DELETED
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5. Message Specifications
NOTE: The ASN.1 collected here should be identical
3.7. User to User Authentication Exchanges
(This entire subsection is new) User to User authentication provides a
method to perform authentication when the contents of
Appendix A.
NOTE: The semantics described here should also verifier does not conflict with those
in section 3, though that still needs checking.
The Kerberos protocol is defined here in terms of Abstract Syntax
Notation One (ASN.1), which provides have a syntax for specifying both access to
long term service key. This might be the
abstract layout of protocol messages case when running a server (for
example a window server) as well a user on a workstation. In such cases, the
server may have access to the ticket granting ticket obtained when the user
logged in to the workstation, but because the server is running as their encodings.
Implementors not utilizing an existing ASN.1 compiler
unprivleged user it might not have access to system keys. Similar situations
may arise when running peer-to-peer applications.
Summary
Message direction Message type Sections
0. Message from application server Not Specified
1. Client to Kerberos KRB_TGS_REQ 3.3 + 5.4.1
2. Kerberos to client KRB_TGS_REP or support library
are cautioned 3.3 + 5.4.2
3. Client to thoroughly understand Application server KRB_AP_REQ 3.2 + 5.5.1
To address this problem, the actual ASN.1 specification Kerberos protocol allows the client to
ensure correct implementation behavior, as there is more complexity in request
that the notation than is immediately obvious, and some tutorials and guides ticket issued by the KDC be encrypted using a session key a ticket
granting ticket issued to ASN.1 are misleading or erroneous.
Note the party that in several places, there have been changes here will verify the authentication.
This ticket granting ticket must be obtained from RFC 1510
that change the abstract types. verifier by a means
exchange external to the Kerberos protococl, usually as part of the
application protocol. This message is shown in part to address widespread
assumptions the summary above as message
0. Note that various implementations have made, in some cases
resulting because the ticket granting ticket is encrypted in unintentional violations of the ASN.1 standard. These will KDC's
secret key, can not be clearly flagged when they occur. The differences between used for authentication without posession of the abstract
types in RFC 1510
corresponding secret key, and abstract types in this document can cause
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
incompatible encodings to be emitted when certain encoding rules, e.g. because the Packed Encoding Rules (PER), are used. This theoretical
incompatibility should verifier does not be relevant for Kerberos, since Kerberos
explicitly specifies give out the use
corresponding secret key, providing a copy of the Distinguished Encoding Rules (DER).
It might be an issue for protocols wishing to use Kerberos types with
other encoding rules. (This practice is verifiers ticket granting
ticket does not recommended.) With very few
exceptions (most notably the usages allow impersonation of BIT STRING), the encodings
emitted verifier.
Once the verifier's ticket granting ticket has been obtained by the DER remain identical between client,
by specifying the types defined ENC-TKT-IN-SKEY option to the KDC, the client can include
the ticket as an additional ticket in RFC
1510 and its KRB_TGS_REQ frequest to the types defined KDC
(message 1 in this document.
The type definitions the table above).
If validated according to the instructions in this section assume an ASN.1 module definition
of 3.3.3, the following form:
Kerberos5 {
iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
} DEFINITIONS ::= BEGIN
-- rest of definitions here
END
This specifies that application ticket
returned to the tagging context for client (message 3 in the module table above) will be explicit
and non-automatic.
Note that in some other publications [RFC1510] [RFC1964], the "dod"
portion of encrypted
using the object identifier is erroneously specified as having session key from the
value "5". In additional ticket and the case of RFC 1964, use of client will note
this when it uses or stores the "correct" OID value would
result in application ticket.
When contacting the server using a change ticket obtained for user to user
authentication (message 3 in the wire protocol; therefore, it remains unchanged
for now.
draft-ietf-krb-wg-kerberos-clarifications-01 table above), the client must specify the
USE-SESSION-KEY flag in the ap-options field. This tells the application
server to use the session key associated with its ticket granting ticket to
decrypt the server ticket provided in the application request.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
Note that elsewhere
4. Encryption and Checksum Specifications
The Kerberos protocols described in this document, nomenclature for various message
types is inconsistent, but seems document are designed to largely follow C language
conventions, including use of underscore (_) characters and all-caps
spelling encrypt
messages of names intended to be numeric constants. Also, in some
places, identifiers (especially ones refering arbitrary sizes, using stream or block encryption ciphers.
Encryption is used to constants) are written
in all-caps prove the identities of the network entities
participating in order to distinguish them from surrounding explanatory text. message exchanges. The ASN.1 notation does not permit underscores in identifiers, so in
actual ASN.1 definitions, underscores are replaced with hyphens (-).
Additionally, structure member names and defined values Key Distribution Center for each
realm is trusted by all principals registered in ASN.1 must
begin with that realm to store a lowercase letter, while type names must begin with an
uppercase letter.
5.1. Specific Compatibility Notes on ASN.1
For compatibility purposes, implementors should heed the following
specific notes regarding the use of ASN.1
secret key in Kerberos. These notes do
not describe deviations from standard usage confidence. Proof of ASN.1. The purpose knowledge of
these notes this secret key is used to instead describe some historical quirks and
non-compliance
verify the authenticity of various implementations, as well as historical
ambiguities, which, while being valid ASN.1, can lead to confusion
during implementation.
5.1.1. ASN.1 Distinguished Encoding Rules a principal.
The encoding of Kerberos protocol messages shall obey KDC uses the Distinguished
Encoding Rules (DER) of ASN.1 as described in X.690 (1997). Some
implementations (believed principal's secret key (in the AS exchange) or a shared
session key (in the TGS exchange) to be primarly ones derived from DCE 1.1 and
earlier) are known encrypt responses to use ticket requests;
the more general Basic Encoding Rules (BER);
in particular, these implementations send indefinite encodings of
lengths. Implementations may accept such encodings in ability to obtain the interests secret key or session key implies the knowledge of
backwards compatibility, though implementors are warned that decoding
fully-general BER is fraught with peril.
5.1.2. Optional Integer Fields
Some implementations do not internally distinguish between an omitted
optional integer value
the appropriate keys and a transmitted value the identity of zero. the KDC. The places in ability of a principal
to decrypt the protocol where this is relevant include various microseconds fields,
nonces, KDC response and sequence numbers. Implementations should treat omitted
optional integer values as having been transmitted present a Ticket and a properly formed
Authenticator (generated with the session key from the KDC response) to a value
service verifies the identity of zero,
if the application is expecting this.
5.1.3. Zero-length SEQUENCE Types
There are places in principal; likewise the protocol where a message contains a SEQUENCE OF
type as an optional member, or a SEQUENCE type where all members are
optional. This can result in an encoding that contains a zero-length
SEQUENCE or SEQUENCE OF encoding. Implementations should not send
zero-length SEQUENCE OF or SEQUENCE encodings that are marked OPTIONAL,
but should accept them as being equivalent to an omitted OPTIONAL type.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.1.4. Unrecognized Tag Numbers
Future revisions to this protocol may include new message types with
different APPLICATION class tag numbers. Such revisions should protect
older implementations by only sending ability of the message types to parties that
are known
service to understand them, e.g. by means of a flag bit set by extract the
receiver session key from the Ticket and prove its knowledge
thereof in a preceding request. In response verifies the interest identity of robust error
handling, implementations should gracefully handle receiving the service.
[RFCEDITOR: Insert citation for KCRYPTO] defines a message
with an unrecognized tag anyway, framework for defining
encryption and return an error message if
appropriate.
5.1.5. Tag Numbers Greater Than 30
A naive implementation of a DER ASN.1 decoder may experience problems checksum mechanisms for use with ASN.1 tag numbers greater than 30, due to Kerberos. It also defines
several such tag numbers being
encoded using mechanisms, and more than one byte. Future revisions of this protocol may
utilize tag numbers greater than 30, and implementations should be
prepared added in future updates to gracefully return an error, if appropriate, if they do not
recognize the tag.
5.2. Basic Kerberos Types
This section defines a number of basic types that are potentially used
in multiple Kerberos protocol messages.
5.2.1. KerberosString
[XXX
document.
The following paragraphs may need some editing, or maybe they want string-to-key operation provided by [KCRYPTO] is used to live in produce a footnote]
[XXX Note that this duplicates much of the contents of jaltman's draft]
long-term key for a principal (generally for a user). The original specification default salt
string, if none is provided via preauthentication data, is the concatenation
of the Kerberos protocol in RFC 1510 uses
GeneralString principal's realm and name components, in numerous places for human-readable string data.
Historical implementations of Kerberos cannot utilize order, with no separators.
Unless otherwise indicated, the full power of
GeneralString. This ASN.1 type requires default string-to-key opaque parameter set
as defined in [KCRYPTO] is used.
Encrypted data, keys and checksums are transmitted using the EncryptedData,
EncryptionKey and Checksum data objects defined in section 5.2.9. The
encryption, decryption, and checksum operations described in this document
use of designation the corresponding encryption, decryption, and
invocation escape sequences as get_mic operations
described in [KCRYPTO], with implicit "specific key" generation using the
"key usage" values specified in ISO 2022 to switch character
sets, and the default character set that is designated for G0 is
basically US ASCII, which mostly works. In practice, many
implementations end up treating GeneralStrings as if they were strings description of whatever character set each EncryptedData or
Checksum object to vary the implementation defaults to, without regard key for correct usage of character set designation escape sequences.
Also, DER prohibits the invocation of character sets into any but each operation. Note that in some cases,
the G0
and C0 sets, which seems value to outright prohibit be used is dependent on the encoding method of characters
with choosing the high bit set. Unfortunately, this seems to have key or the side effect
context of prohibiting the transmission of Latin-1 characters or any other
characters that belong to a 96-character set, since it is prohibited to
invoke them into G0. Some inconclusive discussion has taken place within
the ASN.1 community on this subject. For now, we must assume that the
ASN.1 specification of GeneralString as currently published message.
Key usages are unsigned 32 bit integers; zero is
fundamentally flawed not permitted. The key
usage values for encrypting or checksumming Kerberos messages are indicated
in several ways.
One method of resolving these myriad difficulties is to constrain section 5 along with the
use of GeneralString message definitions. Key usage values 512-1023
are reserved for uses internal to only include IA5String, which is essentially the
US-ASCII. US-ASCII control characters should in general a Kerberos implementation. (For example,
seeding a pseudo-random number generator with a value produced by encrypting
something with a session key and a key usage value not be used in
KerberosString, except for cases such as newlines any other
purpose.) Key usage values between 1024 and 2047 (inclusive) are reserved
for application use; applications should use even values for encryption and
odd values for checksums within this range. Key usage values are also
summarized in lengthy error
messages.
draft-ietf-krb-wg-kerberos-clarifications-01 a table in section 8.4.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
The new (since RFC 1510) type KerberosString, defined below, is a
GeneralString that is constrained to only contain characters
There might exist other documents which define protocols in
IA5String (which are US-ASCII). Note that the ASN.1 standard does not
permit the use terms of escape sequences to change the character sets while
encoding an IA5String.
KerberosString ::= GeneralString (IA5String)
Implementations may choose to accept GeneralString values
RFC1510 encryption types or checksum types. Such documents would not know
about key usages. In order that contain
characters other than those permitted by IA5String, but they should these specifications continue to be
aware that character set designation codes will likely
meaningful until they are updated, key usages 1024 and 1025 must be absent, used to
derive keys for encryption and checksums, respectively. (Of course, this
does not apply to protocols that do their own encryption independent of this
framework, directly using the encoding should probably be treated as locale-specific key resulting from the Kerberos authentication
exchange.) New protocols defined in
almost every way. Implementations may also choose to emit GeneralString
values that are beyond those permitted by IA5String, but terms of the Kerberos encryption and
checksum types should be aware
that doing so use their own key usage values.
Unless otherwise indicated, no cipher state chaining is extraordinarily risky done from an interoperability
perspective.
Some existing implementations use GeneralString one
encryption operation to encode unescaped
locale-specific characters. This is in violation of another.
Implementation note: While we don't recommend it, undoubtedly some
application protocols will continue to use the ASN.1 standard.
Most of these implementations encode US-ASCII key data directly, even if
only in some of the left-hand half, so
as long the currently existing protocol specifications. [4.1]. An
implementation transmits only US-ASCII, intended to support general Kerberos applications may
therefore need to make the ASN.1 standard
is not violated in this regard. As soon key data available, as well as such an implementation
encodes unescaped locale-specific characters with the high bit set, it
violates the attributes and
operations described in [KCRYPTO].
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5. Message Specifications
NOTE: The ASN.1 standard.
Other implementations have been known to use GeneralString collected here should be identical to contain a
UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 contents of
Appendix A. In case of conflict, the contents of Appendix A shall take
precedence.
The Kerberos protocol is defined here in terms of Abstract Syntax Notation
One (ASN.1) [X680], which provides a
different encoding, not a 94 or 96 character "G" set syntax for specifying both the abstract
layout of protocol messages as defined by ISO
2022. It is believed that these implementations do well as their encodings. Implementors not even use the ISO
2022 escape sequence to change the character encoding. Even if
implementations were
utilizing an existing ASN.1 compiler or support library are cautioned to announce the change of encoding by using that
escape sequence,
thoroughly understand the actual ASN.1 standard prohibits specification to ensure correct
implementation behavior, as there is more complexity in the use of any escape
sequences other notation than those used is
immediately obvious, and some tutorials and guides to designate/invoke "G" ASN.1 are misleading
or "C" sets
allowed by GeneralString.
Future revisions to this protocol will almost certainly allow for a more
interoperable representation of principal names, probably including
UTF8String. erroneous.
Note that applying a new constraint in several places, there have been changes here from RFC 1510 that
change the abstract types. This is in part to a previously unconstrained type
constitutes creation address widespread assumptions
that various implementors have made, in some cases resulting in
unintentional violations of a new the ASN.1 type. In this particular case, standard. These will be clearly
flagged when they occur. The differences between the
change does not result abstract types in a changed encoding under DER.
5.2.2. Realm RFC
1510 and PrincipalName
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Kerberos realm names abstract types in this document can cause incompatible encodings to
be emitted when certain encoding rules, e.g. the Packed Encoding Rules
(PER), are encoded as KerberosStrings. Realms shall used. This theoretical incompatibility should not
contain a character with be relevant for
Kerberos, since Kerberos explicitly specifies the code 0 (the ASCII NUL). Most realms will
usually consist use of several components separated by periods (.), in the
style Distinguished
Encoding Rules (DER). It might be an issue for protocols wishing to use
Kerberos types with other encoding rules. (This practice is not
recommended.) With very few exceptions (most notably the usages of Internet Domain Names, or separated by slashes (/) BIT
STRING), the encodings resulting from using the DER remain identical between
the types defined in RFC 1510 and the style
of X.500 names. Acceptable forms for realm names are specified types defined in this document.
The type definitions in this section 7. A PrincipalName is a typed sequence of components consisting assume an ASN.1 module definition of
the following sub-fields:
name-type form:
KerberosV5Spec2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- rest of definitions here
END
This field specifies the type of name that follows. Pre-defined
values the tagging context for this field are specified in section 7.2. The name-type
should be treated as a hint. Ignoring the name type, no two names
can module will be explicit and
non-automatic.
Note that in some other publications [RFC1510] [RFC1964], the same (i.e. at least one "dod" portion
of the components, or the realm,
must be different). This constraint may be eliminated in the future.
name-string
This field encodes a sequence of components that form a name, each
component encoded as a KerberosString. Taken together, a
PrincipalName and a Realm form a principal identifier. Most
PrincipalNames will have only a few components (typically one or two).
5.2.3. KerberosTime
KerberosTime ::= GeneralizedTime -- with no fractional seconds
The timestamps used in Kerberos are encoded object identifier is erroneously specified as GeneralizedTimes. A
KerberosTime having the value shall not include any fractional portions "5".
In the case of RFC 1964, use of the
seconds. As required by "correct" OID value would result in a
change in the DER, it further shall not include any
separators, and wire protocol; therefore, it shall specify the UTC time zone (Z). Example: The
only valid format remains unchanged for UTC time 6 minutes, 27 seconds after 9 pm on 6
November 1985 is 19851106210627Z.
5.2.4. Constrained Integer now.
Note that elsewhere in this document, nomenclature for various message types
Some integer members
is inconsistent, but seems to largely follow C language conventions,
including use of types should be constrained underscore (_) characters and all-caps spelling of names
intended to values
representable be numeric constants. Also, in 32 bits, for compatibility with reasonable
implementation limits.
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable some places, identifiers
(especially ones refering to constants) are written in 32 bits
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
While this results all-caps in changes order to the abstract types
distinguish them from the RFC 1510
version, the encoding surrounding explanatory text.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
The ASN.1 notation does not permit underscores in DER should be unaltered. Historical
implementations were typically limited to 32-bit integer values anyway, identifiers, so in actual
ASN.1 definitions, underscores are replaced with hyphens (-). Additionally,
structure member names and assigned numbers should fall defined values in ASN.1 must begin with a
lowercase letter, while type names must begin with an uppercase letter.
5.1. Specific Compatibility Notes on ASN.1
For compatibility purposes, implementors should heed the space following specific
notes regarding the use of integer values
representable in 32 bits ASN.1 in order Kerberos. These notes do not describe
deviations from standard usage of ASN.1. The purpose of these notes is to promote interoperability anyway.
There are
instead describe some members historical quirks and non-compliance of messages types that are still defined various
implementations, as
unconstrained INTEGER types, but many well as historical ambiguities, which, while being valid
ASN.1, can lead to confusion during implementation.
5.1.1. ASN.1 Distinguished Encoding Rules
The encoding of these have a (non-ASN.1)
constraint applied in Kerberos protocol messages shall obey the descriptive text. There Distinguished
Encoding Rules (DER) of ASN.1 as described in [X690]. Some implementations
(believed to be primarly ones derived from DCE 1.1 and earlier) are specific cases
where more discussion needs known to occur regarding possible constraints,
such as for
use the nonce fields more general Basic Encoding Rules (BER); in various messages.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
There are several integer fields particular, these
implementations send indefinite encodings of lengths. Implementations may
accept such encodings in messages that are constrained to
fixed values.
pvno
also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always the constant integer 5. There is no easy way to make this field into
a useful protocol version number, so its value interests of backwards compatibility, though
implementors are warned that decoding fully-general BER is fixed.
msg-type
this fraught with
peril.
5.1.2. Optional Integer Fields
Some implementations do not internally distinguish between an omitted
optional integer field usually is identical to the application tag
number value and a transmitted value of zero. The places in the containing message type.
5.2.5. HostAddress and HostAddresses
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- XXX HostAddresses
protocol where this is always used as an OPTIONAL field relevant include various microseconds fields, nonces,
and can be
-- zero-length.
HostAddresses -- XXX subtly different from rfc1510,
-- but has sequence numbers. Implementations should treat omitted optional integer
values as having been transmitted with a value mapping and encodes of zero, if the same
::= application
is expecting this.
5.1.3. Empty SEQUENCE OF HostAddress
The host address encodings consists of two fields:
addr-type
This field specifies the type of address that follows. Pre-defined
values for this field Types
There are specified places in section 8.1.
address
This field encodes the protocol where a single address of type addr-type.
The two forms differ slightly. HostAddress contains exactly one address;
HostAddresses message contains a sequence of possibly many addresses.
5.2.6. AuthorizationData
-- XXX AuthorizationData is always used SEQUENCE OF type
as an OPTIONAL field and optional member. This can
-- be zero-length.
AuthorizationData ::= result in an encoding that contains an empty
SEQUENCE OF encoding. The Kerberos protocol does not semantically
distinguish between an absent optional SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
ad-data
This field contains authorization data to be interpreted according
to the value of the corresponding ad-type field.
ad-type
This field specifies the format for the ad-data subfield. All
negative values are reserved for local use. Non-negative values are
reserved for registered use.
Each sequence of OF type and data is referred to as an authorization
element. Elements may be application specific, however, there is a
common set of recursive elements present
optional but empty SEQUENCE OF type. Implementations should not send empty
SEQUENCE OF encodings that are marked OPTIONAL, but should be understood by all
implementations. These elements contain other elements embedded within
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
them, and the interpretation of the encapsulating element determines
which of the embedded elements must be interpreted, and which may be
ignored. Definitions for these common elements may be found in Appendix
B <#ap_adata>.
5.2.7. PA-DATA
Historically, PA-DATA have been known accept them as "pre-authentication data",
meaning that they were used
being equivalent to augment the initial authentication with an omitted OPTIONAL type. In the KDC. Since that time, they have also been used as ASN.1 syntax describing
Kerberos messages, instances of these problematic optional SEQUENCE OF types
are indicated a typed hole with
which comment.
5.1.4. Unrecognized Tag Numbers
Future revisions to extend this protocol exchanges may include new message types with the KDC.
PA-DATA ::= SEQUENCE {
padata-type [1] Int32 -- first
different APPLICATION class tag is [1], not [0] --,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
padata-type
indicates numbers. Such revisions should protect older
implementations by only sending the way message types to parties that the padata-value element is are known
to be
interpreted. Negative values understand them, e.g. by means of padata-type are reserved for
unregistered use; non-negative values are used for a registered
interpretation of the element type.
padata-value
Usually contains flag bit set by the DER encoding of another type; receiver in a
preceding request. In the padata-type
field identifies which type is encoded here.
padata-type name contents interest of padata-value robust error handling, implementations
should gracefully handle receiving a message with an unrecognized tag
anyway, and return an error message if appropriate.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 pa-tgs-req DER encoding May 2003
5.1.5. Tag Numbers Greater Than 30
A naive implementation of AP-REQ
2 pa-enc-timestamp a DER encoding of PA-ENC-TIMESTAMP
3 pa-pw-salt salt (not ASN.1 encoded)
10 pa-etype-info DER encoding decoder may experience problems with
ASN.1 tag numbers greater than 30, due to such tag numbers being encoded
using more than one byte. Future revisions of PA-ETYPE-INFO
[XXX -- the following paragraph needs discussion, as does this protocol may utilize tag
numbers greater than 30, and implementations should be prepared to
gracefully return an error, if appropriate, if they do not recognize the general
concept
tag.
5.2. Basic Kerberos Types
This section defines a number of basic types that are potentially used in
multiple Kerberos protocol messages.
5.2.1. KerberosString
The original specification of authenticating the cleartext pieces Kerberos protocol in RFC 1510 uses
GeneralString in numerous places for human-readable string data. Historical
implementations of Kerberos cannot utilize the protocol] full power of GeneralString.
This field may also contain information needed by certain extensions to ASN.1 type requires the Kerberos protocol. For example, it might be used use of designation and invocation escape
sequences as specified in ISO-2022/ECMA-35 to initially verify switch character sets, and the identity of a client before any response
default character set that is returned.
[XXX -- The following paragraph designated as G0 is subject to change pending the outcome ISO-646/ECMA-6
International Reference Version (IRV) (aka U.S. ASCII), which mostly works.
ISO-2022/ECMA-35 defines four character-set code elements (G0..G3) and two
Control-function code elements (C0..C1). DER prohibits the designation of discussions concerning authenticated cleartext]
When this field is used to authenticate or pre-authenticate a request,
it should contain a keyed checksum over
character sets as any but the KDC-REQ-BODY G0 and C0 sets. Unfortunately, this seems to bind
have the
pre-authentication data to rest side effect of prohibiting the request. The KDC, as a matter use of
policy, may decide whether to honor a KDC-REQ which includes ISO-8859 (ISO Latin)
character-sets or any
pre-authentication data other character-sets that does not contain the checksum field.
It may also be used utilize a 96-character set,
since it is prohibited by the client ISO-2022/ECMA-35 to specify designate them as the version of a key that G0 code
element. This side effect is being used for accompanying preauthentication, and/or which should be
used to encrypt the reply from investigated in the KDC. [XXX ASN.1 standards
community.
In practice, many implementations treat GeneralStrings as if they were 8-bit
strings of whichever character set the following paragraph
should apply perhaps to PA-DATA in general] implementation defaults to, without
regard for correct usage of character-set designation escape sequences. The padata field can also contain information needed to help
default character set is often determined by the KDC or current user's operating
system dependent locale. At least one major implementation places unescaped
UTF-8 encoded Unicode characters in the client select GeneralString. This failure to
adhere to the key needed for generating or decrypting GeneralString specifications results in interoperability
issues when conflicting character encodings are utilized by the
response. Kerberos
clients, services, and KDC.
This form of the padata unfortunate situation is useful for supporting the use result of
certain token cards with Kerberos. The details of such extensions are
specified in separate documents. See [Pat92] for additional uses improper documentation of this
field.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.2.7.1. PA-TGS-REQ
In the case
restrictions of requests for additional tickets (KRB_TGS_REQ),
padata-value will contain an encoded AP-REQ. The checksum in the
authenticator (which must be collision-proof) ASN.1 GeneralString type in prior Kerberos
specifications.
The new (post-RFC 1510) type KerberosString, defined below, is to be computed over the
KDC-REQ-BODY encoding.
5.2.7.2. Encrypted Timestamp Pre-authentication
There are pre-authentication types a
GeneralString that may be used is constrained to pre-authenticate
a client by means of an encrypted timestamp.
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC only contain characters in IA5String
KerberosString ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
Patimestamp contains the client's time, and pausec contains the
microseconds, which may be omitted if a client will not generate more
than one request per second. The ciphertext (padata-value) consists of
the PA-ENC-TS-ENC encoding, encrypted using the client's secret key.
This preauthentication type was GeneralString (IA5String)
US-ASCII control characters should in general not present be used in RFC 1510, but many
implementations support it.
5.2.7.3. PA-PW-SALT
The padata-value for this preauthentication type contains the salt KerberosString,
except for
the string-to-key to cases such as newlines in lengthy error messages. Control
characters should not be used by the client in principal names or realm names.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
For compatibility, implementations may choose to obtain the key for
decrypting the encrypted part of an AS-REP message. Unfortunately, for
historical reasons, the accept GeneralString values
that contain characters other than those permitted by IA5String, but they
should be aware that character set to designation codes will likely be used is unspecified absent,
and that the encoding should probably locale-specific.
This preauthentication type was not present be treated as locale-specific in RFC 1510,
almost every way. Implementations may also choose to emit GeneralString
values that are beyond those permitted by IA5String, but many should be aware
that doing so is extraordinarily risky from an interoperability perspective.
Some existing implementations support it. It use GeneralString to encode unescaped
locale-specific characters. This is necessary a violation of the ASN.1 standard. Most
of these implementations encode US-ASCII in any case where the salt
for left-hand half, so as long
the string-to-key algorithm implementation transmits only US-ASCII, the ASN.1 standard is not
violated in this regard. As soon as such an implementation encodes unescaped
locale-specific characters with the default.
In high bit set, it violates the trivial example, a zero-length salt string is very commonplace
for realms that ASN.1
standard.
Other implementations have converted their principal databases from Kerberos 4.
5.2.7.4. PA-ETYPE-INFO
The ETYPE-INFO preauthentication type is sent by been known to use GeneralString to contain a
UTF-8 encoding. This also violates the KDC in ASN.1 standard, since UTF-8 is a KRB-ERROR
indicating
different encoding, not a requirement for additional preauthentication. 94 or 96 character "G" set as defined by ISO 2022.
It is usually
used believed that these implementations do not even use the ISO 2022
escape sequence to notify a client of which key change the character encoding. Even if implementations
were to use for announce the encryption change of an
encrypted timestamp for encoding by using that escape sequence, the purposes
ASN.1 standard prohibits the use of sending a PA-ENC-TIMESTAMP
preauthentication value.
ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
The salt, like that of PA-PW-SALT, is also completely unspecified with
respect any escape sequences other than those
used to character set and is probably locale-specific.
[XXX -- not clear whether ETYPE-INFO designate/invoke "G" or PW-SALT should take precedence
if they conflict]
This preauthentication type was not present in RFC 1510, but many
implementations that support encrypted timestamps "C" sets allowed by GeneralString.
Future revisions to this protocol will almost certainly allow for preauthentication
need a more
interoperable representation of principal names, probably including
UTF8String.
Note that applying a new constraint to support ETYPE-INFO as well.
5.2.8. KerberosFlags
For several message types, a specific constrained bit string type,
KerberosFlags, is used.
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number previously unconstrained type
constitutes creation of bits
-- shall be sent, but no fewer than 32
Compatibility note: the following paragraphs describe a change from new ASN.1 type. In this particular case, the
RFC1510 description of bit strings that would
change does not result in incompatility in
the case of an implementation that strictly conformed to ASN.1 DER and
RFC1510.
ASN.1 bit strings have multiple uses. The simplest use of a bit string
is to changed encoding under DER.
5.2.2. Realm and PrincipalName
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
Kerberos realm names are encoded as KerberosStrings. Realms shall not
contain a vector of bits, character with no particular meaning attached to
individual bits. This vector the code 0 (the ASCII NUL). Most realms will
usually consist of bits is not necessarily a multiple several components separated by periods (.), in the style
of
eight bits long. The use Internet Domain Names, or separated by slashes (/) in Kerberos the style of a bit string as a compact
boolean vector wherein each element has a distinct meaning poses some
problems. The natural notation X.500
names. Acceptable forms for a compact boolean vector realm names are specified in section 7. A
PrincipalName is a typed sequence of components consisting of the ASN.1
"NamedBit" notation, and following
sub-fields:
name-type
This field specifies the DER require that encodings type of a bit string
using "NamedBit" notation exclude any trailing zero bits. This
truncation is easy to neglect, especially given C language
implementations name that may naturally choose to store boolean vectors as 32
bit integers.
For example, if the notation follows. Pre-defined values
for KDCOptions were to include the
"NamedBit" notation, as this field are specified in RFC 1510, and a KDCOptions value to section 7.2. The name-type should be
encoded had only the "forwardable" (bit number one) bit set,
treated as a hint. Ignoring the DER
encoding must only include name type, no two bits: the first reserved bit ("reserved",
bit number zero, value zero) and names can be the one-valued bit (bit number one) for
"forwardable".
Most existing implementations same
(i.e. at least one of Kerberos unconditionally send 32 bits
on the wire when encoding bit strings used as boolean vectors. This
behavior violates components, or the ASN.1 syntax used for flag values realm, must be different).
This constraint may be eliminated in RFC 1510, but
occurs on such a widely installed base that the protocol description is
being modified to accomodate it.
Consequently, this document removes the "NamedBit" notations for
individual bits, relegating them to comments. The size constraint on the
KerberosFlags type requires future.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
name-string
This field encodes a sequence of components that at least 32 bits be form a name, each
component encoded at all
times, though as a lenient implementation may choose to accept fewer than
32 bits KerberosString. Taken together, a PrincipalName
and to treat the missing bits as set to zero.
Currently, a Realm form a principal identifier. Most PrincipalNames will have
only a few components (typically one or two).
5.2.3. KerberosTime
KerberosTime ::= GeneralizedTime -- with no uses fractional seconds
The timestamps used in Kerberos are encoded as GeneralizedTimes. A
KerberosTime value shall not include any fractional portions of KerberosFlags the seconds.
As required by the DER, it further shall not include any separators, and it
shall specify more than 32 bits worth of
flags, although future revisions the UTC time zone (Z). Example: The only valid format for UTC
time 6 minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z.
5.2.4. Constrained Integer types
Some integer members of this document may do so. When more
than types should be constrained to values representable
in 32 bits, for compatibility with reasonable implementation limits.
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits are to be transmitted
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
While this results in a KerberosFlags value, future
revisions changes to this document will likely specify that the smallest number
of bits needed to encode abstract types from the highest-numbered one-valued bit RFC 1510
version, the encoding in DER should be
sent. This is somewhat similar unaltered. Historical implementations
were typically limited to 32-bit integer values anyway, and assigned numbers
should fall in the DER encoding space of a bit string integer values representable in 32 bits in order
to promote interoperability anyway.
There are several integer fields in messages that are constrained to fixed
values.
pvno
also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is declared with always the "NamedBit" notation.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.2.9. Cryptosystem-related Types
Many Kerberos protocol messages contain an EncryptedData as
constant integer 5. There is no easy way to make this field into a container
for arbitrary encrypted data, which
useful protocol version number, so its value is often fixed.
msg-type
this integer field usually is identical to the encrypted encoding application tag number
of
another data type. Fields within EncryptedData assist the recipient in
selecting a key with which to decrypt the enclosed data.
EncryptedData containing message type.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.5. HostAddress and HostAddresses
HostAddress ::= SEQUENCE {
etype
addr-type [0] Int32 -- EncryptionType --,
kvno Int32,
address [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
}
etype
This field identifies which encryption algorithm was
-- NOTE: HostAddresses is always used to
encipher as an OPTIONAL field and
-- should not be empty.
HostAddresses -- NOTE: subtly different from rfc1510,
-- but has a value mapping and encodes the cipher. Detailed specifications for selected encryption
types appear in section 6. kvno same
::= SEQUENCE OF HostAddress
The host address encodings consists of two fields:
addr-type
This field contains specifies the version number type of the key under which data
is encrypted. It is only present address that follows. Pre-defined
values for this field are specified in messages encrypted under long
lasting keys, such as principals' secret keys. cipher section 8.1.
address
This field contains the enciphered text, encoded as an OCTET STRING.
The EncryptionKey encodes a single address of type addr-type.
The two forms differ slightly. HostAddress contains exactly one address;
HostAddresses contains a sequence of possibly many addresses.
5.2.6. AuthorizationData
-- NOTE: AuthorizationData is the means by which cryptographic keys always used for
encryption are transfered.
EncryptionKey as an OPTIONAL field and
-- should not be empty.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
keytype
ad-type [0] Int32 -- actually encryption type --,
keyvalue Int32,
ad-data [1] OCTET STRING
}
keytype
ad-data
This field specifies the encryption type of the encryption key that
follows in the keyvalue field. While its name is "keytype", it
actually specifies an encryption type. Previously, multiple
cryptosystems that performed encryption differently but were capable
of using keys with the same characteristics were permitted contains authorization data to share
an assigned number be interpreted according to designate
the type value of key; this usage is now
deprecated. keyvalue the corresponding ad-type field.
ad-type
This field contains specifies the key itself, encoded as an octet string. format for the ad-data subfield. All negative
values for the encryption key type are reserved for local use. All non-negative Non-negative values are reserved for officially
assigned
registered use.
Each sequence of type fields and interpretations.
Messages containing cleartext data is referred to as an authorization element.
Elements may be authenticated will usually do
so by using application specific, however, there is a member common set of type Checksum. Most instances
recursive elements that should be understood by all implementations. These
elements contain other elements embedded within them, and the interpretation
of Checksum use a
keyed hash, though exceptions will the encapsulating element determines which of the embedded elements must
be noted.
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
cksumtype
This field indicates interpreted, and which may be ignored.
These common authorization data elements are recursivly defined, meaning the algorithm used to generate
ad-data for these types will itself contain a sequence of authorization data
whose interpretation is affected by the accompanying
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
checksum. checksum
This field contains encapsulating element. Depending on
the checksum itself, encoded meaning of the encapsulating element, the encapsulated elements may be
ignored, might be interpreted as an octet string.
Detailed specification issued directly by the KDC, or they might
be stored in a separate plaintext part of selected checksum the ticket. The types appear in section
6. Negative values for of the checksum type
encapsulating elements are reserved for local use.
All non-negative specified as part of the Kerberos specification
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
because the behavior based on these values are reserved for officially assigned type
fields and interpretations.
5.3. Tickets
This section describes should be understood across
implementations whereas other elements need only be understood by the format and encryption parameters for tickets
and authenticators. When
applications which they affect.
Authorization data elements are considered critical if present in a ticket
or authenticator is included authenticator. Unless encapsulated in a
protocol message known authorization data element
amending the criticality of the elements it is treated as contains, if an opaque object. A ticket unknown
authorization data element type is received by a record
that helps a client authenticate to a service. A Ticket contains server either in an AP-REQ
or in a ticket contained in an AP-REQ, then authentication SHOULD fail.
Authorization data is intended to restrict the
following information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part use of a ticket. If the
service cannot determine whether the restriction applies to that service
then a security weakness may result if the ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must can be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the following used for that
service. Authorization elements that are new since 1510; maybe remove from krb-clarifications?
-- transited-policy-checked(12),
-- ok-as-delegate(13)
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
The encoding of EncTicketPart is encrypted optional can be enclosed in
AD-IF-RELEVANT element.
In the key shared by Kerberos
and definitions that follow, the end server (the server's secret key). See section 6 value of the ad-type for the
format element
will be specified as the least significant part of the ciphertext.
tkt-vno
This field specifies subsection number,
and the version number for value of the ticket format. This
document describes version number 5.
realm
This field specifies ad-data will be as shown in the realm ASN.1 structure that issued a ticket. It also serves
to identify
follows the realm part subsection heading.
contents of ad-data ad-type
DER encoding of AD-IF-RELEVANT 1
DER encoding of AD-KDCIssued 4
DER encoding of AD-OR 5
DER encoding of AD-MANDATORY-FOR-KDC 8
5.2.6.1. IF-RELEVANT
AD-IF-RELEVANT ::= AuthorizationData
AD elements encapsulated within the server's principal identifier.
Since a Kerberos server can only issue tickets if-relevant element are intended for
interpretation only by application servers within
its realm, the two will always be identical.
sname
This field specifies all components of that understand the name part particular
ad-type of the server's
identity, including those parts embedded element. Application servers that identify a specific instance of
a service.
enc-part
This field holds do not understand
the encrypted encoding type of an element embedded within the EncTicketPart sequence.
flags if-relevant element may ignore
the uninterpretable element. This field indicates element promotes interoperability across
implementations which of various options were used or requested
when may have local extensions for authorization.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.6.4. KDCIssued
AD-KDCIssued ::= SEQUENCE {
ad-checksum [0] Checksum,
i-realm [1] Realm OPTIONAL,
i-sname [2] PrincipalName OPTIONAL,
elements [3] AuthorizationData
}
ad-checksum
A checksum over the ticket was issued. It is elements field using a bit-field, where cryptographic checksum
method that is identical to the selected
options are indicated by checksum used to protect the bit being set (1), and ticket
itself (i.e. using the unselected
options same hash function and reserved fields being reset (0). [XXX X.690 ref the same encryption
algorithm used to encrypt the ticket) using the key used to protect the
ticket, and
notes on pitfalls?] a key usage value of 19.
i-realm, i-sname
The meanings name of the flags are:
Bit(s) Name Description
0 reserved Reserved for future expansion of this field.
1 forwardable The FORWARDABLE flag is normally only interpreted by issuing principal if different from the TGS, and can KDC itself.
This field would be ignored used when the KDC can verify the authenticity of
elements signed by end servers. When set, this flag
tells the ticket-granting server that issuing principal and it is OK allows this KDC to issue a new
ticket-granting ticket with a different network address based on
notify the
presented ticket.
2 forwarded When set, this flag indicates that application server of the ticket has either
been forwarded or was validity of those elements.
elements
A sequence of authorization data elements issued based on authentication involving a
forwarded ticket-granting ticket.
3 proxiable The PROXIABLE flag is normally only interpreted by the
TGS, KDC.
The KDC-issued ad-data field is intended to provide a means for Kerberos
principal credentials to embed within themselves privilege attributes and
other mechanisms for positive authorization, amplifying the priveleges of
the principal beyond what can be ignored by end servers. The PROXIABLE flag has done using a credentials without such an
interpretation identical to that
a-data element.
This can not be provided without this element because the definition of the FORWARDABLE flag, except
that
authorization-data field allows elements to be added at will by the PROXIABLE flag tells bearer
of a TGT at the ticket-granting server time that only
non-ticket-granting they request service tickets and elements may also
be issued with different network
addresses.
4 proxy When set, this flag indicates that added to a delegated ticket is a proxy.
5 may-postdate The MAY-POSTDATE flag is normally only interpreted by inclusion in the TGS, and can be ignored authenticator.
For KDC-issued elements this is prevented because the elements are signed by end servers. This flag tells
the
ticket-granting server that KDC by including a post-dated ticket may be issued based
on this ticket-granting ticket.
6 postdated This flag indicates that this ticket has been postdated.
The end-service can check checksum encrypted using the authtime field server's key (the same
key used to see when encrypt the
original authentication occurred.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
7 invalid This flag indicates that a ticket is invalid, and it must - or a key derived from that key). Elements
encapsulated with in the KDC-issued element will be validated ignored by the KDC before use. Application servers must reject
tickets which have
application server if this flag set.
8 renewable The RENEWABLE flag "signature" is normally only not present. Further, elements
encapsulated within this element from a ticket granting ticket may be
interpreted by the
TGS, KDC, and can usually used as a basis according to policy for
including new signed elements within derivative tickets, but they will not
be ignored by end servers (some particularly
careful servers may wish copied to disallow renewable tickets). A renewable a derivative ticket can be used directly. If they are copied directly to obtain a replacement
derivative ticket that expires at by a
later date.
9 initial This flag indicates KDC that is not aware of this ticket was issued using element, the
AS protocol, and signature
will not issued based on a ticket-granting ticket.
10 pre-authent This flag indicates that during initial
authentication, the client was authenticated by be correct for the KDC before a application ticket was issued. The strength of elements, and the preauthentication method is
not indicated, but is acceptable to field will
be ignored by the KDC.
11 hw-authent application server.
This flag indicates that the protocol employed for
initial authentication required element and the use of hardware expected to elements it encapulates may be
possessed solely by the named client. The hardware authentication
method is selected safely ignored by the KDC
applications, application servers, and the strength of the method is not
indicated.
12 transited- policy-checked This flag indicates KDCs that do not implement this
element.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.6.5. AND-OR
AD-AND-OR ::= SEQUENCE {
condition-count [0] INTEGER,
elements [1] AuthorizationData
}
When restrictive AD elements are encapsulated within the KDC for
the realm has checked and-or element are
encountered, only the transited field against a realm defined
policy for trusted certifiers. If this flag is reset (0), then number specified in condition-count of the
application server
encapsulated conditions must check be met in order to satisfy this element. This
element may be used to implement an "or" operation by setting the transited
condition-count field itself, to 1, and if
unable it may specify an "and" operation by setting
the condition count to the number of embedded elements. Application servers
that do so it not implement this element must reject tickets that contain
authorization data elements of this type.
5.2.6.8. MANDATORY-FOR-KDC
AD-MANDATORY-FOR-KDC ::= AuthorizationData
AD elements encapsulated within the authentication. If mandatory-for-kdc element are to be
interpreted by the flag is
set (1) then KDC. KDCs that do not understand the application server may skip its own validation type of an element
embedded within the transited field, relying on if-relevant element must reject the validation performed by request.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.7. PA-DATA
Historically, PA-DATA have been known as "pre-authentication data", meaning
that they were used to augment the KDC.
At its option initial authentication with the application server may still apply its own
validation based on KDC.
Since that time, they have also been used as a separate policy for acceptance.
This flag typed hole with which to
extend protocol exchanges with the KDC.
PA-DATA ::= SEQUENCE {
-- NOTE: first tag is new since RFC 1510.
13 ok-as-delegate This flag [1], not [0]
padata-type [1] Int32,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
padata-type
indicates that the server (not the
client) specified in the ticket has been determined by policy of way that the
realm padata-value element is to be interpreted.
Negative values of padata-type are reserved for unregistered use;
non-negative values are used for a suitable recipient registered interpretation of delegation. A client can use the
presence
element type.
padata-value
Usually contains the DER encoding of this flag another type; the padata-type
field identifies which type is encoded here.
padata-type name contents of padata-value
1 pa-tgs-req DER encoding of AP-REQ
2 pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP
3 pa-pw-salt salt (not ASN.1 encoded)
11 pa-etype-info DER encoding of PA-ETYPE-INFO
19 pa-etype-info2 DER encoding of PA-ETYPE-INFO2
This field may also contain information needed by certain extensions to help the
Kerberos protocol. For example, it make a decision whether might be used to delegate
credentials (either grant a proxy or initially verify the
identity of a forwarded ticket granting
ticket) to this server. The client before any response is free returned.
The padata field can also contain information needed to ignore help the value of
this flag. When setting this flag, an administrator should consider KDC or the Security and placement of
client select the server on which key needed for generating or decrypting the service will
run, as well as whether response. This
form of the service requires padata is useful for supporting the use of delegated
credentials.
This flag is new since RFC 1510.
14-31 reserved Reserved for future use.
key
This field exists certain token cards
with Kerberos. The details of such extensions are specified in separate
documents. See [Pat92] for additional uses of this field.
5.2.7.1. PA-TGS-REQ
In the ticket and case of requests for additional tickets (KRB_TGS_REQ), padata-value
will contain an encoded AP-REQ. The checksum in the KDC response and authenticator (which
must be collision-proof) is used to
pass be computed over the session key from Kerberos KDC-REQ-BODY encoding.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.7.2. Encrypted Timestamp Pre-authentication
There are pre-authentication types that may be used to pre-authenticate a
client by means of an encrypted timestamp.
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
Patimestamp contains the application server client's time, and the
client. The field's encoding is described in section 6.2.
crealm
This field pausec contains the name of the realm in
microseconds, which the may be omitted if a client is
registered and in which initial authentication took place.
cname
This field contains the name part will not generate more than
one request per second. The ciphertext (padata-value) consists of the client's principal identifier.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
transited
This field lists
PA-ENC-TS-ENC encoding, encrypted using the names client's secret key and a key
usage value of the Kerberos realms that took part in
authenticating the user to whom this ticket 1.
This preauthentication type was issued. It does not
specify the order present in which the realms were transited. See section
3.3.3.2 RFC 1510, but many
implementations support it.
5.2.7.3. PA-PW-SALT
The padata-value for details on how this field encodes preauthentication type contains the traversed realms.
When salt for the names of CA's are
string-to-key to be embedded in used by the transited field (as
specified for some extensions client to obtain the protocol), the X.500 names of
the CA's should be mapped into items in the transited field using
the mapping defined by RFC2253.
authtime
This field indicates the time of initial authentication key for decrypting the
named principal. It is the time
encrypted part of issue an AS-REP message. Unfortunately, for historical reasons,
the original ticket on
which this ticket character set to be used is based. unspecified and probably locale-specific.
This preauthentication type was not present in RFC 1510, but many
implementations support it. It is included necessary in any case where the ticket to provide
additional information to the end service, and to provide the
necessary information salt for implementation of a `hot list' service at
the KDC. An end service that string-to-key algorithm is particularly paranoid could refuse
to accept tickets for which not the initial authentication occurred "too
far" in default.
In the past. This field trivial example, a zero-length salt string is also returned as part of the
response very commonplace for
realms that have converted their principal databases from the KDC. When returned as part of the response to
initial authentication (KRB_AS_REP), this is the current time on the Kerberos server[24].
starttime
This field in the ticket specifies the time after which the ticket
is valid. Together with endtime, this field specifies the life of
the ticket. If it is absent from the ticket, its value 4.
A KDC should be
treated as that of the authtime field.
endtime
This field contains the time after which the ticket will not be
honored (its expiration time). Note that individual services may
place their own limits on the life of send PA-PW-SALT when issuing a ticket and may reject
tickets which have not yet expired. As such, this is really KRB-ERROR message that
requests additional preauthentication. Implementation note: some KDC
implementations issue an upper
bound on the expiration time for the ticket.
renew-till
This field is only present in tickets erroneous PA-PW-SALT when issuing a KRB-ERROR
message that have the RENEWABLE flag
set in the flags field. It indicates the maximum endtime requests additional preauthentication. Therefore, clients
should ignore a PA-PW-SALT accompanying a KRB-ERROR message that may be
included requests
additional preauthentication.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.2.7.4. PA-ETYPE-INFO
The ETYPE-INFO preauthentication type is sent by the KDC in a renewal. KRB-ERROR
indicating a requirement for additional preauthentication. It can be thought is usually
used to notify a client of as which key to use for the absolute
expiration time encryption of an
encrypted timestamp for the ticket, including all renewals.
caddr
This field in purposes of sending a ticket contains zero (if omitted) or more (if
present) host addresses. These are PA-ENC-TIMESTAMP
preauthentication value. It may also be sent in an AS-REP to provide
information to the addresses from client about which key salt to use for the
ticket can be used. If there are no addresses, the ticket can string-to-key
to be used from any location. The decision by the KDC client to issue or by obtain the
end server to accept zero-address tickets is a policy decision and
is left to key for decrypting the Kerberos and end-service administrators; they may
refuse to issue or accept such tickets. encrypted part
the AS-REP.
ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
The suggested salt, like that of PA-PW-SALT, is also completely unspecified with
respect to character set and default
policy, however, is that such tickets will only probably locale-specific.
If ETYPE-INFO is sent in an AS-REP, there shall be issued or
accepted when additional information exactly one
ETYPE-INFO-ENTRY, and its etype shall match that can be used to restrict
the use of the ticket is included enc-part in the authorization_data field.
Such a ticket is a capability.
Network addresses are included
AS-REP.
This preauthentication type was not present in the ticket to make it harder RFC 1510, but many
implementations that support encrypted timestamps for
an attacker preauthentication need
to use stolen credentials. Because the session key support ETYPE-INFO as well.
5.2.7.5. PA-ETYPE-INFO2
The ETYPE-INFO2 preauthentication type is
not sent over by the network KDC in cleartext, credentials can't be stolen
simply by listening a KRB-ERROR
indicating a requirement for additional preauthentication. It is usually
used to notify a client of which key to use for the network; encryption of an attacker has
encrypted timestamp for the purposes of sending a PA-ENC-TIMESTAMP
preauthentication value. It may also be sent in an AS-REP to gain access provide
information to the session client about which key (perhaps through operating system security
breaches or a careless user's unattended session) salt to make use of
stolen tickets.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
It is important to note that for the network address from which a
connection is received cannot string-to-key
to be reliably determined. Even if it
could be, an attacker who has compromised the client's workstation
could use used by the credentials from there. Including client to obtain the network
addresses only makes it more difficult, not impossible, key for an
attacker to walk off with stolen credentials and then use them from
a "safe" location.
authorization-data decrypting the encrypted part
the AS-REP.
ETYPE-INFO2-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] KerberosString OPTIONAL,
s2kparams [2] OCTET STRING OPTIONAL
}
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO-ENTRY
The authorization-data field type of the salt is used KerberosString, but existing installations might
have locale-specific characters stored in salt strings, and implementors may
choose to pass authorization data from handle them.
The interpretation of s2kparams is specified in the principal on whose behalf a ticket was issued to cryptosystem description
associated with the application
service. If no authorization data is included, this field will be
left out. Experience etype. Each cryptosystem has shown a default interpretation of
s2kparams that will hold if that element is omitted from the name encoding of this field
ETYPE-INFO2-ENTRY.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
If ETYPE-INFO2 is
confusing, sent in an AS-REP, there shall be exactly one
ETYPE-INFO2-ENTRY, and its etype shall match that a better name for this field would be
restrictions. Unfortunately, it is not possible to change the name
of this field at this time.
This field contains restrictions on any authority obtained on the
basis of authentication using the ticket. It is possible for any
principal enc-part in posession of credentials to add entries to the
authorization
AS-REP.
The preferred ordering of preauthentication data field since these entries further restrict what
can be done with the ticket. Such additions can be made that modify client key
selection is: ETYPE-INFO2, followed by
specifying ETYPE-INFO, followed by PW-SALT. A
KDC shall send all of these preauthentication data that it supports, in the additional entries
preferred ordering, when issuing an AS-REP or when issuing a new ticket KRB-ERROR
requesting additional preauthentication.
The ETYPE-INFO2 preauthentication type was not present in RFC 1510.
5.2.8. KerberosFlags
For several message types, a specific constrained bit string type,
KerberosFlags, is obtained
during the TGS exchange, or they may be added during chained
delegation using the authorization data field used.
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number of the authenticator.
Because entries may bits
-- shall be added to this field by the holder of
credentials, except when an entry is separately authenticated by
encapsulation in sent, but no fewer than 32
Compatibility note: the kdc-issued element, it is not allowable for following paragraphs describe a change from the
presence
RFC1510 description of an entry bit strings that would result in incompatility in the authorization data field
case of a ticket an implementation that strictly conformed to
amplify the privileges one would obtain from using a ticket. ASN.1 DER and RFC1510.
ASN.1 bit strings have multiple uses. The data in this field may be specific simplest use of a bit string is to the end service; the field
will
contain the names a vector of service specific objects, and the rights bits, with no particular meaning attached to those objects. The format for this field is described in section
5.2. Although Kerberos individual
bits. This vector of bits is not concerned with the format necessarily a multiple of the
contents eight bits long.
The use in Kerberos of a bit string as a compact boolean vector wherein each
element has a distinct meaning poses some problems. The natural notation for
a compact boolean vector is the sub-fields, it does carry type information (ad-type).
By using ASN.1 "NamedBit" notation, and the authorization_data field, DER
require that encodings of a principal bit string using "NamedBit" notation exclude any
trailing zero bits. This truncation is able easy to issue
a proxy neglect, especially given C
language implementations that is valid for a specific purpose. may naturally choose to store boolean vectors
as 32 bit integers.
For example, a client
wishing if the notation for KDCOptions were to print a file can obtain include the "NamedBit"
notation, as in RFC 1510, and a file server proxy KDCOptions value to be passed
to the print server. By specifying the name of the file in the
authorization_data field, encoded had only the file server knows that
"forwardable" (bit number one) bit set, the print
server can DER encoding must only use the client's rights when accessing include
two bits: the
particular file to be printed.
A separate service providing authorization or certifying group
membership may be built using first reserved bit ("reserved", bit number zero, value zero)
and the authorization-data field. In this
case, one-valued bit (bit number one) for "forwardable".
Most existing implementations of Kerberos unconditionally send 32 bits on
the entity granting authorization (not wire when encoding bit strings used as boolean vectors. This behavior
violates the authorized entity),
may obtain a ticket ASN.1 syntax used for flag values in its own name (e.g. RFC 1510, but occurs on
such a widely installed base that the ticket protocol description is issued in
the name of a privilege server), and being modified
to accomodate it.
Consequently, this entity adds restrictions
on its own authority and delegates document removes the restricted authority through
a proxy "NamedBit" notations for individual
bits, relegating them to the client. comments. The client would then present this
authorization credential to the application server separately from size constraint on the authentication exchange. Alternatively, such authorization
credentials may KerberosFlags
type requires that at least 32 bits be embedded in the ticket authenticating the
authorized entity, when the authorization is separately
authenticated using the kdc-issued authorization data element (see
B.4).
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Similarly, if one specifies the authorization-data field of encoded at all times, though a proxy
lenient implementation may choose to accept fewer than 32 bits and leaves the host addresses blank, to treat
the resulting ticket and
session key can be treated missing bits as a capability. See [Neu93] for some
suggested set to zero.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
Currently, no uses of KerberosFlags specify more than 32 bits worth of
flags, although future revisions of this field.
The authorization-data field is optional and does not have document may do so. When more than
32 bits are to be
included transmitted in a ticket.
5.4. Specifications for KerberosFlags value, future revisions to
this document will likely specify that the AS and TGS exchanges smallest number of bits needed to
encode the highest-numbered one-valued bit should be sent. This section specifies is somewhat
similar to the format DER encoding of a bit string that is declared with the
"NamedBit" notation.
5.2.9. Cryptosystem-related Types
Many Kerberos protocol messages used in the exchange
between the client and the Kerberos server. The format of possible error
messages appears in section 5.9.1.
5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no application tag number of its own.
Instead, it is incorporated into one of KRB_AS_REQ or KRB_TGS_REQ, which
each have contain an application tag, depending on whether the request is EncryptedData as a container for an
initial ticket or an additional ticket. In either case, the message
arbitrary encrypted data, which is
sent from often the client to encrypted encoding of another
data type. Fields within EncryptedData assist the KDC to request credentials for recipient in selecting a service.
The message fields are:
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
pvno [1] INTEGER (5) -- first tag is [1], not [0] --,
msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
padata [3] SEQUENCE OF PA-DATA OPTIONAL
-- XXX may be zero-length --,
req-body [4] KDC-REQ-BODY
}
KDC-REQ-BODY
key with which to decrypt the enclosed data.
EncryptedData ::= SEQUENCE {
kdc-options [0] KDCOptions,
cname [1] PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] PrincipalName OPTIONAL,
from [4] KerberosTime OPTIONAL,
till [5] KerberosTime,
rtime [6] KerberosTime OPTIONAL,
nonce [7] UInt32,
etype [8] SEQUENCE OF [0] Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses
kvno [1] UInt32 OPTIONAL,
enc-authorization-data [10] EncryptedData -- AuthorizationData --,
additional-tickets [11] SEQUENCE OF Ticket OPTIONAL
cipher [2] OCTET STRING -- XXX may be zero-length ciphertext
}
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- unused11(11),
-- unused12(12),
-- unused13(13),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
The fields in this message are:
pvno
etype
This field is included in each message, and specifies identifies which encryption algorithm was used to encipher
the protocol
version number. This document specifies protocol version 5.
msg-type cipher.
kvno
This field indicates contains the type version number of a protocol message. It will almost
always be the same as the application identifier associated with a
message. key under which data is
encrypted. It is included to make the identifier more readily
accessible to only present in messages encrypted under long lasting
keys, such as principals' secret keys.
cipher
This field contains the application. For enciphered text, encoded as an OCTET STRING.
(Note that the KDC-REQ message, this type
will be KRB_AS_REQ or KRB_TGS_REQ.
padata
Contains pre-authentication data. Requests for additional tickets
(KRB_TGS_REQ) encryption mechanisms defined in [KCRYPTO] must contain a padata of PA-TGS-REQ.
incorporate integrity protection as well, so no additional checksum is
required.)
The padata (pre-authentication data) field contains a sequence of
authentication information EncryptionKey type is the means by which may be needed before credentials
can be issued or decrypted. In most requests cryptographic keys used for initial
authentication (KRB_AS_REQ) and most replies (KDC-REP), the padata
field will be left out.
req-body
encryption are transfered.
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
keytype
This field is a placeholder delimiting specifies the extent encryption type of the remaining
fields. If a checksum is to be calculated over encryption key that
follows in the request, it keyvalue field. While its name is
calculated over "keytype", it actually
specifies an encoding encryption type. Previously, multiple cryptosystems that
performed encryption differently but were capable of using keys with
the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests same characteristics were permitted to the
KDC and indicates the flags that the client wants set on the tickets
as well as other information that is share an assigned number to modify the behavior of the
KDC. Where appropriate,
designate the name type of an option may be the same as the
flag that is set by that option. Although in most case, the bit in
the options field will be the same as that in the flags field, key; this usage is not guaranteed, so it is not acceptable to simply copy the
options now deprecated.
keyvalue
This field to contains the flags field. There are various checks that must
be made before honoring key itself, encoded as an option anyway.
draft-ietf-krb-wg-kerberos-clarifications-01 octet string.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 2003
The kdc_options field is a bit-field, where the selected options are
indicated by the bit being set (1), and the unselected options and
reserved fields being reset (0). The encoding of the bits is
specified in section 5.2. The options are described in more detail
above in section 2. The meanings of the options are:
Bits Name Description
0 RESERVED Reserved for future expansion of this field. 1 FORWARDABLE The FORWARDABLE option indicates that the ticket to be
issued is May 2003
Messages containing cleartext data to have its forwardable flag set. It may only be set on
the initial request, or in authenticated will usually do so by
using a subsequent request if the
ticket-granting ticket on which it is based is also forwardable.
2 FORWARDED The FORWARDED option is only specified in member of type Checksum. Most instances of Checksum use a request to
the ticket-granting server and keyed
hash, though exceptions will only be honored if the
ticket-granting ticket in the request has its FORWARDABLE bit set. noted.
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
cksumtype
This option field indicates that this is a request for forwarding. The
address(es) of the host from which the resulting ticket is algorithm used to be
valid are included in generate the addresses accompanying
checksum.
checksum
This field contains the checksum itself, encoded as an octet string.
See section 4 for a brief description of the request.
3 PROXIABLE The PROXIABLE option indicates that use of encryption and checksums
in Kerberos.
5.3. Tickets
This section describes the format and encryption parameters for tickets and
authenticators. When a ticket to be
issued is to have its proxiable flag set. It may only be set on the
initial request, or authenticator is included in a subsequent request if the ticket-granting
ticket on which protocol
message it is based treated as an opaque object. A ticket is also proxiable.
4 PROXY The PROXY option indicates a record that this is helps a request for
client authenticate to a
proxy. This option will only be honored if the ticket-granting
ticket in the request has its PROXIABLE bit set. The address(es) of service. A Ticket contains the host from which the resulting following
information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part of ticket is to
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must be valid are included
in registered --,
contents [1] OCTET STRING
}
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the addresses following are new since 1510
-- transited-policy-checked(12),
-- ok-as-delegate(13)
tkt-vno
This field of specifies the request.
5 ALLOW-POSTDATE The ALLOW-POSTDATE option indicates that version number for the ticket
to be format. This
document describes version number 5.
realm
This field specifies the realm that issued is to have its MAY-POSTDATE flag set. a ticket. It may only be
set on also serves to
identify the initial request, or in a subsequent request if realm part of the
ticket-granting ticket on which it is based also has its
MAY-POSTDATE flag set.
6 POSTDATED The POSTDATED option indicates that this is server's principal identifier. Since a request
Kerberos server can only issue tickets for a postdated ticket. This option servers within its realm,
the two will only always be honored if identical.
sname
This field specifies all components of the
ticket-granting ticket on which it is based has its MAY-POSTDATE
flag set. The resulting ticket will also have its INVALID flag set,
and name part of the server's
identity, including those parts that flag may be reset by identify a subsequent request to specific instance of a
service.
enc-part
This field holds the KDC after encrypted encoding of the starttime EncTicketPart sequence.
It is encrypted in the ticket has been reached.
7 UNUSED key shared by Kerberos and the end server (the
server's secret key), using a key usage value of 2.
flags
This option is presently unused.
8 RENEWABLE The RENEWABLE option field indicates that the ticket to be
issued is to have its RENEWABLE flag set. It may only be set on the
initial request, which of various options were used or requested
when the ticket-granting ticket on which the
request is based is also renewable. If this option is requested,
then the rtime field in the request contains the desired absolute
expiration time for was issued. The meanings of the ticket.
9 RESERVED Reserved for PK-Cross
10-13 UNUSED These options are presently unused.
14-25 RESERVED flags are:
Bit(s) Name Description
0 reserved Reserved for future use.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
26 DISABLE-TRANSITED-CHECK By default the KDC will check the
transited field expansion of a ticket-granting-ticket against this
field.
The FORWARDABLE flag is normally only
interpreted by the policy of TGS, and can be
ignored by end servers. When set, this
1 forwardable flag tells the local realm before ticket-granting server
that it will is OK to issue derivative tickets a new
ticket-granting ticket with a
different network address based on the
ticket granting
presented ticket. If
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
When set, this flag is set in the request, checking
of indicates that the transited field is disabled. Tickets
ticket has either been forwarded or
2 forwarded was issued without based on authentication
involving a forwarded ticket-granting
ticket.
The PROXIABLE flag is normally only
interpreted by the
performance of this check will TGS, and can be noted
ignored by the reset (0) value end servers. The PROXIABLE
flag has an interpretation identical
3 proxiable to that of the TRANSITED-POLICY-CHECKED FORWARDABLE flag, indicating to
except that the application PROXIABLE flag tells
the ticket-granting server that the tranisted field must only
non-ticket-granting tickets may be checked locally. KDC's are
encouraged but not required to honor the DISABLE-TRANSITED-CHECK
option.
This
issued with different network
addresses.
4 proxy When set, this flag is new since RFC 1510
27 RENEWABLE-OK The RENEWABLE-OK option indicates that a renewable
ticket will be acceptable if is a ticket with proxy.
The MAY-POSTDATE flag is normally only
interpreted by the requested life cannot
otherwise TGS, and can be provided. If a ticket with
5 may-postdate ignored by end servers. This flag
tells the requested life cannot be
provided, then ticket-granting server that
a renewable post-dated ticket may be issued with a renew-till
equal to the the requested endtime.
based on this ticket-granting ticket.
This flag indicates that this ticket
has been postdated. The value of end-service
6 postdated can check the renew-till authtime field may still be limited by local limits, or limits selected by to see
when the individual principal or server.
28 ENC-TKT-IN-SKEY original authentication
occurred.
This option is used only by the ticket-granting
service. The ENC-TKT-IN-SKEY option flag indicates that the a ticket for
the end server is to
invalid, and it must be encrypted in the session key from validated by
7 invalid the
additional ticket-granting ticket provided.
29 RESERVED Reserved for future KDC before use.
30 RENEW This option Application
servers must reject tickets which have
this flag set.
The RENEWABLE flag is used normally only
interpreted by the TGS, and can
usually be ignored by end servers
8 renewable (some particularly careful servers may
wish to disallow renewable tickets). A
renewable ticket can be used to obtain
a replacement ticket that expires at a
later date.
This flag indicates that this ticket
9 initial was issued using the AS protocol, and
not issued based on a ticket-granting service.
The RENEW option
ticket.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
This flag indicates that during
initial authentication, the present request is for client was
authenticated by the KDC before a
renewal. The
10 pre-authent ticket provided is encrypted in the secret key for was issued. The strength of the
server on which it
preauthentication method is valid. not
indicated, but is acceptable to the
KDC.
This option will only be honored if flag indicates that the
ticket protocol
employed for initial authentication
required the use of hardware expected
11 hw-authent to be renewed has its RENEWABLE flag set and if possessed solely by the time in
its renew-till field has not passed. named
client. The ticket to be renewed hardware authentication
method is
passed in selected by the padata field as part KDC and the
strength of the authentication header.
31 VALIDATE This option method is used only by the ticket-granting service.
The VALIDATE option not
indicated.
This flag indicates that the request is to validate a
postdated ticket. It will only be honored if KDC for
the ticket presented is
postdated, presently realm has its INVALID flag set, and would be
otherwise usable at this time. A ticket cannot be validated before
its starttime. The ticket presented checked the transited
field against a realm defined policy
for validation trusted certifiers. If this flag
is encrypted in
the key of reset (0), then the application
server for which it is valid and is passed in must check the
padata transited field as part of the authentication header.
cname
itself, and sname
These fields are the same as those described for if unable to do so it must
reject the ticket in
section 5.3.1. sname may only be absent when authentication. If the ENC-TKT-IN-SKEY
option flag
12 transited- is specified. If absent, set (1) then the name application server
policy-checked
may skip its own validation of the
transited field, relying on the
validation performed by the KDC. At
its option the application server may
still apply its own validation based
on a separate policy for acceptance.
This flag is taken from new since RFC 1510.
This flag indicates that the name of server
(not the client client) specified in the
ticket passed as additional-tickets.
enc-authorization-data
The enc-authorization-data, if present (and it can only has been determined by policy
of the realm to be present
in a suitable
recipient of delegation. A client can
use the TGS_REQ form), presence of this flag to help
it make a decision whether to delegate
credentials (either grant a proxy or a
forwarded ticket granting ticket) to
13 ok-as-delegate this server. The client is free to
ignore the value of this flag. When
setting this flag, an encoding administrator
should consider the Security and
placement of the desired
authorization-data encrypted under server on which the sub-session
service will run, as well as whether
the service requires the use of
delegated credentials.
This flag is new since RFC 1510.
14-31 reserved Reserved for future use.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
key if present
This field exists in the Authenticator, or alternatively from ticket and the KDC response and is used to
pass the session key in the
ticket-granting ticket, both from Kerberos to the padata field in application server and the KRB_AP_REQ.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
realm
client.
crealm
This field specifies contains the realm part name of the server's principal
identifier. In realm in which the AS exchange, this client is also
registered and in which initial authentication took place.
cname
This field contains the realm name part of the client's principal identifier. If
transited
This field lists the CANONICALIZE option is set, names of the realm is used as a hint Kerberos realms that took part in
authenticating the user to whom this ticket was issued. It does not
specify the KDC order in which the realms were transited. See section
3.3.3.2 for its database lookup.
from
This details on how this field is included in encodes the KRB_AS_REQ and KRB_TGS_REQ ticket
requests when traversed realms.
When the requested ticket is names of CA's are to be postdated. It specifies embedded in the desired start time transited field (as
specified for some extensions to the requested ticket. If this field is
omitted then protocol), the KDC X.500 names of the
CA's should use be mapped into items in the current time instead.
till
This transited field contains using the expiration date requested
mapping defined by RFC2253.
authtime
This field indicates the client in a
ticket request. time of initial authentication for the named
principal. It is not optional, but if the requested endtime is
"19700101000000Z", time of issue for the requested original ticket on which
this ticket is to have based. It is included in the maximum
endtime permitted according ticket to KDC policy for. This special
timestamp corresponds provide
additional information to a UNIX time_t value of zero on most systems.
rtime
This field is the requested renew-till time sent from a client end service, and to provide the KDC in necessary
information for implementation of a ticket request. It `hot list' service at the KDC. An
end service that is optional.
nonce particularly paranoid could refuse to accept
tickets for which the initial authentication occurred "too far" in the
past. This field is also returned as part of the KDC request and response. It it intended
to hold a random number generated by the client. If the same number
is included in the encrypted response from the KDC, it provides
evidence that KDC.
When returned as part of the response to initial authentication
(KRB_AS_REP), this is fresh and has not been replayed by an
attacker. Nonces must never be re-used. Ideally, it should be
generated randomly, but if the correct current time is known, it may
suffice[25].
etype
This field specifies the desired encryption algorithm to be used in on the response.
addresses Kerberos server[24].
starttime
This field is included in the initial request for tickets, and
optionally included in requests for additional tickets from the
ticket-granting server. It ticket specifies the addresses from time after which the
requested ticket is to be
valid. Normally it includes the addresses
for the client's host. If a proxy is requested, this field will
contain other addresses. The contents of Together with endtime, this field are usually
copied by the KDC into specifies the caddr field life of the resulting
ticket.
additional-tickets
Additional tickets may be optionally included in a request to the
ticket-granting server. If it is absent from the ENC-TKT-IN-SKEY option has been
specified, then ticket, its value should be treated as
that of the session key from authtime field.
endtime
This field contains the time after which the additional ticket will not be
used in honored
(its expiration time). Note that individual services may place their
own limits on the life of a ticket and may reject tickets which have
not yet expired. As such, this is really an upper bound on the server's key to encrypt
expiration time for the new ticket. When
the ENC-TKT-IN-SKEY option
renew-till
This field is used for user-to-user authentication,
this addional ticket only present in tickets that have the RENEWABLE flag set
in the flags field. It indicates the maximum endtime that may be
included in a TGT issued by renewal. It can be thought of as the local realm or an
inter-realm TGT issued absolute expiration
time for the current KDC's realm by ticket, including all renewals.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
caddr
This field in a remote KDC.
If ticket contains zero (if omitted) or more than one option (if present)
host addresses. These are the addresses from which requires additional tickets has been
specified, then the additional tickets ticket can be
used. If there are no addresses, the ticket can be used in from any
location. The decision by the order
specified KDC to issue or by the ordering of end server to
accept zero-address tickets is a policy decision and is left to the options bits (see kdc-options, above).
Kerberos and end-service administrators; they may refuse to issue or
accept such tickets. The application tag number suggested and default policy, however, is that
such tickets will only be either ten (10) issued or twelve (12)
depending on whether accepted when additional
information that can be used to restrict the use of the request is for an initial ticket (AS-REQ) or
for an additional is
included in the authorization_data field. Such a ticket (TGS-REQ).
The optional fields (addresses, authorization-data and
additional-tickets) is a
capability.
Network addresses are only included if necessary in the ticket to perform make it harder for an
attacker to use stolen credentials. Because the
operation specified in session key is not sent
over the kdc-options field.
It should be noted that network in KRB_TGS_REQ, the protocol version number
appears twice and two different message types appear: cleartext, credentials can't be stolen simply by
listening to the KRB_TGS_REQ
message contains these fields as does network; an attacker has to gain access to the authentication header
(KRB_AP_REQ) that session
key (perhaps through operating system security breaches or a careless
user's unattended session) to make use of stolen tickets.
It is passed in important to note that the padata field.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.4.2. KRB_KDC_REP definition
The KRB_KDC_REP message format network address from which a
connection is used for received cannot be reliably determined. Even if it could
be, an attacker who has compromised the reply client's workstation could use
the credentials from there. Including the KDC network addresses only makes
it more difficult, not impossible, for
either an initial (AS) request or attacker to walk off with
stolen credentials and then use them from a subsequent (TGS) request. There "safe" location.
authorization-data
The authorization-data field is
no message type for KRB_KDC_REP. Instead, used to pass authorization data from
the type principal on whose behalf a ticket was issued to the application
service. If no authorization data is included, this field will be either
KRB_AS_REP or KRB_TGS_REP. The key used to encrypt left
out. Experience has shown that the ciphertext part name of this field is confusing, and
that a better name for this field would be restrictions. Unfortunately,
it is not possible to change the reply depends name of this field at this time.
This field contains restrictions on any authority obtained on the message type. For KRB_AS_REP, basis
of authentication using the ciphertext ticket. It is encrypted possible for any principal in
posession of credentials to add entries to the client's secret key, and authorization data field
since these entries further restrict what can be done with the client's key version
number ticket.
Such additions can be made by specifying the additional entries when a
new ticket is included in obtained during the key version number for TGS exchange, or they may be added
during chained delegation using the encrypted data. For
KRB_TGS_REP, authorization data field of the ciphertext
authenticator.
Because entries may be added to this field by the holder of
credentials, except when an entry is encrypted separately authenticated by
encapsulation in the sub-session key from the
Authenticator, or if absent, kdc-issued element, it is not allowable for the session key from
presence of an entry in the ticket-granting authorization data field of a ticket used to
amplify the privileges one would obtain from using a ticket.
The data in this field may be specific to the request. In that case, no version number end service; the field
will be
present in contain the EncryptedData sequence.
The KRB_KDC_REP message contains names of service specific objects, and the following fields:
AS-REP rights to
those objects. The format for this field is described in section 5.2.
Although Kerberos is not concerned with the format of the contents of
the sub-fields, it does carry type information (ad-type).
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
By using the authorization_data field, a principal is able to issue a
proxy that is valid for a specific purpose. For example, a client
wishing to print a file can obtain a file server proxy to be passed to
the print server. By specifying the name of the file in the
authorization_data field, the file server knows that the print server
can only use the client's rights when accessing the particular file to
be printed.
A separate service providing authorization or certifying group
membership may be built using the authorization-data field. In this
case, the entity granting authorization (not the authorized entity),
may obtain a ticket in its own name (e.g. the ticket is issued in the
name of a privilege server), and this entity adds restrictions on its
own authority and delegates the restricted authority through a proxy to
the client. The client would then present this authorization credential
to the application server separately from the authentication exchange.
Alternatively, such authorization credentials may be embedded in the
ticket authenticating the authorized entity, when the authorization is
separately authenticated using the kdc-issued authorization data
element (see B.4).
Similarly, if one specifies the authorization-data field of a proxy and
leaves the host addresses blank, the resulting ticket and session key
can be treated as a capability. See [Neu93] for some suggested uses of
this field.
The authorization-data field is optional and does not have to be
included in a ticket.
5.4. Specifications for the AS and TGS exchanges
This section specifies the format of the messages used in the exchange
between the client and the Kerberos server. The format of possible error
messages appears in section 5.9.1.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no application tag number of its own. Instead,
it is incorporated into one of KRB_AS_REQ or KRB_TGS_REQ, which each have an
application tag, depending on whether the request is for an initial ticket
or an additional ticket. In either case, the message is sent from the client
to the KDC to request credentials for a service.
The message fields are:
AS-REQ ::= [APPLICATION 11] KDC-REP
TGS-REP 10] KDC-REQ
TGS-REQ ::= [APPLICATION 13] KDC-REP
KDC-REP 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
pvno
-- NOTE: first tag is [1], not [0]
pvno [1] INTEGER (5), (5) ,
msg-type [1] [2] INTEGER (11 (10 -- AS -- | 13 12 -- TGS --),
padata [2] [3] SEQUENCE OF PA-DATA OPTIONAL
-- XXX may be zero length NOTE: not empty --,
crealm [3] Realm,
cname
req-body [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncASRepPart or EncTGSRepPart,
-- as appropriate KDC-REQ-BODY
}
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart
KDC-REQ-BODY ::= SEQUENCE {
key
kdc-options [0] EncryptionKey,
last-req KDCOptions,
cname [1] LastReq,
nonce PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] UInt32,
key-expiration Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] KerberosTime PrincipalName OPTIONAL,
flags
from [4] TicketFlags,
authtime KerberosTime OPTIONAL,
till [5] KerberosTime,
starttime
rtime [6] KerberosTime OPTIONAL,
endtime
nonce [7] KerberosTime,
renew-till UInt32,
etype [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
LastReq ::= SEQUENCE OF Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses OPTIONAL,
enc-authorization-data [10] EncryptedData -- AuthorizationData --,
additional-tickets [11] SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime OF Ticket OPTIONAL
-- NOTE: not empty
}
draft-ietf-krb-wg-kerberos-clarifications-01
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
pvno and msg-type
These
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- opt-hardware-auth(11),
-- unused12(12),
-- unused13(13),
-- 15 is reserved for canonicalize
-- unused15(15),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
The fields are described above in section 5.4.1. msg-type is
either KRB_AS_REP or KRB_TGS_REP.
padata this message are:
pvno
This field is described in detail included in section 5.4.1. One possible use
for this each message, and specifies the protocol
version number. This document specifies protocol version 5.
msg-type
This field is to encode an alternate "mix-in" string to indicates the type of a protocol message. It will almost
always be used the same as the application identifier associated with a string-to-key algorithm (such as is described in section
6.3.2). This ability
message. It is useful to ease transitions if a realm name
needs included to change (e.g. when a company is acquired); in such a case
all existing password-derived entries in make the KDC database would be
flagged as needing a special mix-in string until identifier more readily accessible
to the next password
change.
crealm, cname, srealm and sname
These fields are application. For the same as those described KDC-REQ message, this type will be
KRB_AS_REQ or KRB_TGS_REQ.
padata
Contains pre-authentication data. Requests for the ticket in
section 5.3.1.
ticket additional tickets
(KRB_TGS_REQ) must contain a padata of PA-TGS-REQ.
The newly-issued ticket, from section 5.3.1. enc-part
This padata (pre-authentication data) field is contains a place holder sequence of
authentication information which may be needed before credentials can
be issued or decrypted. In most requests for the ciphertext initial authentication
(KRB_AS_REQ) and related
information that forms most replies (KDC-REP), the encrypted part of padata field will be left
out.
req-body
This field is a message. The
description of placeholder delimiting the encrypted part extent of the message follows each
appearance of this field. The encrypted part remaining
fields. If a checksum is encoded as described
in section 6.1.
Compatibility note: Some implementations unconditionally send to be calculated over the request, it is
calculated over an
encrypted EncTGSRepPart (application tag number 26) in this field
regardless encoding of whether the reply KDC-REQ-BODY sequence which is a AS-REP or a TGS-REP. In
enclosed within the
interests of compatibility, implementors may wish req-body field.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
kdc-options
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to relax the check
KDC and indicates the flags that the client wants set on the tag number tickets as
well as other information that is to modify the behavior of the decrypted ENC-PART.
key
This field is KDC.
Where appropriate, the name of an option may be the same as described for the ticket in section 5.3.1.
last-req
This field flag
that is returned set by that option. Although in most case, the KDC and specifies the time(s) of bit in the
last request by a principal. Depending on what information is
available, this might
options field will be the last time same as that a request for a
ticket-granting ticket was made, or the last time that a request
based on a ticket-granting ticket was successful. It also might
cover all servers for a realm, or just in the particular server. Some
implementations may display flags field, this information to the user to aid in
discovering unauthorized use of one's identity. It is similar in
spirit to the last login time displayed when logging into
timesharing systems.
lr-type
This field indicates how the following lr-value field not
guaranteed, so it is not acceptable to be
interpreted. Negative values indicate that the information pertains
only to simply copy the responding server. Non-negative values pertain options field to all
servers for
the realm.
If the lr-type flags field. There are various checks that must be made before
honoring an option anyway.
The kdc_options field is zero (0), then no information is conveyed a bit-field, where the selected options are
indicated by the lr-value subfield. If bit being set (1), and the absolute value unselected options and
reserved fields being reset (0). The encoding of the lr-type field is
one (1), then the lr-value subfield bits is the time specified
in section 5.2. The options are described in more detail above in
section 2. The meanings of last initial
request the options are:
Bits Name Description
0 RESERVED Reserved for a TGT. If it is two (2), then future expansion of
this field.
The FORWARDABLE option indicates
that the lr-value subfield ticket to be issued is to
have its forwardable flag set. It
1 FORWARDABLE may only be set on the time of last initial request. If
request, or in a subsequent request
if the ticket-granting ticket on
which it is three (3), then the
lr-value subfield based is also
forwardable.
The FORWARDED option is only
specified in a request to the time of issue for
ticket-granting server and will only
be honored if the newest ticket-granting
ticket used. If it in the request has its
2 FORWARDED FORWARDABLE bit set. This option
indicates that this is four (4), then a request for
forwarding. The address(es) of the lr-value
subfield
host from which the resulting ticket
is to be valid are included in the time
addresses field of the last renewal. If it is five (5), then request.
The PROXIABLE option indicates that
the lr-value subfield ticket to be issued is to have
its proxiable flag set. It may only
3 PROXIABLE be set on the time of last initial request, or in
a subsequent request (of any type). If if the
ticket-granting ticket on which it
is (6), then the lr-value subfield based is the time when the password
will expire.
draft-ietf-krb-wg-kerberos-clarifications-01 also proxiable.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
lr-value
The PROXY option indicates that this
is a request for a proxy. This field contains the time of the last request. the time must
option will only be
interpreted according to the contents of honored if the accompanying lr-type
subfield.
nonce
This field is described above
ticket-granting ticket in section 5.4.1.
key-expiration the
4 PROXY request has its PROXIABLE bit set.
The key-expiration field is part address(es) of the response host from
which the KDC and
specifies the time that the client's secret key resulting ticket is due to expire.
The expiration might be
valid are included in the result of password aging or an account
expiration. This addresses
field will usually be left out of the TGS reply
since request.
The ALLOW-POSTDATE option indicates
that the response ticket to be issued is to
have its MAY-POSTDATE flag set. It
5 ALLOW-POSTDATE may only be set on the TGS initial
request, or in a subsequent request
if the ticket-granting ticket on
which it is based also has its
MAY-POSTDATE flag set.
The POSTDATED option indicates that
this is encrypted in a session key
and no client information need request for a postdated
ticket. This option will only be retrieved from
honored if the KDC database.
It ticket-granting
ticket on which it is up based has its
6 POSTDATED MAY-POSTDATE flag set. The resulting
ticket will also have its INVALID
flag set, and that flag may be reset
by a subsequent request to the application client (usually KDC
after the login program) to
take appropriate action (such as notifying starttime in the user) if ticket
has been reached.
7 UNUSED This option is presently unused.
The RENEWABLE option indicates that
the
expiration time ticket to be issued is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in to have
its RENEWABLE flag set. It may only
be set on the encrypted portion
of initial request, or
when the attached ticket-granting ticket (see section 5.3.1), provided so the client
may verify they match on
8 RENEWABLE which the intended request and to assist in proper
ticket caching. is based is also
renewable. If the message this option is of type KRB_TGS_REP,
requested, then the caddr rtime field will only be filled in if
the request was contains the desired
absolute expiration time for a proxy or
forwarded ticket, or if the user
ticket.
9 RESERVED Reserved for PK-Cross
These options are presently unused.
10-13 UNUSED Option 11 is substituting reserved for future is
as opt-hardware-auth.
14-25 RESERVED Reserved for future use.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
By default the KDC will check the
transited field of a subset
ticket-granting-ticket against the
policy of the
addresses from local realm before it
will issue derivative tickets based
on the ticket granting ticket. If
this flag is set in the client-requested
addresses are not present or not used, then request,
checking of the addresses contained
in transited field is
disabled. Tickets issued without the ticket
26 DISABLE-TRANSITED-CHECK performance of this check will be
noted by the same as those included in reset (0) value of the
ticket-granting ticket.
5.5. Client/Server (CS) message specifications
TRANSITED-POLICY-CHECKED flag,
indicating to the application server
that the tranisted field must be
checked locally. KDC's are
encouraged but not required to honor
the DISABLE-TRANSITED-CHECK option.
This section specifies flag is new since RFC 1510
The RENEWABLE-OK option indicates
that a renewable ticket will be
acceptable if a ticket with the format of
requested life cannot otherwise be
provided. If a ticket with the messages used for
requested life cannot be provided,
27 RENEWABLE-OK then a renewable ticket may be
issued with a renew-till equal to
the
authentication the requested endtime. The value
of the client to renew-till field may still be
limited by local limits, or limits
selected by the application individual principal
or server.
5.5.1. KRB_AP_REQ definition
This option is used only by the
ticket-granting service. The KRB_AP_REQ message contains
ENC-TKT-IN-SKEY option indicates
28 ENC-TKT-IN-SKEY that the Kerberos protocol version number, ticket for the message type KRB_AP_REQ, an options field end server
is to indicate any options be encrypted in
use, and the session
key from the additional
ticket-granting ticket and authenticator themselves. The KRB_AP_REQ message
is often referred to as the 'authentication header'.
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (14),
ap-options [2] APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- Authenticator
}
APOptions ::= KerberosFlags
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_AP_REQ. ap-options provided.
29 RESERVED Reserved for future use.
This field appears in the application request (KRB_AP_REQ) and
affects option is used only by the way
ticket-granting service. The RENEW
option indicates that the present
request is processed. It is for a bit-field, where renewal. The ticket
provided is encrypted in the selected options are indicated by secret
key for the bit being server on which it is
30 RENEW valid. This option will only be
honored if the ticket to be renewed
has its RENEWABLE flag set (1), and if
the
unselected options and reserved fields being reset (0). time in its renew-till field has
not passed. The encoding
of the bits ticket to be renewed
is specified passed in section 5.2. The meanings of the options
are:
Bit(s) Name Description
0 reserved Reserved for future expansion padata field as
part of this field. the authentication header.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 use-session-key May 2003
This option is used only by the
ticket-granting service. The USE-SESSION-KEY
VALIDATE option indicates that the
ticket the client
request is presenting to validate a server postdated
ticket. It will only be honored if
the ticket presented is postdated,
presently has its INVALID flag set,
31 VALIDATE and would be otherwise usable at
this time. A ticket cannot be
validated before its starttime. The
ticket presented for validation is
encrypted in the
session key from of the server's ticket-granting ticket. When this
option server
for which it is not specified, the ticket valid and is encrypted passed
in the server's
secret key.
2 mutual-required The MUTUAL-REQUIRED option tells the server that padata field as part of the client requires mutual authentication,
authentication header.
cname and that it must respond
with a KRB_AP_REP message.
3-31 reserved Reserved sname
These fields are the same as those described for future use.
ticket
This field is a the ticket authenticating in section
5.3. sname may only be absent when the client to ENC-TKT-IN-SKEY option is
specified. If absent, the server.
authenticator
This contains name of the encrypted authenticator, which includes server is taken from the
client's choice name of a subkey.
The encrypted authenticator is included
the client in the AP-REQ; ticket passed as additional-tickets.
enc-authorization-data
The enc-authorization-data, if present (and it certifies to a
server that can only be present in
the sender has recent knowledge TGS_REQ form), is an encoding of the encryption desired authorization-data
encrypted under the sub-session key if present in the
accompanying Authenticator, or
alternatively from the session key in the ticket-granting ticket, to help both
from the server detect replays. It also assists padata field in the selection of KRB_AP_REQ. The key usage value used when
encrypting is 5 if a "true sub-session key is used, or 4 if the session key" to use with key
is used.
realm
This field specifies the particular
session. The DER encoding realm part of the following server's principal
identifier. In the AS exchange, this is encrypted also the realm part of the
client's principal identifier.
from
This field is included in the ticket's
session key:
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno [0] INTEGER (5),
crealm [1] Realm,
cname [2] PrincipalName,
cksum [3] Checksum OPTIONAL,
cusec [4] Microseconds,
ctime [5] KerberosTime,
subkey [6] EncryptionKey OPTIONAL,
seq-number [7] UInt32 OPTIONAL,
authorization-data [8] AuthorizationData OPTIONAL
}
authenticator-vno
This field KRB_AS_REQ and KRB_TGS_REQ ticket
requests when the requested ticket is to be postdated. It specifies the version number
desired start time for the format of the
authenticator. This document specifies version 5. crealm and cname
These fields are requested ticket. If this field is omitted
then the same as those described for KDC should use the ticket in
section 5.3.1.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
cksum current time instead.
till
This field contains a checksum of the expiration date requested by the application data that
accompanies client in a
ticket request. It is not optional, but if the KRB_AP_REQ.
cusec
This field contains requested endtime is
"19700101000000Z", the microsecond part of requested ticket is to have the client's timestamp.
Its value (before encryption) ranges from 0 maximum endtime
permitted according to 999999. It often
appears along with ctime. The two fields are used together KDC policy for. This special timestamp
corresponds to
specify a reasonably accurate timestamp.
ctime UNIX time_t value of zero on most systems.
rtime
This field contains is the current requested renew-till time on sent from a client to the client's host. subkey
KDC in a ticket request. It is optional.
nonce
This field contains the client's choice for an encryption key which is part of the KDC request and response. It it intended to be used to protect this specific application session. Unless
an application specifies otherwise, if this field
hold a random number generated by the client. If the same number is left out
included in the
session key encrypted response from the ticket will KDC, it provides evidence
that the response is fresh and has not been replayed by an attacker.
Nonces must never be used.
seq-number re-used.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
etype
This optional field includes the initial sequence number to be used
by specifies the KRB_PRIV or KRB_SAFE messages when sequence numbers are used desired encryption algorithm to detect replays (It may also be used by application specific
messages). When included in the authenticator this
response.
addresses
This field specifies is included in the initial sequence number request for messages tickets, and
optionally included in requests for additional tickets from the client
ticket-granting server. It specifies the addresses from which the
requested ticket is to be valid. Normally it includes the
server. When addresses for
the client's host. If a proxy is requested, this field will contain
other addresses. The contents of this field are usually copied by the
KDC into the caddr field of the resulting ticket.
additional-tickets
Additional tickets may be optionally included in a request to the AP-REP message,
ticket-granting server. If the initial sequence
number is that for messages ENC-TKT-IN-SKEY option has been
specified, then the session key from the server additional ticket will be used
in place of the server's key to encrypt the client. new ticket. When the
ENC-TKT-IN-SKEY option is used
in KRB_PRIV for user-to-user authentication, this
addional ticket may be a TGT issued by the local realm or KRB_SAFE messages, it is incremented an
inter-realm TGT issued for the current KDC's realm by a remote KDC. If
more than one after
each message is sent. Sequence numbers fall option which requires additional tickets has been
specified, then the additional tickets are used in the range order specified
by the ordering of 0
through 2^32 - 1 and wrap to zero following the value 2^32 - 1.
For sequence numbers to adequately support the detection of replays
they should be non-repeating, even across connection boundaries. options bits (see kdc-options, above).
The
initial sequence application tag number should will be random and uniformly distributed
across either ten (10) or twelve (12) depending
on whether the full space of possible sequence numbers, so that it
cannot be guessed by request is for an attacker initial ticket (AS-REQ) or for an
additional ticket (TGS-REQ).
The optional fields (addresses, authorization-data and so additional-tickets)
are only included if necessary to perform the operation specified in the
kdc-options field.
It should be noted that it in KRB_TGS_REQ, the protocol version number appears
twice and two different message types appear: the successive
sequence numbers do not repeat other sequences.
Implmentation note: historically, some implementations transmit
signed twos-complement numbers for sequence numbers. In KRB_TGS_REQ message
contains these fields as does the
interests of compatibility, implementations may accept authentication header (KRB_AP_REQ) that is
passed in the
equivalent negative number where a positive number greater than 2^31
- 1 padata field.
5.4.2. KRB_KDC_REP definition
The KRB_KDC_REP message format is expected.
Implementation note: as noted before, some implementations omit used for the
optional sequence number when its value would be zero.
Implementations may accept an omitted sequence number when expecting
a value of zero, and should not transmit reply from the KDC for either
an Authenticator with initial (AS) request or a
sequence number of zero.
authorization-data
This field subsequent (TGS) request. There is the same as described no message
type for KRB_KDC_REP. Instead, the ticket in section 5.3.1.
It is optional and type will only appear when additional restrictions are
to be placed on either KRB_AS_REP or
KRB_TGS_REP. The key used to encrypt the use ciphertext part of a ticket, beyond those carried in the
ticket itself.
5.5.2. KRB_AP_REP definition
The KRB_AP_REP reply
depends on the message contains type. For KRB_AS_REP, the Kerberos protocol version number, ciphertext is encrypted in
the message type, client's secret key, and an the client's key version number is included in
the key version number for the encrypted time- stamp. The message data. For KRB_TGS_REP, the
ciphertext is sent encrypted in
response to an application request (KRB_AP_REQ) where the mutual
authentication option has been selected sub-session key from the Authenticator, or if
absent, the session key from the ticket-granting ticket used in the ap-options field.
draft-ietf-krb-wg-kerberos-clarifications-01 request.
In that case, no version number will be present in the EncryptedData
sequence.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
AP-REP
The KRB_KDC_REP message contains the following fields:
AS-REP ::= [APPLICATION 15] 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part (11 -- AS -- | 13 -- TGS --),
padata [2] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
crealm [3] Realm,
cname [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncAPRepPart EncASRepPart or EncTGSRepPart,
-- as appropriate
}
EncAPRepPart
EncASRepPart ::= [APPLICATION 27] 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
ctime
key [0] KerberosTime,
cusec EncryptionKey,
last-req [1] Microseconds,
subkey LastReq,
nonce [2] EncryptionKey OPTIONAL,
seq-number UInt32,
key-expiration [3] UInt32 KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
The encoded EncAPRepPart is encrypted in the shared session key of the
ticket. The optional subkey field can be used in an application-arranged
negotiation to choose a per association session key.
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_AP_REP.
enc-part either
KRB_AS_REP or KRB_TGS_REP.
padata
This field is described above in detail in section 5.4.2.
ctime
This 5.4.1. One possible use
for this field contains is to encode an alternate "salt" string to be used with
a string-to-key algorithm. This ability is useful to ease transitions
if a realm name needs to change (e.g. when a company is acquired); in
such a case all existing password-derived entries in the current time on KDC database
would be flagged as needing a special salt string until the client's host.
cusec next
password change.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
crealm, cname, srealm and sname
These fields are the same as those described for the ticket in section
5.3.
ticket
The newly-issued ticket, from section 5.3.
enc-part
This field contains is a place holder for the microsecond ciphertext and related information
that forms the encrypted part of a message. The description of the client's
timestamp.
subkey
This field contains an encryption key which is to be used to protect
encrypted part of the message follows each appearance of this specific application session. See section 3.2.6 field.
The key usage value for specifics
on how encrypting this field is used to negotiate a key. Unless 3 in an application
specifies otherwise, if this field is left out, AS-REP
message, using the sub-session client's long-term key
from the authenticator, or another key selected via
preauthentication mechanisms. In a TGS-REP message, the key usage value
is 8 if also left out, the TGS session key from
the ticket will be used.
seq-number
This field is described above in section 5.3.2.
5.5.3. Error message reply
If used, or 9 if a TGS authenticator subkey
is used.
Compatibility note: Some implementations unconditionally send an error occurs while processing the application request, the
KRB_ERROR message will be sent
encrypted EncTGSRepPart (application tag number 26) in response. See section 5.9.1 for the
format this field
regardless of whether the error message. The cname and crealm fields reply is a AS-REP or a TGS-REP. In the
interests of compatibility, implementors may be left out
if wish to relax the server cannot determine their appropriate values from check on
the
corresponding KRB_AP_REQ message. If tag number of the authenticator was decipherable, decrypted ENC-PART.
key
This field is the ctime and cusec fields will contain same as described for the values from it.
5.6. KRB_SAFE message specification
This ticket in section 5.3.
last-req
This field is returned by the KDC and specifies the format time(s) of the last
request by a message that can principal. Depending on what information is available,
this might be used by
either side (client the last time that a request for a ticket-granting ticket
was made, or server) of an application to send the last time that a tamper-proof
message to its peer. request based on a ticket-granting
ticket was successful. It presumes that also might cover all servers for a session key has previously been
exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
There are two KRB_SAFE messages; realm, or
just the KRB-SAFE message is particular server. Some implementations may display this
information to the one
specified user to aid in RFC 1510. The KRB-SAFE2 message discovering unauthorized use of one's
identity. It is new with this document,
and shares a number of fields with the old KRB-SAFE message.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.6.1. KRB_SAFE definition
The KRB_SAFE message contains user data along with a collision-proof
checksum keyed with similar in spirit to the last encryption key negotiated via subkeys, or login time displayed when
logging into timesharing systems.
lr-type
This field indicates how the session key if no negotiation has occurred. The message fields are:
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress,
r-address [5] HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type following lr-value field is
KRB_SAFE or KRB_SAFE2, respectively, to be
interpreted. Negative values indicate that the information
pertains only to the responding server. Non-negative values
pertain to all servers for the KRB-SAFE and KRB-SAFE2
messages.
safe-body
This realm.
If the lr-type field is a placeholder for zero (0), then no information is conveyed
by the body lr-value subfield. If the absolute value of the KRB-SAFE message.
cksum
This lr-type
field contains the checksum of is one (1), then the application data. Checksum
details are described in section 6.4.
The checksum lr-value subfield is computed over the encoding time of last
initial request for a TGT. If it is two (2), then the KRB-SAFE sequence.
First, lr-value
subfield is the cksum time of last initial request. If it is set to a type zero, zero-length value and three (3),
then the
checksum lr-value subfield is computed over the encoding time of issue for the KRB-SAFE sequence, newest
ticket-granting ticket used. If it is four (4), then the checksum lr-value
subfield is set to the result time of that computation, and
finally the KRB-SAFE sequence last renewal. If it is encoded again. This method, while
different than five (5), then
the one specified in RFC 1510, corresponds to
existing practice.
user-data
This field lr-value subfield is part of the KRB_SAFE and KRB_PRIV messages and contain time of last request (of any type).
If it is (6), then the application specific data that lr-value subfield is being passed from the sender
to time when the recipient.
timestamp
password will expire.
lr-value
This field is part of the KRB_SAFE and KRB_PRIV messages. Its
contents are contains the current time as known by the sender of the message.
By checking the timestamp, the recipient of last request. the message is able time must be
interpreted according to
make sure that it was recently generated, and is not a replay.
usec
This field is part of the KRB_SAFE and KRB_PRIV headers. It contains the microsecond part contents of the timestamp.
seq-number accompanying lr-type
subfield.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
nonce
This field is described above in section 5.3.2.
s-address
Sender's address.
This 5.4.1.
key-expiration
The key-expiration field specifies is part of the address in use by response from the sender of KDC and
specifies the
message. It may time that the client's secret key is due to expire. The
expiration might be omitted if not required by the application protocol.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
r-address result of password aging or an account
expiration. This field specifies the address in use by the recipient will usually be left out of the
message. It may be omitted for some uses (such as broadcast
protocols), but TGS reply since
the recipient may arbitrarily reject such messages.
This field, along with s-address, can response to the TGS request is encrypted in a session key and no
client information need be used retrieved from the KDC database. It is up to help detect
messages which have been incorrectly or maliciously delivered
the application client (usually the login program) to take appropriate
action (such as notifying the user) if the expiration time is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in the encrypted portion of
the attached ticket (see section 5.3), provided so the client may
verify they match the intended request and to assist in proper ticket
caching. If the
wrong recipient.
5.7. KRB_PRIV message specification is of type KRB_TGS_REP, the caddr field will
only be filled in if the request was for a proxy or forwarded ticket,
or if the user is substituting a subset of the addresses from the
ticket granting ticket. If the client-requested addresses are not
present or not used, then the addresses contained in the ticket will be
the same as those included in the ticket-granting ticket.
5.5. Client/Server (CS) message specifications
This section specifies the format of a message that can be the messages used by
either side (client or server) for the
authentication of an application to securely and
privately send a message the client to its peer. It presumes that a session key has
previously been exchanged (for example, by using the
KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV application server.
5.5.1. KRB_AP_REQ definition
The KRB_PRIV KRB_AP_REQ message contains user data encrypted the Kerberos protocol version number, the
message type KRB_AP_REQ, an options field to indicate any options in use,
and the Session Key. ticket and authenticator themselves. The KRB_AP_REQ message fields are:
KRB-PRIV is often
referred to as the 'authentication header'.
AP-REQ ::= [APPLICATION 21] 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- there is no (14),
ap-options [2] tag
enc-part APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- EncKrbPrivPart Authenticator
}
EncKrbPrivPart
APOptions ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress KerberosFlags
-- sender's addr --,
r-address [5] HostAddress OPTIONAL reserved(0),
-- recip's addr
} use-session-key(1),
-- mutual-required(2)
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_PRIV.
enc-part
KRB_AP_REQ.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
ap-options
This field holds an encoding of appears in the EncKrbPrivPart sequence
encrypted under application request (KRB_AP_REQ) and affects
the session key[32]. This encrypted encoding is used
for way the enc-part field of request is processed. It is a bit-field, where the KRB-PRIV message. See section 6 for selected
options are indicated by the format of bit being set (1), and the ciphertext.
user-data, timestamp, usec, s-address unselected
options and r-address
These reserved fields are described above in section 5.6.1. seq-number
This field being reset (0). The encoding of the bits
is described above specified in section 5.3.2.
5.8. KRB_CRED message specification
This section specifies 5.2. The meanings of the format options are:
Bit(s) Name Description
0 reserved Reserved for future expansion of a message this field.
The USE-SESSION-KEY option indicates that can be used to send
Kerberos credentials from one principal to another. It the
ticket the client is presented here
to encourage a common mechanism to be used by applications when
forwarding tickets or providing proxies presenting to subordinate servers. It
presumes that a server
1 use-session-key is encrypted in the session key has already been exchanged perhaps by using from the KRB_AP_REQ/KRB_AP_REP messages.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.8.1. KRB_CRED definition
server's ticket-granting ticket. When this
option is not specified, the ticket is
encrypted in the server's secret key.
The KRB_CRED message contains MUTUAL-REQUIRED option tells the server
2 mutual-required that the client requires mutual
authentication, and that it must respond with
a sequence KRB_AP_REP message.
3-31 reserved Reserved for future use.
ticket
This field is a ticket authenticating the client to the server.
authenticator
This contains the encrypted authenticator, which includes the client's
choice of tickets a subkey.
The encrypted authenticator is included in the AP-REQ; it certifies to be sent and
information needed a
server that the sender has recent knowledge of the encryption key in the
accompanying ticket, to use help the tickets, including server detect replays. It also assists in
the selection of a "true session key from
each. The information needed key" to use with the tickets particular session.
The DER encoding of the following is encrypted under an
encryption in the ticket's session key,
with a key previously exchanged usage value of 11 in normal application exchanges, or transferred alongside 7 when used
as the
KRB_CRED message. The message fields are:
KRB-CRED PA-TGS-REQ PA-DATA field of a TGS-REQ exchange (see section 5.4.1):
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 22] 2] SEQUENCE {
pvno
authenticator-vno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm
crealm [1] Realm OPTIONAL,
pname Realm,
cname [2] PrincipalName OPTIONAL,
flags PrincipalName,
cksum [3] TicketFlags Checksum OPTIONAL,
authtime
cusec [4] KerberosTime OPTIONAL,
starttime Microseconds,
ctime [5] KerberosTime OPTIONAL,
endtime KerberosTime,
subkey [6] KerberosTime EncryptionKey OPTIONAL,
renew-till
seq-number [7] KerberosTime UInt32 OPTIONAL,
srealm
authorization-data [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses AuthorizationData OPTIONAL
}
pvno
authenticator-vno
This field specifies the version number for the format of the
authenticator. This document specifies version 5.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
crealm and msg-type cname
These fields are the same as those described above for the ticket in section 5.4.1. msg-type is
KRB_CRED.
tickets
These are the tickets obtained from the KDC specifically for use by
5.3.
cksum
This field contains a checksum of the intended recipient. Successive tickets are paired with the
corresponding KrbCredInfo sequence from application data that
accompanies the enc-part KRB_AP_REQ, computed using a key usage value of 10 in
normal application exchanges, or 6 when used in the KRB-CRED
message.
enc-part TGS-REQ PA-TGS-REQ
AP-DATA field.
cusec
This field holds an encoding of contains the EncKrbCredPart sequence
encrypted under microsecond part of the session key shared between client's timestamp. Its
value (before encryption) ranges from 0 to 999999. It often appears
along with ctime. The two fields are used together to specify a
reasonably accurate timestamp.
ctime
This field contains the sender and current time on the
intended recipient. client's host.
subkey
This encrypted encoding field contains the client's choice for an encryption key which is
to be used for the enc-part to protect this specific application session. Unless an
application specifies otherwise, if this field of is left out the KRB-CRED message. See section 6 for session
key from the format of ticket will be used.
seq-number
This optional field includes the
ciphertext.
Implementation note: implementations of certain applications, most
notably of the Kerberos GSS-API mechanism, do not encrypt initial sequence number to be used by
the
KRB-CRED message KRB_PRIV or KRB_SAFE messages when sending it. In the case of the GSS-API
mechanism, this is not a security vulnerability, as the KRB-CRED
message itself is itself encrypted inside an Authenticator.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
nonce
If practical, an application sequence numbers are used to
detect replays (It may require the inclusion of a nonce
generated also be used by application specific messages).
When included in the recipient of authenticator this field specifies the message. If initial
sequence number for messages from the same value is
included as client to the nonce server. When
included in the AP-REP message, it provides evidence that the
message initial sequence number is fresh and has not been replayed by an attacker. A nonce
must never be re-used; that for
messages from the server to the client. When used in KRB_PRIV or
KRB_SAFE messages, it should be generated randomly is incremented by one after each message is sent.
Sequence numbers fall in the
recipient range of the message 0 through 2^32 - 1 and provided wrap to
zero following the sender value 2^32 - 1.
For sequence numbers to adequately support the detection of replays
they should be non-repeating, even across connection boundaries. The
initial sequence number should be random and uniformly distributed
across the message
in full space of possible sequence numbers, so that it cannot
be guessed by an application specific manner.
timestamp attacker and usec
These fields specify the time so that it and the KRB-CRED message was
generated. The time is used to provide assurance that successive sequence
numbers do not repeat other sequences.
Implmentation note: historically, some implementations transmit signed
twos-complement numbers for sequence numbers. In the message interests of
compatibility, implementations may accept the equivalent negative
number where a positive number greater than 2^31 - 1 is
fresh.
s-address expected.
Implementation note: as noted before, some implementations omit the
optional sequence number when its value would be zero. Implementations
may accept an omitted sequence number when expecting a value of zero,
and r-address
These fields are should not transmit an Authenticator with a sequence number of
zero.
authorization-data
This field is the same as described above for the ticket in section 5.6.1. They 5.3. It
is optional and will only appear when additional restrictions are used
optionally to provide additional assurance of be
placed on the integrity use of the
KRB-CRED message.
key
This field exists a ticket, beyond those carried in the corresponding ticket passed by
itself.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.5.2. KRB_AP_REP definition
The KRB_AP_REP message contains the Kerberos protocol version number, the KRB-CRED
message type, and an encrypted time- stamp. The message is used sent in response
to pass the session key from an application request (KRB_AP_REQ) where the sender to mutual authentication
option has been selected in the
intended recipient. ap-options field.
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part [2] EncryptedData -- EncAPRepPart
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] Microseconds,
subkey [2] EncryptionKey OPTIONAL,
seq-number [3] UInt32 OPTIONAL
}
The field's encoding encoded EncAPRepPart is described encrypted in section 6.2. the shared session key of the
ticket. The following fields are optional. If present, they optional subkey field can be associated
with the credentials used in the remote ticket file. If left out, then it is
assumed that the recipient of the credentials already knows their value.
prealm and pname
The name and realm of the delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname, an application-arranged
negotiation to choose a per association session key.
pvno and caddr msg-type
These fields contain the values are described above in section 5.4.1. msg-type is
KRB_AP_REP.
enc-part
This field is described above in section 5.4.2. It is computed with a
key usage value of 12.
ctime
This field contains the corresponding fields from current time on the
ticket found in client's host.
cusec
This field contains the ticket field. Descriptions microsecond part of the fields are
identical to the descriptions in the KDC-REP message.
5.9. Error message specification client's timestamp.
subkey
This field contains an encryption key which is to be used to protect
this specific application session. See section specifies the format 3.2.6 for the KRB_ERROR message. The fields
included in the message are intended specifics on
how this field is used to return as much information as
possible about negotiate a key. Unless an error. It application
specifies otherwise, if this field is not expected that all left out, the information
required by sub-session key
from the fields authenticator, or if also left out, the session key from the
ticket will be available for all types of errors. used.
seq-number
This field is described above in section 5.3.2.
5.5.3. Error message reply
If an error occurs while processing the
appropriate information is not available when application request, the KRB_ERROR
message is composed,
the corresponding field will be left out sent in response. See section 5.9.1 for the format of the
error message.
Note that since The cname and crealm fields may be left out if the KRB_ERROR message is not integrity protected, it is
quite possible for an intruder to synthesize or modify such a server
cannot determine their appropriate values from the corresponding KRB_AP_REQ
message.
In particular, this means that If the client should not use any authenticator was decipherable, the ctime and cusec fields in
this
will contain the values from it.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.6. KRB_SAFE message for security-critical purposes, such as setting a system
clock or generating specification
This section specifies the format of a fresh authenticator. The message that can be useful,
however, for advising used by either
side (client or server) of an application to send a user on tamper-proof message to
its peer. It presumes that a session key has previously been exchanged (for
example, by using the reason for some failure.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.9.1. KRB_ERROR KRB_AP_REQ/KRB_AP_REP messages).
5.6.1. KRB_SAFE definition
The KRB_ERROR KRB_SAFE message consists of contains user data along with a collision-proof
checksum keyed with the following fields:
KRB-ERROR last encryption key negotiated via subkeys, or the
session key if no negotiation has occurred. The message fields are:
KRB-SAFE ::= [APPLICATION 30] 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
cusec [3]
usec [2] Microseconds OPTIONAL,
stime
seq-number [3] UInt32 OPTIONAL,
s-address [4] KerberosTime,
susec HostAddress,
r-address [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- Correct realm --,
sname [10] PrincipalName -- Correct name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_ERROR.
ctime
KRB_SAFE.
safe-body
This field is described above in section 5.4.1.
cusec
This field is described above in section 5.5.2.
stime a placeholder for the body of the KRB-SAFE message.
cksum
This field contains the current time on checksum of the server. It application data, computed with
a key usage value of 15.
The checksum is computed over the encoding of the KRB-SAFE sequence.
First, the cksum is set to a type
KerberosTime.
susec zero, zero-length value and the
checksum is computed over the encoding of the KRB-SAFE sequence, then
the checksum is set to the result of that computation, and finally the
KRB-SAFE sequence is encoded again. This field contains method, while different than
the microsecond one specified in RFC 1510, corresponds to existing practice.
user-data
This field is part of the server's timestamp.
Its value ranges KRB_SAFE and KRB_PRIV messages and contain
the application specific data that is being passed from 0 to 999999. It appears along with stime. The
two fields are used in conjunction the sender to specify a reasonably accurate
timestamp.
error-code
the recipient.
timestamp
This field contains is part of the error code returned by Kerberos or KRB_SAFE and KRB_PRIV messages. Its contents
are the
server when a request fails. To interpret current time as known by the value sender of this field
see the list message. By checking
the timestamp, the recipient of error codes in section 8. Implementations are
encouraged to provide for national language support in the display message is able to make sure that
it was recently generated, and is not a replay.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
usec
This field is part of error messages.
crealm, cname, srealm the KRB_SAFE and sname
These fields are KRB_PRIV headers. It contains
the microsecond part of the timestamp.
seq-number
This field is described above in section 5.3.1. e-text 5.3.2.
s-address
Sender's address.
This field contains additional text to help explain specifies the error code
associated with address in use by the failed request (for example, it might include a
principal name which was unknown).
e-data sender of the message.
It may be omitted if not required by the application protocol.
r-address
This field contains additional data about specifies the error for address in use by the
application recipient of the
message. It may be omitted for some uses (such as broadcast protocols),
but the recipient may arbitrarily reject such messages. This field,
along with s-address, can be used to help it recover from detect messages which have
been incorrectly or handle the error. If maliciously delivered to the
errorcode is KDC_ERR_PREAUTH_REQUIRED, then wrong recipient.
5.7. KRB_PRIV message specification
This section specifies the e-data field will
contain an encoding format of a sequence message that can be used by either
side (client or server) of padata fields, each
corresponding to an acceptable pre- authentication method application to securely and
optionally containing privately send a
message to its peer. It presumes that a session key has previously been
exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV definition
The KRB_PRIV message contains user data for encrypted in the method:
METHOD-DATA Session Key. The
message fields are:
KRB-PRIV ::= [APPLICATION 21] SEQUENCE OF PA-DATA
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
5.10. Application Tag Numbers
The following table lists the application class {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- NOTE: there is no [2] tag numbers used by
various data types defined in this section.
Tag Number(s) Type Name Comments
0 unused
1 Ticket
2 Authenticator
3 EncTicketPart
4-10 unused
10 AS-REQ
11 AS-REP
12 TGS-REQ
13 TGS-REP
14 AP-REQ
15 AP-REP
16 TGT-REQ
17-19 unused
20 KRB-SAFE
21 KRB-PRIV
22 KRB-PRIV
23-24 unused
25 EncASRepPart
26 EncTGSRepPart
27 EncApRepPart
28
enc-part [3] EncryptedData -- EncKrbPrivPart
29 EncKrbCredPart
30 KRB-ERROR
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
6. Encryption
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress -- sender's addr --,
r-address [5] HostAddress OPTIONAL -- recip's addr
}
pvno and Checksum Specifications
The Kerberos protocols msg-type
These fields are described above in this document are designed to
encrypt blocks section 5.4.1. msg-type is
KRB_PRIV.
enc-part
This field holds an encoding of arbitrary sizes, using stream or block encryption
ciphers. Encryption the EncKrbPrivPart sequence encrypted
under the session key[32], with a key usage value of 13. This encrypted
encoding is used to prove for the identities enc-part field of the network
entities participating KRB-PRIV message.
user-data, timestamp, usec, s-address and r-address
These fields are described above in message exchanges. The Key Distribution Center
for each realm section 5.6.1.
seq-number
This field is trusted by all principals registered described above in section 5.3.2.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.8. KRB_CRED message specification
This section specifies the format of a message that realm can be used to
store a secret key in confidence. Proof of knowledge of this secret key send
Kerberos credentials from one principal to another. It is used presented here to verify the authenticity of
encourage a principal.
The KDC uses the principal's secret key (in the AS exchange) common mechanism to be used by applications when forwarding
tickets or providing proxies to subordinate servers. It presumes that a shared
session key (in has already been exchanged perhaps by using the TGS exchange)
KRB_AP_REQ/KRB_AP_REP messages.
5.8.1. KRB_CRED definition
The KRB_CRED message contains a sequence of tickets to encrypt responses be sent and
information needed to ticket
requests; use the ability to obtain tickets, including the secret key or session key implies
the knowledge of the appropriate keys and the identity of the KDC. from each.
The
ability of a principal information needed to decrypt use the KDC response and present a Ticket
and a properly formed Authenticator (generated with tickets is encrypted under an encryption
key previously exchanged or transferred alongside the session KRB_CRED message. The
message fields are:
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm [1] Realm OPTIONAL,
pname [2] PrincipalName OPTIONAL,
flags [3] TicketFlags OPTIONAL,
authtime [4] KerberosTime OPTIONAL,
starttime [5] KerberosTime OPTIONAL,
endtime [6] KerberosTime OPTIONAL,
renew-till [7] KerberosTime OPTIONAL,
srealm [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_CRED.
tickets
These are the tickets obtained from the KDC response) to a service verifies specifically for use by the identity of
intended recipient. Successive tickets are paired with the principal;
likewise
corresponding KrbCredInfo sequence from the ability enc-part of the service to extract KRB-CRED
message.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
enc-part
This field holds an encoding of the EncKrbCredPart sequence encrypted
under the session key from shared between the
Ticket sender and prove its knowledge thereof in a response verifies the
identity of the service.
[KCRYPTO] defines a framework for defining encryption and checksum
mechanisms for use intended
recipient, with Kerberos. It also defines several such
mechanisms, and more may be added in future updates to that document.
The string-to-key operation provided by [KCRYPTO] is used to produce a
long-term key usage value of 14. This encrypted encoding is
used for the enc-part field of the KRB-CRED message.
Implementation note: implementations of certain applications, most
notably of the Kerberos GSS-API mechanism, do not encrypt the KRB-CRED
message when sending it. In the case of the GSS-API mechanism, this is
not a principal (generally for security vulnerability, as the KRB-CRED message itself is itself
encrypted inside an Authenticator.
nonce
If practical, an application may require the inclusion of a user). The default salt
string, if none nonce
generated by the recipient of the message. If the same value is provided via preauthentication data,
included as the nonce in the message, it provides evidence that the
message is fresh and has not been replayed by an attacker. A nonce must
never be re-used; it should be generated randomly by the
concatenation recipient of
the principal's realm message and name components, in order,
with no separators. Unless otherwise indicated, provided to the default
string-to-key opaque parameter set as defined sender of the message in [KCRYPTO] is used.
The encryption, decryption, an application
specific manner.
timestamp and checksum operations usec
These fields specify the time that the KRB-CRED message was generated.
The time is used in this
document use to provide assurance that the corresponding encryption, decryption, message is fresh.
s-address and get_mic
operations r-address
These fields are described above in [KCRYPTO], with implicit "specific key"
generation using section 5.6.1. They are used
optionally to provide additional assurance of the integrity of the
KRB-CRED message.
key usage values outlined
This field exists in section 6.1. Unless
otherwise indicated, no chaining of cipher state the corresponding ticket passed by the KRB-CRED
message and is done used to pass the session key from one
encryption operation the sender to another. the
intended recipient. The EncryptedData object's "etype" and "cipher" field's encoding is described in section 5.2.9.
The following fields are optional. If present, they can be associated with
the
encryption mechanism type number credentials in the remote ticket file. If left out, then it is assumed
that the recipient of the credentials already knows their value.
prealm and encryption operation output. pname
The
EncryptionKey object's "keytype" name and "keyvalue" fields are realm of the
encryption mechanism type number and protocol key representation. The
Checksum object's "cksumtype" delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname, and "checksum" caddr
These fields are contain the checksum
mechanism type number and get_mic operation output.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
6.1. Key Usage Values
The encryption and checksum specifications in [KCRYPTO] require as input
a "key usage number", to alter values of the encryption key used corresponding fields from the
ticket found in any specific
message, to make certain types of cryptographic attack more difficult.
This is a list the ticket field. Descriptions of key usage number definitions and reserved ranges,
including values for all places keys the fields are used
identical to the descriptions in the Kerberos protocol
and associated KDC-REP message.
5.9. Error message specification
This section numbers.
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the client
key (section 5.4.1)
2. AS-REP Ticket and TGS-REP Ticket (includes TGS session key or
application session key), encrypted with the service key (section 5.4.2)
3. AS-REP encrypted part (includes TGS session key or application
session key), encrypted with the client key (section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
session key (section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with specifies the TGS
authenticator subkey (section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed with format for the
TGS session key (sections 5.3.2, 5.4.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes TGS
authenticator subkey), encrypted with KRB_ERROR message. The fields
included in the TGS session key (section 5.3.2)
8. TGS-REP encrypted part (includes application session key), encrypted
with message are intended to return as much information as
possible about an error. It is not expected that all the TGS session key (section 5.4.2)
9. TGS-REP encrypted part (includes application session key), encrypted
with information
required by the TGS authenticator subkey (section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with fields will be available for all types of errors. If the application session key
(section 5.3.2)
11. AP-REQ Authenticator (includes application authenticator subkey),
encrypted with
appropriate information is not available when the application session key (section 5.3.2)
12. AP-REP encrypted part (includes application session subkey),
encrypted with message is composed, the application session key (section 5.5.2)
13. KRB-PRIV encrypted part, encrypted with a key chosen by
corresponding field will be left out of the
application (section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by message.
Note that since the
application (section 5.6.1)
15. KRB-SAFE cksum, keyed with KRB_ERROR message is not integrity protected, it is
quite possible for an intruder to synthesize or modify such a key chosen by message. In
particular, this means that the application (section
5.8.1)
18. KRB-ERROR checksum (e-cksum in section 5.9.1)
19. AD-KDCIssued checksum (ad-checksum in appendix B.4)
20. Checksum for Mandatory Ticket Extensions (appendix B.6)
21. Checksum in Authorization Data in Ticket Extensions (appendix B.7)
22-24. Reserved for use in GSSAPI mechanisms derived from RFC 1964.
(raeburn/MIT)
25-511. Reserved for future client should not use any fields in Kerberos and related protocols.
512-1023. Reserved this
message for uses internal to a Kerberos implementation. [6.1]
A few of these key usages need security-critical purposes, such as setting a little clarification. A service which
receives an AP-REQ has no way to know if the enclosed Ticket was part of
an AS-REP system clock or TGS-REP. Therefore, key usage 2 must always be used for
generating a Ticket, whether it is in response to an AS-REQ or TGS-REQ.
Key usage values between 1024 and 2047 (inclusive) are reserved for
application use. Applications should use even values fresh authenticator. The message can be useful, however, for encryption and
odd values
advising a user on the reason for checksums within this range.
draft-ietf-krb-wg-kerberos-clarifications-01 some failure.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
There might exist other documents which define protocols in terms
5.9.1. KRB_ERROR definition
The KRB_ERROR message consists of the
RFC1510 encryption types or checksum types. Such documents would not
know about key usages. In order that these specifications continue to be
meaningful until they are updated, key usages 1024 and 1025 must be used
to derive keys for encryption following fields:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime [2] KerberosTime OPTIONAL,
cusec [3] Microseconds OPTIONAL,
stime [4] KerberosTime,
susec [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- service realm --,
sname [10] PrincipalName -- service name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING OPTIONAL
}
pvno and checksums, respectively.[6.2] New
protocols defined msg-type
These fields are described above in terms of section 5.4.1. msg-type is
KRB_ERROR.
ctime
This field is described above in section 5.4.1.
cusec
This field is described above in section 5.5.2.
stime
This field contains the Kerberos encryption and checksum types
should use their own key usage values. Key usages are unsigned 32 bit
integers; zero current time on the server. It is not permitted. Usage numbers may be registered with
IANA of type
KerberosTime.
susec
This field contains the microsecond part of the server's timestamp. Its
value ranges from 0 to avoid conflicts.
6.2. Implementation Notes
While we don't recommend it, undoubtedly some application protocols will
continue 999999. It appears along with stime. The two
fields are used in conjunction to use specify a reasonably accurate
timestamp.
error-code
This field contains the key data directly, even if only in some error code returned by Kerberos or the server
when a request fails. To interpret the value of this field see the
currently existing protocol specifications. An implementation intended list
of error codes in section 8. Implementations are encouraged to provide
for national language support general Kerberos applications may therefore need to make the
key data available, as well as in the attributes display of error messages.
crealm, cname, srealm and operations sname
These fields are described above in [KCRYPTO].
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
7. Naming Constraints
7.1. Realm Names
Although realm names are encoded as GeneralStrings and although section 5.3.
e-text
This field contains additional text to help explain the error code
associated with the failed request (for example, it might include a realm
can technically select any
principal name it chooses, interoperability across
realm boundaries requires agreement on how realm names are which was unknown).
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
e-data
This field contains additional data about the error for use by the
application to be
assigned, and what information they imply.
To enforce these conventions, help it recover from or handle the error. If the
errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
contain an encoding of a sequence of padata fields, each realm must conform corresponding
to an acceptable pre-authentication method and optionally containing
data for the conventions
itself, method:
METHOD-DATA ::= SEQUENCE OF PA-DATA
For error codes defined in this document other than
KDC_ERR_PREAUTH_REQUIRED, the format and it must require that any realms with which inter-realm keys contents of the e-data field
are shared also conform to implementation-defined. Similarly, for future error codes, the conventions
format and require contents of the same from its
neighbors.
Kerberos realm names e-data field are case sensitive. Realm names that differ only implementation-defined
unless specified. Whether defined by the implementation or in a future
document, the case of e-data field MAY take the characters are not equivalent. There are presently four
styles of realm names: domain, X500, other, and reserved. Examples form of
each style follow:
domain: ATHENA.MIT.EDU TYPED-DATA:
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
data-type [0] INTEGER,
data-value [1] OCTET STRING OPTIONAL
}
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
5.10. Application Tag Numbers
The following table lists the application class tag numbers used by various
data types defined in this section.
Tag Number(s) Type Name Comments
0 unused
1 Ticket PDU
2 Authenticator non-PDU
3 EncTicketPart non-PDU
4-10 unused
10 AS-REQ PDU
11 AS-REP PDU
12 TGS-REQ PDU
13 TGS-REP PDU
14 AP-REQ PDU
15 AP-REP PDU
16 RESERVED16 TGT-REQ
17 RESERVED17 TGT-REP
18-19 unused
20 KRB-SAFE PDU
21 KRB-PRIV PDU
22 KRB-CRED PDU
23-24 unused
25 EncASRepPart non-PDU
26 EncTGSRepPart non-PDU
27 EncApRepPart non-PDU
28 EncKrbPrivPart non-PDU
29 EncKrbCredPart non-PDU
30 KRB-ERROR PDU
The ASN.1 types marked as "PDU" (Protocol Data Unit) in the above are the
only ASN.1 types intended as top-level types of the Kerberos protcol, and
are the only types that may be used as elements in another protocol that
makes use of Kerberos.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
6. Naming Constraints
6.1. Realm Names
Although realm names are encoded as GeneralStrings and although a realm can
technically select any name it chooses, interoperability across realm
boundaries requires agreement on how realm names are to be assigned, and
what information they imply.
To enforce these conventions, each realm must conform to the conventions
itself, and it must require that any realms with which inter-realm keys are
shared also conform to the conventions and require the same from its
neighbors.
Kerberos realm names are case sensitive. Realm names that differ only in the
case of the characters are not equivalent. There are presently four styles
of realm names: domain, X500, other, and reserved. Examples of each style
follow:
domain: ATHENA.MIT.EDU (example)
X500: C=US/O=OSF (example)
other: NAMETYPE:rest/of.name=without-restrictions (example)
reserved: reserved, but will not conflict with above
Domain syle realm names must look like domain names: they consist of
components separated by periods (.) and they contain neither colons (:) nor
slashes (/). Though domain names themselves are case insensitive, in order
for realms to match, the case must match as well. When establishing a new
realm name based on an internet domain name it is recommended by convention
that the characters be converted to upper case.
X.500 names contain an equal (=) and cannot contain a colon (:) before the
equal. The realm names for X.500 names will be string representations of the
names with components separated by slashes. Leading and trailing slashes
will not be included. Note that the slash separator is consistent with
Kerberos implementations based on RFC1510, but it is different from the
separator recommended in RFC2253.
Names that fall into the other category must begin with a prefix that
contains no equal (=) or period (.) and the prefix must be followed by a
colon (:) and the rest of the name. All prefixes must be assigned before
they may be used. Presently none are assigned.
The reserved category includes strings which do not fall into the first
three categories. All names in this category are reserved. It is unlikely
that names will be assigned to this category unless there is a very strong
argument for not using the 'other' category.
These rules guarantee that there will be no conflicts between the various
name styles. The following additional constraints apply to the assignment of
realm names in the domain and X.500 categories: the name of a realm for the
domain or X.500 formats must either be used by the organization owning (to
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
whom it was assigned) an Internet domain name or X.500 name, or in the case
that no such names are registered, authority to use a realm name may be
derived from the authority of the parent realm. For example, if there is no
domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
authorize the creation of a realm with that name.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
This is acceptable because the organization to which the parent is assigned
is presumably the organization authorized to assign names to its children in
the X.500 and domain name systems as well. If the parent assigns a realm
name without also registering it in the domain name or X.500 hierarchy, it
is the parent's responsibility to make sure that there will not in the
future exist a name identical to the realm name of the child unless it is
assigned to the same entity as the realm name.
7.2.
6.2. Principal Names
As was the case for realm names, conventions are needed to ensure that all
agree on what information is implied by a principal name. The name-type
field that is part of the principal name indicates the kind of information
implied by the name. The name-type should be treated only as a hint to
interpreting the meaning of a name. It is not significant when checking for
equivalence. Principal names that differ only in the name-type identify the
same principal. The name type does not partition the name space. Ignoring
the name type, no two names can be the same (i.e. at least one of the
components, or the realm, must be different). The following name types are
defined:
name-type value meaning
NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 General principal name (e.g. username,
or DCE principal)
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance (telnet, rcommands)
NT-SRV-XHST 4 Service with slash-separated host name components
NT-UID 5 Unique ID
NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779]
NT-SMTP-NAME 7 Name in form of SMTP email name (e.g. user@foo.com)
NT-ENTERPRISE 10 Enterprise name - may be mapped to principal name
When a name implies no information other than its uniqueness at a particular
time the name type PRINCIPAL should be used. The principal name type should
be used for users, and it might also be used for a unique server. If the
name is a unique machine generated ID that is guaranteed never to be
reassigned then the name type of UID should be used (note that it is
generally a bad idea to reassign names of any type since stale entries might
remain in access control lists).
If the first component of a name identifies a service and the remaining
components identify an instance of the service in a server specified manner,
then the name type of SRV-INST should be used. An example of this name type
is the Kerberos ticket-granting service whose name has a first component of
krbtgt and a second component identifying the realm for which the ticket is
valid.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
If instance is a single component following the service name and the
instance identifies the host on which the server is running, then the name
type SRV-HST should be used. This type is typically used for Internet
services such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components following the
name of the service, then the name type SRV-XHST should be used. This type
might be used to identify servers on hosts with X.500 names where the slash
(/) might otherwise be ambiguous.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
A name type of NT-X500-PRINCIPAL should be used when a name from an X.509
certificate is translated into a Kerberos name. The encoding of the X.509
name as a Kerberos principal shall conform to the encoding rules specified
in RFC 2253.
A name type of SMTP allows a name to be of a form that resembles a SMTP
email name. This name, including an "@" and a domain name, is used as the
one component of the principal name.
A name type of UNKNOWN should be used when the form of the name is not
known. When comparing names, a name of type UNKNOWN will match principals
authenticated with names of any type. A principal authenticated with a name
of type UNKNOWN, however, will only match other names of type UNKNOWN.
Names of any type with an initial component of 'krbtgt' are reserved for the
Kerberos ticket granting service. See section 8.2.4 for the form of such
names.
7.2.1.
6.2.1. Name of server principals
The principal identifier for a server on a host will generally be composed
of two parts: (1) the realm of the KDC with which the server is registered,
and (2) a two-component name of type NT-SRV-HST if the host name is an
Internet domain name or a multi-component name of type NT-SRV-XHST if the
name of the host is of a form such as X.500 that allows slash (/)
separators. The first component of the two- or multi-component name will
identify the service and the latter components will identify the host. Where
the name of the host is not case sensitive (for example, with Internet
domain names) the name of the host must be lower case. If specified by the
application protocol for services such as telnet and the Berkeley R commands
which run with system privileges, the first component may be the string
'host' instead of a service specific identifier. When a host has an official
name and one or more aliases and the official name can be reliably
determined, the official name of the host should be used when constructing
the name of the server principal.
8.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7. Constants and other defined values
8.1.
7.1. Host address types
All negative values for the host address type are reserved for local use.
All non-negative values are reserved for officially assigned type fields and
interpretations.
The values of the types for the following addresses are chosen to match the
defined address family constants in the Berkeley Standard Distributions of
Unix. They can be found in with symbolic names AF_xxx (where xxx is an
abbreviation of the address family name).
Internet (IPv4) Addresses
Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in MSB
order. The IPv4 loopback address should not appear in a Kerberos packet. The
type of IPv4 addresses is two (2).
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Internet (IPv6) Addresses [Westerlund]
IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order. The
type of IPv6 addresses is twenty-four (24). [RFC2373]. The following
addresses (see [RFC1884]) MUST not appear in any Kerberos packet:
* the Unspecified Address
* the Loopback Address
* Link-Local addresses
IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2.
DECnet Phase IV addresses
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The
type of DECnet Phase IV addresses is twelve (12).
Netbios addresses
Netbios addresses are 16-octet addresses typically composed of 1 to 15
characters, trailing blank (ascii char 20) filled, with a 16th octet of 0x0.
The type of Netbios addresses is 20 (0x14).
Directional Addresses
In many environments, including the sender address in KRB_SAFE and KRB_PRIV
messages is undesirable because the addresses may be changed in transport by
network address translators. However, if these addresses are removed, the
messages may be subject to a reflection attack in which a message is
reflected back to its originator. The directional address type provides a
way to avoid transport addresses and reflection attacks. Directional
addresses are encoded as four byte unsigned integers in network byte order.
If the message is originated by the party sending the original KRB_AP_REQ
message, then an address of 0 should be used. If the message is originated
by the party to whom that KRB_AP_REQ was sent, then the address 1 should be
used. Applications involving multiple parties can specify the use of other
addresses.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
Directional addresses MUST only be used for the sender address field in the
KRB_SAFE or KRB_PRIV messages. They MUST NOT be used as a ticket address or
in a KRB_AP_REQ message. This address type SHOULD only be used in situations
where the sending party knows that the receiving party supports the address
type. This generally means that directional addresses may only be used when
the application protocol requires their support. Directional addresses are
type XX (0xXX).
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
8.2.
7.2. KDC messaging
8.2.1
7.2.1 IP Transports
Kerberos defines two IP transport mechanisms for communication between
clients and servers: UDP/IP and TCP/IP.
8.2.1.1.
7.2.1.1. UDP/IP transport
Kerberos servers (KDCs) supporting IP transports MUST accept UDP requests
and SHOULD listen for such requests on port 88 (decimal) unless specifically
configured to listen on an alternative UDP port. Alternate ports MAY be used
when running multiple KDCs for multiple realms on the same host.
Kerberos clients supporting IP transports SHOULD (XXX or is this a
MUST?) support the sending of UDP
requests. Clients SHOULD use KDC discovery
[8.2.1.3] [7.2.1.3] to identify the IP
address and port to which they will send their request.
When contacting a KDC for a KRB_KDC_REQ request using UDP/IP transport, the
client shall send a UDP datagram containing only an encoding of the request
to the KDC. The KDC will respond with a reply datagram containing only an
encoding of the reply message (either a KRB_ERROR or
8a a KRB_KDC_REP) to the
sending port at the sender's IP address. The response to a request made
through UDP/IP transport must also use UDP/IP transport. If the response can
not be handled using UDP (for example because it is too large), the KDC must
return an error forcing the client to retry the request using the TCP
transport.
8.2.1.2.
7.2.1.2. TCP/IP transport [Westerlund,Danielsson]
Kerberos servers (KDCs) supporting IP transports MUST accept TCP requests
and SHOULD listen for such requests on port 88 (decimal) unless specifically
configured to listen on an alternate TCP port. Alternate ports MAY be used
when running multiple KDCs for multiple realms on the same host.
Clients must support the sending of TCP requests, but may choose to intially
try a request using the UDP transport. Clients SHOULD use KDC discovery [8.2.1.3]
[7.2.1.3] to identify the IP address and port to which they will send their
request.
Implementation note: Some extensions to the Kerberos protocol will not
succeed if any client or KDC not supporting the TCP transport is involved.
Implementations of RFC 1510 were not required to support TCP/IP transports.
[XXX as discussed we believe that the consensus was to allow multiple
requests over a single TCP connection between a client and KDC.]
When the KRB_KDC_REQ message is sent to the KDC over a TCP stream, the
response (KRB_KDC_REP or KRB_ERROR message) MUST be returned to the client
on the same TCP stream that was established for the request. The KDC may
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
close the TCP stream after sending a response, but may leave the stream open
for a reasonable period of time if it expects a followup. Care must be taken
in managing TCP/IP connections on the KDC to prevent denial of service
attacks based on the number of open TCP/IP connections.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
The client must MUST be prepared to have the stream closed by the KDC at anytime
after the receipt of a response. A stream closure should not be treated as a
fatal error. Instead, if multiple exchanges are required (e.g., certain
forms of preauthentication) the client should may need to establish a new
connection when it is ready to send subsequent messages. A client may close
the stream after receiving a response, and should close the stream if it
does not expect to send followup messages.
The first four octets of the TCP stream used
A client MAY send multiple requests before receiving responses, though it
must be prepared to transmit the request
request will encode in network byte order handle the length of connection being closed after the first
response.
Each request
(KRB_KDC_REQ), (KRB_KDC_REQ) and the length will be followed by the request itself.
The response will similarly be preceded by a 4 octet encoding in network
byte order of the length of the KRB_KDC_REP or the KRB_ERROR message and
will be followed by the KRB_KDC_REP (KRB_KDC_REP or KRB_ERROR) sent over
the KRB_ERROR response. If the
sign bit TCP stream is set on the integer represented preceded by the first 4 octets, then
the next 4 octets will be read, extending the length of the field by
another request as 4 octets (less the sign in
network byte order. The high bit of the additional four octets which length is reserved for future
expansion and which at present must currently be zero).
8.2.1.3 KDC Discovery on IP Networks
Kerberos client implementations must provide set to zero.
If multiple requests are sent over a means for single TCP connection, and the client KDC
sends multiple responses, the KDC is not required to
determine send the location responses in
the order of the Kerberos Key Distribution Centers (KDCs).
Traditionally, Kerberos corresponding requests. This may permit some
implementations have stored such configuration
information in a file on to send each response as soon as it is ready even if earlier
requests are still being processed (for example, waiting for a response from
an external device or database).
7.2.1.3 KDC Discovery on IP Networks
Kerberos client implementations must provide a means for the client to
determine the location of the Kerberos Key Distribution Centers (KDCs).
Traditionally, Kerberos implementations have stored such configuration
information in a file on each client machine. Experience has shown this
method of storing configuration information presents problems with
out-of-date information and scaling problems, especially when using
cross-realm authentication. This section describes a method for using the
Domain Name System [RFC 1035] for storing KDC location information.
8.2.1.3.1.
7.2.1.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names
In Kerberos, realm names are case sensitive. While it is strongly encouraged
that all realm names be all upper case this recommendation has not been
adopted by all sites. Some sites use all lower case names and other use
mixed case. DNS on the other hand is case insensitive for queries. Since
"MYREALM", "myrealm", and "MyRealm" are all different it is necessary that
only one of the possible combinations of upper and lower case characters be
used. This restriction may be lifted in the future as the DNS naming scheme
is expanded to support non-ASCII names.
8.2.1.3.2.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7.2.1.3.2. Specifying KDC Location information with DNS SRV records
KDC location information is to be stored using the DNS SRV RR [RFC 2052].
The format of this RR is as follows:
Service.Proto.Realm TTL Class SRV Priority Weight Port Target
The Service name for Kerberos is always "_kerberos".
The Proto can be one of "_udp", "_tcp". If these SRV records are to be used,
both "_udp" and "_tcp" records MUST be specified for all KDC deployments.
The Realm is the Kerberos realm that this record corresponds to.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
TTL, Class, SRV, Priority, Weight, and Target have the standard meaning as
defined in RFC 2052.
As per RFC 2052 the Port number used for "_udp" and "_tcp" SRV records
SHOULD be the value assigned to "kerberos" by the Internet Assigned Number
Authority: 88 (decimal) unless the KDC is configured to listen on an
alternate TCP port.
Implementation note: Many existing client implementations do not support KDC
Discovery and are configured to send requests to the IANA assigned port (88
decimal), so it is strongly recommended that KDCs be configured to listen on
that port.
8.2.1.3.3. Example DNS SRV records specifying
7.2.1.3.3. KDC location
information Discovery for Domain Style Realm Names on IP Networks
These are DNS records for a Kerberos realm ASDF.COM. EXAMPLE.COM. It has two Ker
beros Kerberos
servers, kdc1.asdf.com kdc1.example.com and kdc2.asdf.com. kdc2.example.com. Queries should be directed
to kdc1.asdf.com kdc1.example.com first as per the specified priority. Weights are not
used in these sample records.
_kerberos._udp.ASDF.COM.
_kerberos._udp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.asdf.com.
_kerberos._udp.ASDF.COM. kdc1.example.com.
_kerberos._udp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.asdf.com.
_kerberos._tcp.ASDF.COM. kdc2.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.asdf.com.
_kerberos._tcp.ASDF.COM. kdc1.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.asdf.com.
8.2.2. OSI transport
During authentication of an OSI client to an OSI server, the mutual
authentication of an OSI server to an OSI client, the transfer of
credentials from an OSI client to an OSI server, or during exchange of
private or integrity checked messages, Kerberos protocol messages may be
treated as opaque objects and the type of the authentication mechanism
will be:
OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), security(5),kerberosv5(2)}
Depending on the situation, the opaque object will be an authentication
header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe
message (KRB_SAFE), a private message (KRB_PRIV), or a credentials
message (KRB_CRED). The opaque data contains an application code as
specified in the ASN.1 description for each message. The application
code may be used by Kerberos to determine the message type.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
8.3. kdc2.example.com.
7.3. Name of the TGS
The principal identifier of the ticket-granting service shall be composed of
three parts: (1) the realm of the KDC issuing the TGS ticket (2) a two-part
name of type NT-SRV-INST, with the first part "krbtgt" and the second part
the name of the realm which will accept the ticket-granting ticket. For
example, a ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be
used to get tickets from the ATHENA.MIT.EDU KDC has a principal identifier
of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A
ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get
tickets from the MIT.EDU realm has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name).
8.4.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7.4. OID arc for KerberosV5
This OID may be used to identify Kerberos protocol messages encapsulated in
other protocols. It also designates the OID arc for KerberosV5-related OIDs
assigned by future IETF action. Implementation note:: RFC 1510 had an
incorrect value (5) for "dod" in its OID.
id-krb5 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2)
}
Assignment of OIDs beneath the id-krb5 arc must be obtained by contacting
krb5-oid-registrar@mit.edu.
7.5. Protocol constants and associated values
The following tables list constants used in the protocol and define their
meanings. Ranges are specified in the "specification" section that limit the
values of constants for which values are defined here. This allows
implementations to make assumptions about the maximum values that will be
received for these constants. Implementation receiving values outside the
range specified in the "specification" section may reject the request, but
they must recover cleanly.
padata and data types padata-type value comment
PA-TGS-REQ
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PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
[reserved] 4
PA-ENC-UNIX-TIME 5 (depricated)
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
PA-SAM-CHALLENGE 12 (sam/otp)
PA-SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ 14 (pkinit)
PA-PK-AS-REP 15 (pkinit)
PA-USE-SPECIFIED-KVNO 20
PA-SAM-REDIRECT 21 (sam/otp)
PA-GET-FROM-TYPED-DATA 22 (embedded May 2003
7.5.1. Key usage numbers
The encryption and checksum specifications in typed data)
TD-PADATA 22 (embeds padata)
PA-SAM-ETYPE-INFO 23 (sam/otp)
PA-ALT-PRINC 24 (crawdad@fnal.gov)
PA-SAM-CHALLENGE2 30 (kenh@pobox.com)
PA-SAM-RESPONSE2 31 (kenh@pobox.com)
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
TD-KRB-PRINCIPAL 102 PrincipalName (see Sec.5.9.1)
TD-KRB-REALM 103 Realm (see Sec.5.9.1)
TD-TRUSTED-CERTIFIERS 104 from PKINIT
TD-CERTIFICATE-INDEX 105 from PKINIT
TD-APP-DEFINED-ERROR 106 application [KCRYPTO] require as input a
"key usage number", to alter the encryption key used in any specific (see Sec.5.9.1)
TD-REQ-NONCE 107 INTEGER (see Sec.5.9.1)
TD-REQ-SEQ 108 INTEGER (see Sec.5.9.1)
PA-PAC-REQUEST 128 (jbrezak@exchange.microsoft.com)
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Address type value
IPV4 2
ChaosNet 5
XNS 6
ISO 7
DECNET Phase IV 12
AppleTalk DDP 16
NetBios 20
IPV6 24
authorization data type ad-type value
AD-IF-RELEVANT 1
AD-INTENDED-FOR-SERVER 2
AD-INTENDED-FOR-APPLICATION-CLASS 3
AD-KDC-ISSUED 4
AD-OR 5
AD-MANDATORY-TICKET-EXTENSIONS 6
AD-IN-TICKET-EXTENSIONS 7
AD-MANDATORY-FOR-KDC 8
reserved
message, to make certain types of cryptographic attack more difficult. These
are the key usage values 9-63
OSF-DCE 64
SESAME 65
AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com)
AD-WIN2K-PAC 128 (jbrezak@exchange.microsoft.com) assigned in this document:
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted
with the client key (section 5.2.7.2)
2. AS-REP Ticket Extension Types
TE-TYPE-NULL 0 Null ticket extension
TE-TYPE-EXTERNAL-ADATA 1 Integrity protected authorization data
[reserved] 2 TE-TYPE-PKCROSS-KDC (I have reservations)
TE-TYPE-PKCROSS-CLIENT 3 PKCROSS cross realm and TGS-REP Ticket (includes TGS session
key ticket
TE-TYPE-CYBERSAFE-EXT 4 Assigned to CyberSafe Corp
[reserved] 5 TE-TYPE-DEST-HOST (I have reservations)
transited encoding type tr-type value
DOMAIN-X500-COMPRESS 1
reserved values all others
Label or application session key), encrypted with the
service key (section 5.3)
3. AS-REP encrypted part (includes TGS session key or
application session key), encrypted with the client key
(section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS session key (section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS authenticator subkey (section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
keyed with the TGS session key (sections 5.5.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator
(includes TGS authenticator subkey), encrypted with the
TGS session key (section 5.5.1)
8. TGS-REP encrypted part (includes application session
key), encrypted with the TGS session key (section
5.4.2)
9. TGS-REP encrypted part (includes application session
key), encrypted with the TGS authenticator subkey
(section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with the application
session key (section 5.5.1)
11. AP-REQ Authenticator (includes application
authenticator subkey), encrypted with the application
session key (section 5.5.1)
12. AP-REP encrypted part (includes application session
subkey), encrypted with the application session key
(section 5.5.2)
13. KRB-PRIV encrypted part, encrypted with a key chosen by
the application (section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by
the application (section 5.8.1)
15. KRB-SAFE cksum, keyed with a key chosen by the
application (section 5.8.1)
19. AD-KDCIssued checksum (ad-checksum in appendix B.4)
22-24. Reserved for use in GSSAPI mechanisms derived from RFC
1964. (raeburn/MIT)
18, 20-21, 25-511. Reserved for future use in Kerberos and related
protocols.
512-1023. Reserved for uses internal to a Kerberos
implementation.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7.5.2. PreAuthentication Data Types
padata and data types padata-type value comment
PA-TGS-REQ 1
PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
[reserved] 4
PA-ENC-UNIX-TIME 5 (depricated)
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
PA-SAM-CHALLENGE 12 (sam/otp)
PA-SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ 14 (pkinit)
PA-PK-AS-REP 15 (pkinit)
PA-ETYPE-INFO2 19 (replaces pa-etype-info)
PA-USE-SPECIFIED-KVNO 20
PA-SAM-REDIRECT 21 (sam/otp)
PA-GET-FROM-TYPED-DATA 22 (embedded in typed data)
TD-PADATA 22 (embeds padata)
PA-SAM-ETYPE-INFO 23 (sam/otp)
PA-ALT-PRINC 24 (crawdad@fnal.gov)
PA-SAM-CHALLENGE2 30 (kenh@pobox.com)
PA-SAM-RESPONSE2 31 (kenh@pobox.com)
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
TD-KRB-PRINCIPAL 102 PrincipalName
TD-KRB-REALM 103 Realm
TD-TRUSTED-CERTIFIERS 104 from PKINIT
TD-CERTIFICATE-INDEX 105 from PKINIT
TD-APP-DEFINED-ERROR 106 application specific
TD-REQ-NONCE 107 INTEGER
TD-REQ-SEQ 108 INTEGER
PA-PAC-REQUEST 128 (jbrezak@exchange.microsoft.com)
7.5.3. Address Types
Address type value
IPV4 2
ChaosNet 5
XNS 6
ISO 7
DECNET Phase IV 12
AppleTalk DDP 16
NetBios 20
IPV6 24
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7.5.4. Authorization Data Types
authorization data type ad-type value
AD-IF-RELEVANT 1
AD-INTENDED-FOR-SERVER 2
AD-INTENDED-FOR-APPLICATION-CLASS 3
AD-KDC-ISSUED 4
AD-OR 5
AD-MANDATORY-TICKET-EXTENSIONS 6
AD-IN-TICKET-EXTENSIONS 7
AD-MANDATORY-FOR-KDC 8
reserved values 9-63
OSF-DCE 64
SESAME 65
AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com)
AD-WIN2K-PAC 128 (jbrezak@exchange.microsoft.com)
7.5.5. Transited Encoding Types
transited encoding type tr-type value
DOMAIN-X500-COMPRESS 1
reserved values all others
7.5.6. Protocol Version Number
Label Value Meaning or MIT code
pvno 5 current Kerberos protocol version number
7.5.7. Kerberos Message Types
message types (Will be updated to match section 5)
KRB_AS_REQ 10 Request for initial authentication
KRB_AS_REP 11 Response to KRB_AS_REQ request
KRB_TGS_REQ 12 Request for authentication based on TGT
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
KRB_AP_REQ 14 application request to server
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
KRB_RESERVED16 16 Reserved for user-to-user krb_tgt_request
KRB_RESERVED17 17 Reserved for user-to-user krb_tgt_reply
KRB_SAFE 20 Safe (checksummed) application message
KRB_PRIV 21 Private (encrypted) application message
KRB_CRED 22 Private (encrypted) message to forward credentials
KRB_ERROR 30 Error response
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
7.5.8. Name Types
name types
KRB_NT_UNKNOWN 0 Name type not known
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for users
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
KRB_NT_SRV_HST 3 Service with host name as instance (telnet, rcommands)
KRB_NT_SRV_XHST 4 Service with host as remaining components
KRB_NT_UID 5 Unique ID
KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
7.5.9. Error Codes
error codes
KDC_ERR_NONE 0 No error
KDC_ERR_NAME_EXP 1 Client's entry in database has expired
KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired
KDC_ERR_BAD_PVNO 3 Requested protocol version number
not supported
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key
KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database
KDC_ERR_NULL_KEY 9 The client or server has a null key
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating
KDC_ERR_NEVER_VALID 11 Requested start time is later than end time
KDC_ERR_POLICY 12 KDC policy rejects request
KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later
KDC_ERR_KEY_EXPIRED 23 Password has expired
- change password to reset
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was invalid
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired [40]
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user only
KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path
KDC_ERR_SVC_UNAVAILABLE 29 A service is not available
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field failed
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
KRB_AP_ERR_REPEAT 34 Request is a replay
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match
KRB_AP_ERR_SKEW 37 Clock skew too great
KRB_AP_ERR_BADADDR 38 Incorrect net address
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
KRB_AP_ERR_MODIFIED 41 Message stream modified
KRB_AP_ERR_BADORDER 42 Message out of order
KRB_AP_ERR_BADKEYVER 44 Specified version of key is not available
KRB_AP_ERR_NOKEY 45 Service key not available
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
KRB_AP_ERR_METHOD 48 Alternative authentication method required
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message
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KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in message
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KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path
KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP, retry with TCP
KRB_ERR_GENERIC 60 Generic error (description in e-text)
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this implementation
KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit)
KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit)
KDC_ERROR_INVALID_SIG 64 (pkinit)
KDC_ERR_KEY_TOO_WEAK 65 (pkinit)
KDC_ERR_CERTIFICATE_MISMATCH 66 (pkinit)
KRB_AP_ERR_NO_TGT 67 (user-to-user)
KDC_ERR_WRONG_REALM 68 (user-to-user)
KRB_AP_ERR_USER_TO_USER_REQUIRED 69 (user-to-user)
KDC_ERR_CANT_VERIFY_CERTIFICATE 70 (pkinit)
KDC_ERR_INVALID_CERTIFICATE 71 (pkinit)
KDC_ERR_REVOKED_CERTIFICATE 72 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 (pkinit)
KDC_ERR_CLIENT_NAME_MISMATCH 75 (pkinit)
KDC_ERR_KDC_NAME_MISMATCH 76 (pkinit)
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9.
8. Interoperability requirements
Version 5 of the Kerberos protocol supports a myriad of options. Among these
are multiple encryption and checksum types, alternative encoding schemes for
the transited field, optional mechanisms for pre-authentication, the
handling of tickets with no addresses, options for mutual authentication,
user to user authentication, support for proxies, forwarding, postdating,
and renewing tickets, the format of realm names, and the handling of
authorization data.
In order to ensure the interoperability of realms, it is necessary to define
a minimal configuration which must be supported by all implementations. This
minimal configuration is subject to change as technology does. For example,
if at some later date it is discovered that one of the required encryption
or checksum algorithms is not secure, it will be replaced.
9.1.
8.1. Specification 2
This section defines the second specification of these options.
Implementations which are configured in this way can be said to support
Kerberos Version 5 Specification 2 (5.1). (5.2). Specification 1 (deprecated) may
be found in RFC1510.
Transport
TCP/IP and UDP/IP transport must be supported by clients and KDCs claiming
conformance to specification 2.
Encryption and checksum methods
The following encryption and checksum mechanisms must be supported.
Encyrption: AES256-CTS-HMAC-SHA1-96
Checksums: HMAC-SHA1-96-AES256
Implementations may should support other mechanisms as well, but the additional
mechanisms may only be used when communicating with principals known to also
support them: This list is to be determined and them. The mechanisms that should correspond
to section 6. be supported are:
Encryption: DES-CBC-MD5, DES3-CBC-SHA1-KD, RIJNDAEL(decide identifier) DES3-CBC-SHA1-KD
Checksums: CRC-32, DES-MAC, DES-MAC-K, DES-MD5, HMAC-SHA1-DES3-KD
Realm Names
All implementations must understand hierarchical realms in both the
Internet Domain and
Implementations may support other mechanisms as well, but the X.500 style. When a ticket granting ticket additional
mechanisms may only be used when communicating with principals known to also
support them.
Implementation note: earlier implementations of Kerberos generate messages
using the CRC-32, RSA-MD5 checksum methods. For interoperability with these
earlier releases implementors may consider supporting these checksum methods
but should carefully analyze the security impplications to limit the
situations within which these methods are accepted.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
Realm Names
All implementations must understand hierarchical realms in both the Internet
Domain and the X.500 style. When a ticket granting ticket for an unknown
realm is requested, the KDC must be able to determine the names of the
intermediate realms between the KDCs realm and the requested realm.
Transited field encoding
DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported.
Alternative encodings may be supported, but they may be used only when that
encoding is supported by ALL intermediate realms.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Pre-authentication methods
The TGS-REQ method must be supported. The TGS-REQ method is not used on the
initial request. The PA-ENC-TIMESTAMP method must be supported by clients
but whether it is enabled by default may be determined on a realm by realm
basis. If not used in the initial request and the error
KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an
acceptable method, the client should retry the initial request using the
PA-ENC-TIMESTAMP preauthentication method. Servers need not support the
PA-ENC-TIMESTAMP method, but if not supported the server should ignore the
presence of PA-ENC-TIMESTAMP pre-authentication in a request.
The ETYPE-INFO2 method must be supported; this method is used to communicate
the set of supported encryption types, and corresponding salt and string to
key paramters. The ETYPE-INFO method should be supported for
interoperability with older implementation.
Mutual authentication
Mutual authentication (via the KRB_AP_REP message) must be supported.
Ticket addresses and flags
All KDC's must pass through tickets that carry no addresses (i.e. if a TGT
contains no addresses, the KDC will return derivative tickets), but
each realm may set its own policy for issuing such tickets, and each
application server will set its own policy with respect tickets).
Implementations SHOULD default to accepting them. requesting addressless tickets as this
significantly increases interoperability with network address translation.
In some cases realms or application servers MAY require that tickets have an
address.
Proxies and forwarded tickets must be supported. Individual realms and
application servers can set their own policy on when such tickets will be
accepted.
All implementations must recognize renewable and postdated tickets, but need
not actually implement them. If these options are not supported, the
starttime and endtime in the ticket shall specify a ticket's entire useful
life. When a postdated ticket is decoded by a server, all implementations
shall make the presence of the postdated flag visible to the calling server.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
User-to-user authentication
Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC option)
must be provided by implementations, but individual realms may decide as a
matter of policy to reject such requests on a per-principal or realm-wide
basis.
Authorization data
Implementations must pass all authorization data subfields from
ticket-granting tickets to any derivative tickets unless directed to
suppress a subfield as part of the definition of that registered subfield
type (it is never incorrect to pass on a subfield, and no registered
subfield types presently specify suppression at the KDC).
Implementations must make the contents of any authorization data subfields
available to the server when a ticket is used. Implementations are not
required to allow clients to specify the contents of the authorization data
fields.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
Constant ranges
All protocol constants are constrained to 32 bit (signed) values unless
further constrained by the protocol definition. This limit is provided to
allow implementations to make assumptions about the maximum values that will
be received for these constants. Implementation receiving values outside
this range may reject the request, but they must recover cleanly.
9.2.
8.2. Recommended KDC values
Following is a list of recommended values for a KDC implementation,
based on the list of suggested configuration constants (see section 4.4). configuration.
minimum lifetime 5 minutes
maximum renewable lifetime 1 week
maximum ticket lifetime 1 day
empty addresses only when suitable restrictions appear
in authorization data
proxiable, etc. Allowed.
draft-ietf-krb-wg-kerberos-clarifications-01
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10.
9. IANA considerations
Appendix with all the tables that IANA will need to start maintaining?
* cryptosystem registration
* usage number registration
11. ACKNOWLEDGEMENTS
T.B.S.
12. REFERENCES
[Blumenthal96]
Blumenthal, U., "A Better Key Schedule
Section 7 of this document specifies protocol constants and other defined
values required for DES-Like Ciphers",
Proceedings the interoperability of PRAGOCRYPT '96, 1996. [Bellare98]
Bellare, M., Desai, A., Pointcheval, D., Rogaway, P., "Relations
Among Notions multiple implementations. Until
otherwise specified in a subsequent RFC, allocations of Security additional protocol
constants and other defined values required for Public-Key Encryption Schemes".
Extended abstract published extensions to the Kerberos
protocol will be administered by the Kerberos Working Group.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 1 May 2003
10. Security Considerations
As an authentication service, Kerberos provides a means of verifying the
identity of principals on a network. Kerberos does not, by itself, provide
authorization. Applications should not accept the issuance of a service
ticket by the Kerberos server as granting authority to use the service,
since such applications may become vulnerable to the bypass of this
authorization check in Advances an environment if they inter-operate with other KDCs
or where other options for application authentication are provided.
Denial of service attacks are not solved with Kerberos. There are places in Cryptology- Crypto 98
Proceedings, Lecture Notes
the protocols where an intruder can prevent an application from
participating in Computer Science Vol. 1462, H.
Krawcyzk ed., Springer-Verlag, 1998. [DES77]
National Bureau the proper authentication steps. Because authentication is
a required step for the use of Standards, U.S. Department many services, successful denial of Commerce, "Data
Encryption Standard," Federal Information Processing Standards
Publication 46, Washington, DC (1977). [DESM80]
National Bureau service
attacks on a Kerberos server might result in the denial of Standards, U.S. Department other network
services that rely on Kerberos for authentication. Kerberos is vulnerable to
many kinds of Com- merce, "DES
Modes denial of Operation," Federal Information Processing Standards
Publication 81, Springfield, VA (December 1980). [Dolev91]
Dolev, D., Dwork, C., Naor, M., "Non-malleable cryptography",
Proceedings service attacks: denial of the 23rd Annual Symposium service attacks on Theory of Computing,
ACM, 1991. [DS81]
Dorothy E. Denning and Giovanni Maria Sacco, "Time- stamps in Key
Distribution Protocols," Communications the
network which would prevent clients from contacting the KDC; denial of
service attacks on the domain name system which could prevent a client from
finding the IP address of the ACM, Vol. 24(8), pp.
533-536 (August 1981). [DS90]
Don Davis and Ralph Swick, "Workstation Services and Kerberos
Authentication at Project Athena," Technical Memorandum TM-424, MIT
Laboratory for Computer Science (February 1990). [Horowitz96]
Horowitz, M., "Key Derivation for Authentication, Integrity, and
Privacy", draft-horowitz-key-derivation-02.txt, August 1998. [HorowitzB96]
Horowitz, M., "Key Derivation for Kerberos V5", draft-
horowitz-kerb-key-derivation-01.txt, September 1998. [IS3309]
International Organization for Standardization, "ISO Information
Processing Systems - Data Communication - High-Level Data Link
Control Procedure - Frame Struc- ture," IS 3309 (October 1984). 3rd
Edition. [KBC96]
H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed- Hashing for
Message Authentication," Working Draft
draft-ietf-ipsec-hmac-md5-01.txt, (August 1996). [KNT92]
John T. Kohl, B. Clifford Neuman, server; and Theodore Y. Ts'o, "The
Evolution denial of service attack
by overloading the Kerberos Authentication Service," KDC itself with repeated requests.
Interoperability conflicts caused by incompatible character-set usage (see
5.2.1) can result in an IEEE
Computer Society Text soon to be published (June 1992). [Krawczyk96]
Krawczyk, H., Bellare, and M., Canetti, R., "HMAC: Keyed-Hashing for
Message Authentication", draft-ietf-ipsec-hmac- md5-01.txt, August,
1996. [LGDSR87]
P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som- merfeld, and
K. Raeburn, Section E.1: Service Manage- ment System, M.I.T. Project
Athena, Cambridge, Mas- sachusetts (1987). [MD4-92]
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
R. Rivest, "The MD4 Message Digest Algorithm," RFC 1320, MIT
Laboratory for Computer Science (April 1992). [MD5-92]
R. Rivest, "The MD5 Message Digest Algorithm," RFC 1321, MIT
Laboratory denial of service for Computer Science (April 1992). [MNSS87]
S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
Section E.2.1: clients that utilize
character-sets in Kerberos strings other than those stored in the KDC
database.
Authentication servers maintain a database of principals (i.e., users and Authorization System,
M.I.T. Project Athena, Cambridge, Massachusetts (December 21, 1987).
[Neu93]
B. Clifford Neuman, "Proxy-Based Authorization
servers) and Accounting for
Distributed Systems," in Proceedings their secret keys. The security of the 13th International
Conference on Distributed Com- puting Systems, Pittsburgh, PA (May,
1993). [NS78]
Roger M. Needham and Michael D. Schroeder, "Using Encryption for
Authentication in Large Networks authentication server
machines is critical. The breach of Com- puters," Communications security of an authentication server
will compromise the ACM, Vol. 21(12), pp. 993-999 (December, 1978). [NT94]
B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti- cation
Service for Computer Networks," IEEE Communica- tions Magazine, Vol.
32(9), pp. 33-38 (September 1994). [Pat92].
J. Pato, Using Pre-Authentication to Avoid Password Guessing
Attacks, Open Software Foundation DCE Request for Comments 26
(December 1992). [SG92]
Stuart G. Stubblebine and Virgil D. Gligor, "On Message Integrity in
Cryptographic Protocols," in Proceedings security of all servers that rely upon the IEEE Symposium on
Research in Security and Privacy, Oakland, California (May 1992). [SNS88]
J. G. Steiner, B. C. Neuman, compromised
KDC, and J. I. Schiller, "Ker- beros: An
Authentication Service for Open Network Sys- tems," pp. 191-202 will compromise the authentication of any principals registered in
Usenix Conference Proceedings, Dallas, Texas (February, 1988). [X509-88]
CCITT, Recommendation X.509: The Directory Authentica- tion
Framework, December 1988.
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
A. ASN.1 module
Kerberos5 {
iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
} DEFINITIONS ::= BEGIN
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
KerberosString ::= GeneralString (IA5String)
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
KerberosTime ::= GeneralizedTime
-- with no fractional seconds
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- XXX HostAddresses is always used as
the realm of the compromised KDC.
Principals must keep their secret keys secret. If an OPTIONAL field and can intruder somehow steals
a principal's key, it will be
-- zero-length.
HostAddresses ::= SEQUENCE OF HostAddress -- XXX subtly different from 1510
-- but encodes able to masquerade as that principal or
impersonate any server to the same
-- XXX AuthorizationData legitimate principal.
Password guessing attacks are not solved by Kerberos. If a user chooses a
poor password, it is always used as possible for an OPTIONAL field and attacker to successfully mount an
off-line dictionary attack by repeatedly attempting to decrypt, with
successive entries from a dictionary, messages obtained which are encrypted
under a key derived from the user's password.
Unless pre-authentication options are required by the policy of a realm, the
KDC will not know whether a request for authentication succeeds. An attacker
can
-- be zero-length.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
PA-DATA ::= SEQUENCE {
padata-type [1] Int32 -- first tag is [1], request a reply with credentials for any principal. These credentials
will likely not [0] --,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
KerberosFlags ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
-- shall be sent, much use to the attacker unless it knows the client's
secret key, but no fewer than 32
EncryptedData1510 ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- the availability of the response encrypted in the client's
secret key provides the attacker with ciphertext
}
draft-ietf-krb-wg-kerberos-clarifications-01 that may be used to mount
brute force or dictionary attacks to decrypt the credentials, by guessing
the user's password. For this reason it is strongly encouraged that Kerberos
realms require the use of pre-authentication. Even with preauthentication,
attackers may try brute force or dictionary attacks against credentials that
are observed by eavesdropping on the network.
draft-ietf-krb-wg-kerberos-clarifications-02 Expires 9 March 1 May 2003
EncryptedData {Type, UInt32:KeyUsages} ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING (CONSTRAINED BY {
--
Each host on the network must be encrypted DER encoding have a clock which is loosely synchronized to
the time of -- Type,
-- with key usage being one the other hosts; this synchronization is used to reduce the
bookkeeping needs of -- UInt32:KeyUsages
})
}
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
Checksum {UInt32:KeyUsages} ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING (CONSTRAINED BY {
-- with key usage being one application servers when they do replay detection. The
degree of -- UInt32:KeyUsages
})
}
-- key usage numbers
keyuse-pa-enc-ts Int32 ::= 1
keyuse-Ticket Int32 ::= 2
keyuse-EncASRepPart Int32 ::= 3
keyuse-TGSReqAuthData-sesskey Int32 ::= 4
keyuse-TGSReqAuthData-subkey Int32 ::= 5
keyuse-pa-TGSReq-cksum Int32 ::= 6
keyuse-pa-TGSReq-authenticator Int32 ::= 7
keyuse-EncTGSRepPart-sesskey Int32 ::= 8
keyuse-EncTGSRepPart-subkey Int32 ::= 9
keyuse-APReq-cksum Int32 ::= 10
keyuse-APReq-authenticator Int32 ::= 11
keyuse-EncAPRepPart Int32 ::= 12
keyuse-EncKrbPrivPart Int32 ::= 13
keyuse-EncKrbCredPart Int32 ::= 14
keyuse-KrbSafe-cksum Int32 ::= 15
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData {EncTicketPart, {keyuse-Ticket}}
}
-- Encrypted part "looseness" can be configured on a per-server basis, but is
typically on the order of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
draft-ietf-krb-wg-kerberos-clarifications-01 Expires 9 March 2003
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- 5 minutes. If the clocks are synchronized over the
network, the clock synchronization protocol must itself be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- secured from
network attackers.
Principal identifiers must not recycled on a short-term basis. A typical
mode of access control will use access control lists (ACLs) to grant
permissions to particular principals. If a stale ACL entry remains for a
deleted principal and the principal identifier is reused, the following are new since 1510; maybe remove principal
will inherit rights specified in the stale ACL entry. By not re-using
principal identifiers, the danger of inadvertent access is removed.
Proper decryption of an KRB_AS_REP message from krb-clarifications?
-- transited-policy-checked(12),
-- ok-as-delegate(13),
-- anonymous(14),
-- cksummed-ticket(15)
AS-REQ ::= KDC-REQ {10}
TGS-REQ ::= KDC-REQ {12}
KDC-REQ {INTEGER:tagnum} ::= [APPLICATION tagnum] SEQUENCE {
pvno [1] INTEGER (5) -- first tag the KDC is not sufficient
for the host to verify the identity of the user; the user and an attacker
could cooperate to generate a KRB_AS_REP format message which decrypts
prope