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Public Key Cryptography for Initial Authentication in Kerberos
0. Status Of this This Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
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To learn the current status of any Internet-Draft, please check
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The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-pk-init-03.txt,
draft-ietf-cat-kerberos-pk-init-04.txt, and expires September 30,
1997. January 31,
1998. Please send comments to the authors.
1. Abstract
This document defines extensions (PKINIT) to the Kerberos protocol
specification (RFC 1510 [1]) to provide a method for using public
key cryptography during initial authentication. The methods
defined specify the ways in which preauthentication data fields and
error data fields in Kerberos messages are to be used to transport
public key data.
2. Introduction
The popularity of public key cryptography has produced a desire for
its support in Kerberos [2]. The advantages provided by public key
cryptography include simplified key management (from the Kerberos
perspective) and the ability to leverage existing and developing
public key certification infrastructures.
Public key cryptography can be integrated into Kerberos in a number
of ways. One is to to associate a key pair with each realm, which can
then be used to facilitate cross-realm authentication; this is the
topic of another draft proposal. Another way is to allow users with
public key certificates to use them in initial authentication. This
is the concern of the current document.
One of the guiding principles in the design of PKINIT is that
changes should be as minimal as possible. As a result, the basic
mechanism of PKINIT is as follows: The user sends a request to the
KDC as before, except that if that user is to use public key
cryptography in the initial authentication step, his certificate
accompanies the initial request, in the preauthentication fields.
Upon receipt of this request, the KDC verifies the certificate and
issues a ticket granting ticket (TGT) as before, except that instead
of being encrypted in the user's long-term key (which is derived
from a password), it is encrypted in a randomly-generated key. This
random key is in turn encrypted using the public key from the
certificate that came with the request and signed using the KDC's signature
private key, and accompanies the reply, in the preauthentication
fields.
PKINIT also allows for users with only digital signature keys to
authenticate using those keys, and for users to store and retrieve
private keys on the KDC.
The PKINIT specification may also be used for direct peer to peer
authentication without contacting a central KDC. This application
of PKINIT is described in PKTAPP [4] and is based on concepts
introduced in [5, 6]. For direct client-to-server authentication,
the client uses PKINIT to authenticate to the end server (instead
of a central KDC), which then issues a ticket for itself. This
approach has an advantage over SSL [7] in that the server does not
need to save state (cache session keys). Furthermore, an
additional benefit is that Kerberos tickets can facilitate
delegation (see [8]).
3. Proposed Extensions
This section describes extensions to RFC 1510 for supporting the
use of public key cryptography in the initial request for a ticket
granting ticket (TGT).
In summary, the following changes to RFC 1510 are proposed:
--> Users may authenticate using either a public key pair or a
conventional (symmetric) key. If public key cryptography is
used, public key data is transported in preauthentication
data fields to help establish identity.
--> Users may store private keys on the KDC for retrieval during
Kerberos initial authentication.
This proposal addresses two ways that users may use public key
cryptography for initial authentication. Users may present public
key certificates, or they may generate their own session key,
signed by their digital signature key. In either case, the end
result is that the user obtains an ordinary TGT that may be used for
subsequent authentication, with such authentication using only
conventional cryptography.
Section 3.1 provides definitions to help specify message formats.
Section 3.2 and 3.3 describe the extensions for the two initial
authentication methods. Section 3.3 3.4 describes a way for the user to
store and retrieve his private key on the KDC. KDC, as an adjunct to the
initial authentication.
3.1. Definitions
Hash and
The extensions involve new encryption types will be specified using ENCTYPE tags; methods; we propose the
addition of the following types:
#define ENCTYPE_SIGN_DSA_GENERATE 0x0011
#define ENCTYPE_SIGN_DSA_VERIFY 0x0012
#define ENCTYPE_ENCRYPT_RSA_PRIV 0x0021
#define ENCTYPE_ENCRYPT_RSA_PUB 0x0022
allowing further signature types to be defined in the range 0x0011
through 0x001f, and further
dsa-sign 8
rsa-priv 9
rsa-pub 10
rsa-pub-md5 11
rsa-pub-sha1 12
The proposal of these encryption types to be defined in notwithstanding, we do not
mandate the
range 0x0021 through 0x002f. use of any particular public key encryption method.
The extensions involve new preauthentication fields. The
preauthentication data types are in fields; we propose the range 17 through 21.
These values are also specified along with their corresponding
ASN.1 definition.
#define
addition of the following types:
PA-PK-AS-REQ 17
#define 14
PA-PK-AS-REP 18
#define 15
PA-PK-AS-SIGN 19
#define 16
PA-PK-KEY-REQ 20
#define 17
PA-PK-KEY-REP 21 18
The extensions also involve new error types. The new error types
are types; we propose the addition
of the following types:
KDC_ERR_CLIENT_NOT_TRUSTED 62
KDC_ERR_KDC_NOT_TRUSTED 63
KDC_ERR_INVALID_SIG 64
KDC_ERR_KEY_TOO_WEAK 65
In many cases, PKINIT requires the encoding of an X.500 name as a
Realm. In these cases, the realm will be represented using a
different style, specified in RFC 1510 with the range 227 through 229. They are:
#define KDC_ERROR_CLIENT_NOT_TRUSTED 227
#define KDC_ERROR_KDC_NOT_TRUSTED 228
#define KDC_ERROR_INVALID_SIG 229 following example:
NAMETYPE:rest/of.name=without-restrictions
For a realm derived from an X.500 name, NAMETYPE will have the value
X500-ASN1-BASE64. The full realm name will appear as follows:
X500-ASN1-BASE64:Base64Encode(DistinguishedName)
where Base64 is an ASCII encoding of binary data as per RFC 1521,
and DistinguishedName is the ASN.1 encoding of the X.500
Distinguished Name from the X.509 certificate.
Similarly, PKINIT may require the encoding of an X.500 name as a
PrincipalName. In these cases, the name-type of the principal name
shall be set to NT-X500-PRINCIPAL, and the name-string shall be set
as follows:
Base64Encode(DistinguishedName)
as described above.
[Similar description needed on how realm names and principal names
are to be derived from PGP names.]
3.1.1. Encryption and Key Formats
In the exposition below, we use the following terms: encryption key,
decryption key, signature key, verification key. terms public key and private
key generically. It should be understood that the term "public
key" may be used to refer to either a public encryption and key or a
signature verification keys are essentially
public keys, and decryption key, and signature keys are essentially
private keys. The fact that they the term "private key" may be
used to refer to either a private decryption key or a signature
generation key. The fact that these are logically distinct does
not preclude the assignment of bitwise identical keys.
All additional symmetric keys specified in this draft shall use the
same encryption type as the session key in the response from the
KDC. These include the temporary keys used to encrypt the signed
random key encrypting the response, as well as the key derived from
Diffie-Hellman agreement. In the case of Diffie-Hellman, the key
shall be produced from the agreed bit string as follows:
* Truncate the bit string to the appropriate length.
* Rectify parity in each byte (if necessary) to obtain the key.
For instance, in the case of a DES key, we take the first eight
bytes of the bit stream, and then adjust the least significant bit
of each byte to ensure that each byte has odd parity.
RFC 1510, Section 6.1, defines EncryptedData as follows:
EncryptedData ::= SEQUENCE {
etype [0] INTEGER,
kvno [1] INTEGER OPTIONAL,
cipher [2] OCTET STRING
-- is CipherText
}
RFC 1510 suggests that ciphertext is coded as follows:
CipherText ::= ENCRYPTED SEQUENCE {
confounder [0] UNTAGGED OCTET STRING(conf_length)
OPTIONAL,
check [1] UNTAGGED OCTET STRING(checksum_length)
OPTIONAL,
msg-seq [2] MsgSequence,
pad [3] UNTAGGED OCTET STRING(pad_length)
OPTIONAL
}
The PKINIT protocol introduces several new types of encryption.
Data that is encrypted with a public key will allow only the check
optional field, as it is defined above. This type of the checksum
will be specified in the etype field, together with the encryption
type.
In order to identify the checksum type, etype will have the
following values:
rsa-pub-MD5
rsa-pub-SHA1
In the case that etype is set to rsa-pub, the optional 'check'
field will not be provided.
Data that is encrypted with a private key will not use any optional
fields. PKINIT uses private key encryption only for signatures,
which are encrypted checksums. Therefore, the optional check field
is not needed.
3.2. Standard Public Key Authentication
Implementation of the changes in this section is REQUIRED for
compliance with pk-init. PKINIT.
It is assumed that all public keys are signed by some certification
authority (CA). The initial authentication request is sent as per
RFC 1510, except that a preauthentication field containing data
signed by the user's signature private key accompanies the request:
PA-PK-AS-REQ ::- ::= SEQUENCE {
-- PA TYPE 17
signedPKAuth 14
signedAuthPack [0] SignedPKAuthenticator, SignedAuthPack
userCert [1] SEQUENCE OF Certificate OPTIONAL,
-- the user's certificate
-- optionally followed by that
-- certificate's certifier chain
trustedCertifiers [2] SEQUENCE OF PrincipalName OPTIONAL
-- CAs that the client trusts
}
SignedPKAuthenticator
SignedAuthPack ::= SEQUENCE {
pkAuth
authPack [0] PKAuthenticator,
pkAuthSig AuthPack,
authPackSig [1] Signature,
-- of pkAuth authPack
-- using user's signature private key
}
AuthPack ::= SEQUENCE {
pkAuthenticator [0] PKAuthenticator,
clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
-- if client is using Diffie-Hellman
}
PKAuthenticator ::= SEQUENCE {
cusec
kdcName [0] PrincipalName,
kdcRealm [1] Realm,
cusec [2] INTEGER,
-- for replay prevention
ctime [1] [3] KerberosTime,
-- for replay prevention
nonce [2] INTEGER,
-- binds response to this request
kdcName [3] PrincipalName,
clientPubValue [4] SubjectPublicKeyInfo OPTIONAL,
-- for Diffie-Hellman algorithm INTEGER
}
Signature ::= SEQUENCE {
signedHash [0] EncryptedData
-- of type Checksum
-- encrypted under signature key
}
Checksum ::= SEQUENCE {
cksumtype [0] INTEGER,
checksum [1] OCTET STRING
} -- as specified by RFC 1510
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm [0] algorithmIdentifier, AlgorithmIdentifier,
subjectPublicKey [1] BIT STRING
-- for DH, equals
-- public exponent (INTEGER encoded
-- as payload of BIT STRING)
} -- as specified by the X.509 recommendation [9]
AlgorithmIdentifier ::= SEQUENCE {
algorithm [0] ALGORITHM.&id,
-- for DH, equals
-- dhKeyAgreement
-- ({iso(1) member-body(2) US(840)
-- rsadsi(113549) pkcs(1) pkcs-3(3)
-- 1})
parameters [1] ALGORITHM.&type
-- for DH, is DHParameter
} -- as specified by the X.509 recommendation [9]
DHParameter ::= SEQUENCE {
prime [0] INTEGER,
-- p
base [1] INTEGER,
-- g
privateValueLength [2] INTEGER OPTIONAL
}
Certificate ::= SEQUENCE {
CertType
certType [0] INTEGER,
-- type of certificate
-- 1 = X.509v3 (DER encoding)
-- 2 = PGP (per PGP draft)
CertData specification)
certData [1] OCTET STRING
-- actual certificate
-- type determined by CertType certType
}
Note: If the signature uses RSA keys, then it is to be performed
as per PKCS #1.
The PKAuthenticator carries information to foil replay attacks,
to bind the request and response, and to optionally pass the
client's Diffie-Hellman public value (i.e. for using DSA in
combination with Diffie-Hellman). The PKAuthenticator is signed
with the private key corresponding to the public key in the
certificate found in userCert (or cached by the KDC).
In the PKAuthenticator, the client may specify the KDC name in one
of two ways: 1) a
* The Kerberos principal name, or 2) name krbtgt/<realm_name>@<realm_name>,
where <realm_name> is replaced by the applicable realm name.
* The name in the KDC's certificate (e.g., an X.500 name, or a
PGP name).
Note that the first case #1 requires that the certificate name and the
Kerberos principal name be bound together (e.g., via an X.509v3
extension).
The userCert field is a sequence of certificates, the first of which
must be the user's public key certificate. Any subsequent
certificates will be certificates of the certifiers of the user's
certificate. These cerificates may be used by the KDC to verify the
user's public key. This field is may be left empty if the KDC already
has the user's certifcate. certificate.
The trustedCertifiers field contains a list of certification
authorities trusted by the client, in the case that the client does
not possess the KDC's public key certificate.
Upon receipt of the AS_REQ with PA-PK-AS-REQ pre-authentication
type, the KDC attempts to verify the user's certificate chain
(userCert), if one is provided in the request. This is done by
verifying the certification path against the KDC's policy of
legitimate certifiers. This may be based on a certification
hierarchy, or it may be simply a list of recognized certifiers in a
system like PGP.
If the certification path does not match one verification of the KDC's trusted certifiers, user's certificate fails, the KDC sends back
an error message of type KDC_ERROR_CLIENT_NOT_TRUSTED, and it includes in the error data KDC_ERR_CLIENT_NOT_TRUSTED. The e-data
field a list of its own trusted certifiers, upon which the client
resends contains additional information pertaining to this error, and
is formatted as follows:
METHOD-DATA ::= SEQUENCE {
method-type [0] INTEGER,
-- 1 = cannot verify public key
-- 2 = invalid certificate
-- 3 = revoked certificate
-- 4 = invalid KDC name
method-data [1] OCTET STRING OPTIONAL
} -- syntax as for KRB_AP_ERR_METHOD (RFC 1510)
The values for the request. method-type and method-data fields are described
in Section 3.2.1.
If trustedCertifiers is provided in the PA-PK-AS-REQ, the KDC
verifies that it has a certificate issued by one of the certifiers
trusted by the client. If it does not have a suitable certificate,
the KDC returns an error message of type KDC_ERROR_KDC_NOT_TRUSTED KDC_ERR_KDC_NOT_TRUSTED to
the client.
If a trust relationship exists, the KDC then verifies the client's
signature on PKAuthenticator. If that fails, the KDC returns an
error message of type KDC_ERROR_INVALID_SIG. KDC_ERR_INVALID_SIG. Otherwise, the KDC uses
the timestamp in the PKAuthenticator to assure that the request is
not a replay. The KDC also verifies that its name is specified in
the PKAuthenticator.
Assuming no errors,
If the clientPublicValue field is filled in, indicating that the
client wishes to use Diffie-Hellman key agreement, then the KDC replies as per RFC 1510, except
checks to see that it
encrypts the reply parameters satisfy its policy. If they do
not with (e.g., the user's key, but with a random key
generated only for this particular response. This random key prime size is sealed insufficient for the expected
encryption type), then the KDC sends back an error message of type
KDC_ERR_KEY_TOO_WEAK. Otherwise, it generates its own public and
private values for the response.
The KDC also checks that the timestamp in the PKAuthenticator is
within the allowable window. If the local (server) time and the
client time in the authenticator differ by more than the allowable
clock skew, then the KDC returns an error message of type
KRB_AP_ERR_SKEW.
Assuming no errors, the KDC replies as per RFC 1510, except as
follows: The user's name in the ticket is as represented in the
certificate, unless a Kerberos principal name is bound to the name
in the certificate (e.g., via an X.509v3 extension). The user's
realm in the ticket shall be the name of the Certification
Authority that issued the user's public key certificate.
The KDC encrypts the reply not with the user's long-term key, but
with a random key generated only for this particular response. This
random key is sealed in the preauthentication field:
PA-PK-AS-REP ::= SEQUENCE {
-- PA TYPE 18
kdcCert 15
encSignedReplyKeyPack [0] SEQUENCE OF Certificate OPTIONAL, EncryptedData,
-- the KDC's certificate of type SignedReplyKeyPack
-- optionally followed by that using the temporary key
-- certificate's certifier chain
encPaReply in encTmpKey
encTmpKeyPack [1] EncryptedData,
-- of type PaReply TmpKeyPack
-- using either the client public
-- key or the Diffie-Hellman key
-- specified by SignedDHPublicValue
signedDHPublicValue
signedKDCPublicValue [2] SignedDHPublicValue SignedKDCPublicValue OPTIONAL
-- if one was passed in the request
kdcCert [3] SEQUENCE OF Certificate OPTIONAL,
-- the KDC's certificate chain
}
PaReply
SignedReplyKeyPack ::= SEQUENCE {
replyEncKeyPack
replyKeyPack [0] ReplyEncKeyPack,
replyEncKeyPackSig ReplyKeyPack,
replyKeyPackSig [1] Signature,
-- of replyEncKeyPack
-- using KDC's signature private key
}
ReplyEncKeyPack
ReplyKeyPack ::= SEQUENCE {
replyEncKey
replyKey [0] EncryptionKey,
-- used to encrypt main reply
nonce [1] INTEGER
-- binds response to the request
-- must be same as the nonce
-- passed in the PKAuthenticator
}
SignedDHPublicValue
TmpKeyPack ::= SEQUENCE {
tmpKey [0] EncryptionKey,
-- used to encrypt the
-- SignedReplyKeyPack
}
SignedKDCPublicValue ::= SEQUENCE {
dhPublicValue
kdcPublicValue [0] SubjectPublicKeyInfo,
dhPublicValueSig
-- as described above
kdcPublicValueSig [1] Signature
-- of dhPublicValue kdcPublicValue
-- using KDC's signature private key
}
The kdcCert field is a sequence of certificates, the first of which
must be the KDC's public key certificate. Any subsequent
certificates will be certificates of the certifiers of the KDC's
certificate. The last of these must have as its root certifier one of
the certifiers sent to the KDC in the PA-PK-AS-REQ. Any subsequent certificates will be
certificates of the certifiers of the KDC's certificate. These
cerificates may be used by the client to verify the KDC's public
key. This field is empty if the client did not send to the KDC a
list of trusted certifiers (the trustedCertifiers field was empty).
Since each certifier in the certification path of a user's
certificate is essentially a separate realm, the name of each
certifier shall be added to the transited field of the ticket. The
format of these realm names shall follow the naming constraints set
forth is defined in RFC 1510 (sections 7.1 and 3.3.3.1). Note that Section 3.1 of this will
require new nametypes to
document. If applicable, the transit-policy-checked flag should be defined for PGP certifiers and other
types of realms as they arise.
set in the issued ticket.
The KDC's certificate must bind the public key to a name derivable
from the name of the realm for that KDC. For an X.509 certificate,
this is done as follows. The certificate will contain a
distinguished X.500 name contains, in addition to other attributes,
an extended attribute, called principalName, with the KDC's
principal name as its value (as the text string
krbtgt/<realm_name>@<realm_name>, where <realm_name> is the realm
name of the KDC):
principalName ATTRIBUTE ::= {
WITH SYNTAX PrintableString(SIZE(1..ub-prinicipalName))
EQUALITY MATCHING RULE caseExactMatch
ORDERING MATCHING RULE caseExactOrderingMatch
SINGLE VALUE TRUE
ID id-at-principalName
}
ub-principalName INTEGER ::= 1024
id-at OBJECT IDENTIFIER ::= {joint-iso-ccitt(2) ds(5) 4}
id-at-principalName OBJECT IDENTIFIER ::= {id-at 60}
where ATTRIBUTE is as defined in X.501, and the matching rules are
as defined in X.520.
[Still need to register principalName.]
[Still need to discuss what is done for a PGP certificate.]
The client then extracts the random key used to encrypt the main
reply. This random key (in encPaReply) is encrypted with either the
client's public key or with a key derived from the DH values
exchanged between the client and the KDC.
3.2.1. Additional Information for Errors
This section describes the interpretation of the method-type and
method-data fields of the KDC_ERR_CLIENT_NOT_TRUSTED error.
If method-type=1, the client's public key certificate chain does not
contain a certificate that is signed by a certification authority
trusted by the KDC. The format of the method-data field will be an
ASN.1 encoding of a list of trusted certifiers, as defined above:
TrustedCertifiers ::= SEQUENCE OF PrincipalName
If method-type=2, the signature on one of the certificates in the
chain cannot be verified. The format of the method-data field will
be an ASN.1 encoding of the integer index of the certificate in
question:
CertificateIndex ::= INTEGER
-- 0 = 1st certificate,
-- 1 = 2nd certificate, etc
If method-type=3, one of the certificates in the chain has been
revoked. The format of the method-data field will be an ASN.1
encoding of the integer index of the certificate in question:
CertificateIndex ::= INTEGER
-- 0 = 1st certificate,
-- 1 = 2nd certificate, etc
If method-type=4, the KDC name or realm in the PKAuthenticator does
not match the principal name of the KDC. There is no method-data
field in this case.
3.3. Digital Signature
Implementation of the changes in this section are OPTIONAL for
compliance with pk-init. PKINIT.
We offer this option with the warning that it requires the client to
generate a random key; the client may not be able to guarantee the
same level of randomness as the KDC.
If the user registered registered, or presents a certificate for, a digital
signature key with the KDC instead of an encryption key, then a
separate exchange must be used. The client sends a request for a
TGT as usual, except that it (rather than the KDC) generates the
random key that will be used to encrypt the KDC response. This key
is sent to the KDC along with the request in a preauthentication field:
field, encrypted with the KDC's public key:
PA-PK-AS-SIGN ::= SEQUENCE {
-- PA TYPE 19
encSignedKeyPack 16
encSignedRandomKeyPack [0] EncryptedData EncryptedData,
-- of type SignedRandomKeyPack
-- using the key in encTmpKeyPack
encTmpKeyPack [1] EncryptedData,
-- of SignedKeyPack type TmpKeyPack
-- using the KDC's public key
userCert [2] SEQUENCE OF Certificate OPTIONAL
-- the user's certificate chain
}
SignedKeyPack
SignedRandomKeyPack ::= SEQUENCE {
signedKey
randomkeyPack [0] KeyPack,
signedKeyAuth RandomKeyPack,
randomkeyPackSig [1] PKAuthenticator,
signedKeySig [2] Signature
-- of signedKey.signedKeyAuth keyPack
-- using user's signature private key
}
KeyPack
RandomKeyPack ::= SEQUENCE {
randomKey [0] EncryptionKey,
-- will be used to encrypt reply
nonce
randomKeyAuth [1] INTEGER
}
where the PKAuthenticator
-- nonce is copied from AS-REQ
}
If the request. KDC does not accept client-generated random keys as a matter
of policy, then it sends back an error message of type
KDC_ERR_KEY_TOO_WEAK. Otherwise, it extracts the random key as
follows.
Upon receipt of the PA-PK-AS-SIGN, the KDC decrypts then verifies
the randomKey. It then replies as per RFC 1510, except that the
reply is encrypted not with a password-derived user key, but with
the randomKey sent in the request. Since the client already knows
this key, there is no need to accompany the reply with an extra
preauthentication field. The transited field of the ticket should
specify the certification path as described in Section 3.2.
3.4. Retrieving the User's Private Key From from the KDC
Implementation of the changes described in this section is RECOMMENDED are OPTIONAL
for compliance with pk-init. PKINIT.
When the user's private key is not stored local to the user, he may
choose to store the private key (normally encrypted using a
password-derived key) on the KDC. In this case, the client makes a
request as described above, except that instead of preauthenticating
with his private key, he uses a symmetric key shared with the KDC.
For simplicity's sake, this shared key is derived from the password-
derived key used to encrypt the private key, in such a way that the
KDC can authenticate the user with the shared key without being able
to extract the private key.
We provide this option to present the user with an alternative to
storing the private key on local disk at each machine where he
expects to authenticate himself using
pk-init. PKINIT. It should be noted
that it replaces the added risk of long-term storage of the private
key on possibly many workstations with the added risk of storing the
private key on the KDC in a form vulnerable to brute-force attack.
In order
Denote by K1 the symmetric key used to obtain a encrypt the private key.
Then construct symmetric key K2 as follows:
* Perform a hash on K1.
* Truncate the digest to Length(K1) bytes.
* Rectify parity in each byte (if necessary) to obtain K2.
The KDC stores K2, the public key, and the encrypted private key.
This key pair is designated as the "primary" key pair for that user.
This primary key pair is the one used to perform initial
authentication using the PA-PK-AS-REP preauthentication field. If
he desires, he may also store additional key pairs on the KDC; these
may be requested in addition to the primary. When the client includes
requests initial authentication using public key cryptography, it
must then include in its request, instead of a
preauthentication field with PA-PK-AS-REQ, the AS-REQ message:
following preauthentication sequence:
PA-PK-KEY-REQ ::= SEQUENCE {
-- PA TYPE 20
patimestamp 17
signedPKAuth [0] KerberosTime OPTIONAL,
-- used to address replay attacks.
pausec SignedPKAuth,
trustedCertifiers [1] INTEGER SEQUENCE OF PrincipalName OPTIONAL,
-- used to address replay attacks.
nonce [2] INTEGER,
-- binds CAs that the reply to this request
privkeyID [3] client trusts
keyIDList [2] SEQUENCE OF KeyID Checksum OPTIONAL
-- constructed as a payload is hash of
-- public key
-- corresponding to
-- desired
-- private key
-- if absent, KDC will return all
-- stored private keys
}
KeyID
SignedPKAuth ::= SEQUENCE {
KeyIdentifier
pkAuth [0] OCTET STRING PKAuthenticator,
pkAuthSig [1] Signature
-- of pkAuth
-- using the symmetric key K2
}
The client may request
If a specific keyIDList is present, the first identifier should indicate
the primary private key. No public key by sending certificate is required,
since the
corresponding ID. KDC stores the public key along with the private key.
If this field there is left empty, then no keyIDList, all the user's private keys are returned.
Upon receipt, the KDC verifies the signature using K2. If the
verification fails, the KDC sends back an error of type
KDC_ERR_INVALID_SIG. If the signature verifies, but the requested
keys are not found on the KDC, then the KDC sends back an error of
type KDC_ERR_PREAUTH_FAILED. If all checks out, the KDC responds as
described in the above
sections, Section 3.2, except that an additional preauthentication field,
containing in addition, the user's private key, accompanies KDC appends
the reply: following preauthentication sequence:
PA-PK-KEY-REP ::= SEQUENCE {
-- PA TYPE 21
nonce 18
encKeyRep [0] INTEGER, EncryptedData
-- binds the reply to of type EncKeyReply
-- using the request
KeyData [1] symmetric key K2
}
EncKeyReply ::= SEQUENCE {
keyPackList [0] SEQUENCE OF KeyPack,
-- the first KeyPair is
-- the primary key pair
nonce [1] INTEGER
-- binds reply to request
-- must be identical to the nonce
-- sent in the SignedAuthPack
}
KeyPair
KeyPack ::= SEQUENCE {
privKeyID
keyID [0] OCTET STRING,
-- corresponding to encPrivKey Checksum,
encPrivKey [1] OCTET STRING
}
3.4.1. Additional Protection
Upon receipt of Retrieved Private Keys
We solicit discussion on the following proposal: that reply, the client extracts the encrypted private
keys (and may
optionally include in its request additional data to encrypt store them, at the client's option). The primary
private key, which is currently only protected by must be the user's
password. One possibility first private key in the keyPack
SEQUENCE, is that used to decrypt the client might generate a random string of bits, encrypt it with the public key of in the KDC (as PA-PK-AS-REP;
this key in turn is used to decrypt the SignedKeyPack, but with an ordinary OCTET STRING main reply as described in place of
an EncryptionKey),
Section 3.2.
4. Logistics and include this with Policy
This section describes a way to define the policy on the use of
PKINIT for each principal and request.
The KDC then
XORs each returned key with this random bit string. (If the bit
string is too short, the KDC could not required to contain a database record for users
that use either return an error, or XOR the returned key Standard Public Key Authentication or Public Key
Authentication with a repetition of Digital Signature. However, if these users
are registered with the bit string.)
In order KDC, it is recommended that the database
record for these users be modified to make this work, include three additional means of preauthentication
need to be devised flags
in order to prevent attackers from simply
inserting their own bit string. One way to do this is to store
a hash of the password-derived attributes field.
The first flag, use_standard_pk_init, indicates that the user should
authenticate using standard PKINIT as described in Section 3.2. The
second flag, use_digital_signature, indicates that the user should
authenticate using digital signature PKINIT as described in Section
3.3. The third flag, store_private_key, indicates that the user
has stored his private key (the on the KDC and should retrieve it using
the exchange described in Section 3.4.
If one used to encrypt of the
private key). This hash preauthentication fields defined above is included in
the request, then used the KDC shall respond as described in turn to derive a second
key (called Sections 3.2
through 3.4, ignoring the hash-key); aforementioned database flags. If more
than one of the hash-key preauthentication fields is used to encrypt present, the KDC shall
respond with an ASN.1
structure containing error of type KDC_ERR_PREAUTH_FAILED.
In the generated bit string and a nonce value event that binds it to none of the request.
Since preauthentication fields defined above
are included in the request, the KDC possesses checks to see if any of the
above flags are set. If the hash, first flag is set, then it can generate sends back
an error of type KDC_ERR_PREAUTH_REQUIRED indicating that a
preauthentication field of type PA-PK-AS-REQ must be included in the hash-key and
verify this (weaker) preauthentication, and yet cannot reproduce
request.
Otherwise, if the private key itself, since first flag is clear, but the hash second flag is set,
then the KDC sends back an error of type KDC_ERR_PREAUTH_REQUIRED
indicating that a one-way function.
4. Logistics and Policy Issues
We solicit discussion on how clients and KDCs should preauthentication field of type PA-PK-AS-SIGN must
be configured included in order to determine which of the options described above (if any)
should be used. One possibility is to set request.
Lastly, if the user's database
record to indicate that authentication first two flags are clear, but the third flag is to use public key
cryptography; this will not work, however, in set,
then the event KDC sends back an error of type KDC_ERR_PREAUTH_REQUIRED
indicating that a preauthentication field of type PA-PK-KEY-REQ must
be included in the
client needs to know before making the initial request.
5. Compatibility with One-Time Passcodes
We solicit discussion Dependence on how the protocol changes proposed in Transport Mechanisms
Certificate chains can potentially grow quite large and span several
UDP packets; this
draft will interact with in turn increases the proposed use of one-time passcodes
discussed probability that a Kerberos
message involving PKINIT extensions will be broken in draft-ietf-cat-kerberos-passwords-00.txt.
6. Strength of Cryptographic Schemes transit. In
light of recent findings on the strength of MD5 and DES, possibility that the Kerberos specification will
allow TCP as a transport mechanism, we solicit discussion on which encryption types to incorporate
into whether
using PKINIT should encourage the protocol changes.
7. use of TCP.
6. Bibliography
[1] J. Kohl, C. Neuman. The Kerberos Network Authentication Service
(V5). Request for Comments: 1510 Comments 1510.
[2] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
for Computer Networks, IEEE Communications, 32(9):33-38. September
1994.
[3] A. Medvinsky, M. Hur. Addition of Kerberos Cipher Suites to
Transport Layer Security (TLS).
draft-ietf-tls-kerb-cipher-suites-00.txt
[4] A. Medvinsky, M. Hur, B. Clifford Neuman. Public Key Utilizing
Tickets for Application Servers (PKTAPP).
draft-ietf-cat-pktapp-00.txt
[5] M. Sirbu, J. Chuang. Distributed Authentication in Kerberos
Using Public Key Cryptography. Symposium On Network and Distributed
System Security, 1997.
[6] B. Cox, J.D. Tygar, M. Sirbu. NetBill Security and Transaction
Protocol. In Proceedings of the USENIX Workshop on Electronic
Commerce, July 1995.
[7] Alan O. Freier, Philip Karlton and Paul C. Kocher. The SSL
Protocol, Version 3.0 - IETF Draft.
[8] B.C. Neuman, Proxy-Based Authorization and Accounting for
Distributed Systems. In Proceedings of the 13th International
Conference on Distributed Computing Systems, May 1993 1993.
[9] ITU-T (formerly CCITT) Information technology - Open Systems
Interconnection - The Directory: Authentication Framework
Recommendation X.509 ISO/IEC 9594-8
8.
7. Acknowledgements
Sasha Medvinsky contributed several ideas to the protocol changes
and specifications in this document. His additions have been most
appreciated.
Some of the ideas on which this proposal is based arose during
discussions over several years between members of the SAAG, the IETF
CAT working group, and the PSRG, regarding integration of Kerberos
and SPX. Some ideas have also been drawn from the DASS system.
These changes are by no means endorsed by these groups. This is an
attempt to revive some of the goals of those groups, and this
proposal approaches those goals primarily from the Kerberos
perspective. Lastly, comments from groups working on similar ideas
in DCE have been invaluable.
9.
8. Expiration Date
This draft expires September 30, January 31, 1997.
10.
9. Authors
Clifford Neuman
Brian Tung
Clifford Neuman
USC Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey CA 90292-6695
Phone: +1 310 822 1511
E-mail: {bcn, brian}@isi.edu {brian, bcn}@isi.edu
John Wray
Digital Equipment Corporation
550 King Street, LKG2-2/Z7
Littleton, MA 01460
Phone: +1 508 486 5210
E-mail: wray@tuxedo.enet.dec.com
Ari Medvinsky
Matthew Hur
CyberSafe Corporation
1605 NW Sammamish Road Suite 310
Issaquah WA 98027-5378
Phone: +1 206 391 6000
E-mail: {ari.medvinsky, matt.hur}@cybersafe.com
Jonathan Trostle
Novell Corporation
Provo UT
E-mail: jonathan.trostle@novell.com jtrostle@novell.com
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