WDIFF

draft-ietf-sip-identity-02.txt --> draft-ietf-sip-identity-03.txt

SIP WG J. Peterson
Internet-Draft NeuStar
Expires: November 15, 2004 March 30, 2005 C. Jennings
Cisco Systems
May 17,
September 29, 2004

Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)
draft-ietf-sip-identity-02
draft-ietf-sip-identity-03

Status of this Memo

This document is an Internet-Draft

By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and is any of which I become aware will be disclosed, in full conformance accordance with
all provisions of Section 10 of RFC2026.
RFC 3668.

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Abstract

The existing security mechanisms in the Session Initiation Protocol
are inadequate for cryptographically assuring the identity of the end
users that originate SIP requests and responses, requests, especially in an interdomain
context. This document recommends practices and conventions for
identifying end users in SIP messages, and proposes a way to
distribute cryptographically secure cryptographically-secure authenticated identities.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Overview of Operations . . . . . . . . . . . . . . . . . . . . 6
6. User Agent Behavior: Sending Messages . . . Authentication Service Behavior . . . . . . . . . 7
7. Authentication Service Behavior . . . . . 7
7. Verifying Identity . . . . . . . . . . 7
7.1 UAs acting as an Authentication service . . . . . . . . . . . 9
8. Verifying Identity . . User Agent Behavior . . . . . . . . . . . . . . . . . . . . 9 10
9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . . 10
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 12 11
11. Security Considerations Compliance Tests and Examples . . . . . . . . . . . . . . . 13
11.1 Identity-Info with a Singlepart MIME body . . . . . 13 . . 14
11.2 Identity for a Request with no MIME body or Contact . . 16
12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 19
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
14. Security Considerations . . . . . . . . . . . . . . . . . . 21
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25
15.1 Header Field Names . 16 . . . . . . . . . . . . . . . . . . 25
15.2 Response Code . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17 26
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 26
B. Bit-exact archive of example messages . . . . . 17
Normative References . . . . . . 27
B.1 Encoded Reference Files . . . . . . . . . . . . . . . 16
Informative . . 27
16. References . . . . . . . . . . . . . . . . . . . . 16
B. Changelog . . . . . 25
16.1 Normative References . . . . . . . . . . . . . . . . . . . 25
16.2 Informative References . . 17
Full Copyright Statement . . . . . . . . . . . . . . . . 26
C. Changelog . . . 19

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1. Introduction

This document provides enhancements . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . 32

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1. Introduction

This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP [1]). An identity, for the purposes of this document, is
defined as a canonical SIP address-of-record URI employed to reach a
user (such as 'sip:alice@atlanta.com'). 'sip:alice@atlanta.example.com').

RFC3261 enumerates a number of places within a SIP request that a
user can express an identity for themselves, notably the user-
populated
user-populated From header field. However, the recipient of a SIP
request has no way to verify that the From header field has been
populated accurately, in the absence of some sort of cryptographic
authentication mechanism.

RFC3261 specifies a number of security mechanisms that can be
employed by SIP UAs, including Digest, TLS and S/MIME
(implementations may support other security schemes as well).
However, few SIP user agents today support the end-user certificates
necessary to authenticate themselves via TLS or S/MIME, and
furthermore Digest authentication is limited by the fact that the
originator and destination must share a pre-arranged secret. It is
desirable for SIP user agents to be able to send requests to
destinations with they have no previous association - just as in the
telephone network today, one can receive a call from someone with
whom one has no previous association, and still have a reasonable
assurance that their displayed Caller-ID is accurate.

2. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC2119 [2] and indicate requirement levels for
compliant SIP implementations.

3. Background

All RFC3261-compliant SIP user agents support a means of
authenticating themselves to a SIP registrar - registrar, commonly with a shared
secret (Digest
secret; Digest authentication, which MUST be supported by SIP user
agents, is typically used for this purpose). purpose. Registration allows a
user agent to express that it is the proper entity to which requests
should be sent for a particular address-of-record SIP URI.

Coincidentally, the URI (e.g.,
'sip:alice@atlanta.example.com').

The address-of-record URI of a SIP user used for registration is also the URI with
which a user UA commonly populates the From header of requests
- in other words, the address-of-record is an identity. So in this order to

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context, users already have

provide their 'return address' identity to recipients. If you can
prove you are eligible to register in a means domain under a particular
address-of-record, you are proving that you are capable of providing their identity,
which makes good sense: since
legitimately receiving requests for that address-of-record, and
accordingly, when you place that address-of-record in the contents of a From header
field of a SIP request other than a registration (like an INVITE),
you are
essentially providing a 'return address' for SIP requests, being able to prove
that where you can legitimately be
reached. In other words, if you are eligible authorized to receive requests
for that 'return address'
should be identical to proving that address', you are also authorized to assert this
identity. that
'return address' in your From header field.

In the context of registration, users already have a means of proving
their identity to a registrar. However, the credentials with which a
user agent proves their identity to a registrar cannot be validated
by a just any user agent or proxy server outside your local domain - these credentials are currently only useful for registration.
shared between the user agent and their domain administrator. For
the purposes of determining whether or not the 'return address' of a
request can legitimately be asserted as in the identity From header field of the user, a
request, SIP entities in other domains that are not operated by the domain
administrator require an assurance that the sender of a message is
capable of authenticating themselves to a registrar in their own
domain.

Ideally, then, SIP user agents should have some way of proving to
recipients of SIP messages requests that their local domain has authenticated
them. In the absence of end-user certificates in user agents, it is
possible to implement a mediated authentication architecture for SIP
in which requests are sent to a server in the user's local domain
which authenticates such requests (using the same practices by which
the domain would authenticate REGISTER requests). Once a message has
been authenticated, the local domain then needs some way to
communicate to other SIP entities that the sending user has been
authenticated. This draft addresses how that imprimatur of
authentication can be shared.

RFC3261 already describes an architecture very similar to this in
Section 26.3.2.2, in which a user agent authenticates itself to a
local proxy server which in turn authenticates itself to a remote
proxy server via mutual TLS, creating a two-link chain of transitive
authentication between the originator and the remote domain. While
this works well in some architectures, there are a few respects in
which this is impractical. For one, transitive trust in is inherently
weaker than an assertion that can be validated end-to-end. It is
possible for SIP requests to cross multiple intermediaries in
separate administrative domains, in which case transitive trust
becomes even less compelling. It also requires intermediaries to act
as proxies, rather than redirecting requests to their destinations
(redirection lightens loads on SIP intermediaries).

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One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP
header. A mechanism for doing so (with the P-Asserted-Identity
header) is given in [6]. [8]. However, this solution allows only hop-by-
hop
hop-by-hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict

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mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment.

Accordingly, this document specifies a new tactic is required for means of sharing a
cryptographic assurance of end-user SIP identity in an intradomain context.
Furthermore, this interdomain
context based on the concept of an 'authentication service' and a new mechanism must work for both
SIP requests and
responses. However, there header, the Identity header. Note that the scope of this
document is an additional wrinkle specific limited to providing this identity in a response. While the original address-of-
record to which a request assurance for SIP
requests; solving this problem for SIP responses is sent more complicated,
and is stored in the a subject for future work.

This specification allows either a user agent or a proxy server to
act as an authentication service. To header field of
the request, maximize end-to-end security,
it is possible, due obviously preferable for end users to retargeting at intermediaries, it
is possible that the request will hold their own
certificates; if they do, they can act as an authentication service.
However, end-user certificates may be forwarded neither practical nor
affordable, given the difficulties of establishing a PKI that extends
to an entity end users, and moreover, given the potentially large number of SIP
user agents (phones, PCs, laptops, PDAs, gaming devices) that has may be
employed by a different AoR (i.e. identity). Since single user. In such environments, synchronizing
certificates across multiple devices may be very complex, and
requires quite a good deal of additional endpoint behavior. Managing
several certificates for the To header various devices is not changed also quite
problematic and unpopular with users. Accordingly, in responses to a SIP request, the UAC has no way initial
use of discovering that
new AoR. This this mechanism, it is generally known as likely that intermediaries will
instantiate the "response identity" or
"connected party" problem. authentication service role.

4. Requirements

This draft addresses the following requirements:
o The mechanism must allow a UAC to provide a strong cryptographic
identity assurance to the UAS in a request.

The mechanism must allow a UAS to provide a strong cryptographic
identity assurance to the UAC in request that can be verified by a response. proxy
server or UAS.
o User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup.

Proxy servers
o User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to
requests or responses. requests.
o Proxy servers that hold certificates on behalf of their domain
must be capable of adding this identity assurance to requests; a
UAC is not required to support the Identity header in order for
identity to be added to a request in this fashion.

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o The mechanism must prevent replay of the identity assurance by an
attacker.
o The mechanism must be capable of protecting the integrity of SIP
message bodies (to ensure that media offers and answers are linked
to the signaling identity).
o It must be possible for a user to have multiple AoRs (i.e.
accounts or aliases) under which it is known at a domain, and for
the UAC to assert one identity while authenticating itself as
another, related, identity, as permitted by the local policy of
the domain.
o It must be possible, in cases where a request has been retargeted
to a different AoR than the one designated in the To header field,
for the UAC to ascertain the AoR to which the request has been

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sent.

5. Overview of Operations

This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document.

Imagine the case where Alice, who has the home proxy of example.com
and the address-of-record sip:alice@example.com, wants to communicate
with sip:bob@example.org.

Alice generates an INVITE and places her identity in the From header
field of the request. She then sends an INVITE over TLS to an
authentication service proxy for her domain.

The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized
to populate the value of the From header field (which may be Alice's
AoR, or it may be some other value that the policy of the proxy
server permits her to use). It then computes a hash over some
particular headers, including the From header field and the bodies in
the message. This hash is signed with the certificate for the domain
(example.com, in Alice's case) and inserted in a new header field (the
new Identity header) in
the SIP message. message, the 'Identity' header.

The proxy, as the holder the private key of its domain, is asserting
that the originator of this request has been authenticated and that
she is authorized to claim the identity (the SIP address-of-record)
which appears in the From header field. The proxy also inserts a
companion header field that tells Bob how to acquire its certificate,
if he doesn't already have it.

When Bob returns a response to the INVITE (such as a 200 OK), a
similar set of steps happen. Bob's home proxy asserts his identity
in the response. In this instance, the proxy has to insert the
header directly into the request - redirection of responses is not
possible. When Alice domain receives the response, she verifies Bob's
identity.

If Alice's request for Bob were retargeted, one of two things might
happened. If request, it were retargeted to a domain that was also verifies the
responsibility of Bob's home proxy (for example, retargeted from
sip:bob@example.com to sip:carol@example.com), then signature
provided in the request would
proceed normally Identity header, and receive an Identity. If Bob's home proxy would
retarget thus can authenticate that the request to some other
domain (e.g.
sip:bob@example.ORG), then his home proxy would redirect indicated by the request
rather than proxying it, and Alice would send a new request that
could receive a response with an Identity from host portion of the new domain. AoR in the From header

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field authenticated the user, and permitted them to assert that From
header field value.

6. User Agent Behavior: Sending Messages Authentication Service Behavior

This mechanism requires one important change to existing user agent
behavior document defines a new role for sending requests and responses: user agents using this
mechanism to send requests or responses MUST support TLS; moreover,
they MUST be capable of establishing a persistent TLS connection with
a proxy server that acts as SIP entities called an
authentication service. Additionally,
there are several practices that should The authentication service role can be highlighted in the context
of this identity solution.

When
instantiated by a UAC sends proxy server, redirect server or a request, it MUST accurately populate the header
field user agent. Any
entity that asserts its identity (for a SIP request, this is instantiates the From
header field). In a request it authentication service role MUST set possess
the URI portion private key of its From
header to match a SIP, SIPS domain certificate, and MUST be capable of
authenticating one or TEL URI AoR under which the UAC more SIP users that can register (including anonymous URIs, as described in RFC 3323 [3]).
The UAC MUST also that
domain. Commonly, this role will be capable of sending requests through an
'outbound' instantiated by a proxy (the authentication service), server
or redirect server, since these entities are more likely to have a
static hostname, hold a corresponding certificate, and of course MUST
support the Digest authentication mechanism described in RFC3261.
Because this mechanism does not provide integrity protection for the
first hop access to the authentication service, the UAC MUST send requests SIP
registrar capabilities that allow them to authenticate users in their
domain.

SIP entities that act as an authentication service only over MUST add a TLS connection.
Additionally, in order Date
header field to provide identity for responses, user agents SIP requests if one is not already present.
Similarly, authentication services MUST form add a persistent TLS connection Content-Length header
field to a proxy server when a
REGISTER SIP requests if one is sent.

Since a UAS cannot send a response that does not replicate the
contents of the To and From header fields in the corresponding
request, UAS response-sending behavior is unchanged. Again, because already present; this mechanism does not provide integrity protection for can help
the first
hop verifier to double-check that they are hashing exactly as many
bytes of message-body as the response path, authentication service when they verify
the UAS SHOULD send responses only over a
TLS connection.

7. Authentication Service Behavior message.

The authentication service authenticates the identity of the message
sender and validates that the identity given in the message can
legitimately be asserted by the sender. Then it computes a signature
over the canonical form of several headers and all the bodies, and
inserts this signature into the message.

First, an authentication service MUST extract the identity of the
sender. For requests, it inspects
sender from the request. The authentication service takes this value
from the From header field; for
responses, the To header field (henceforth the result of this
inspection AoR will be referred to here as the "identity field).
'identity field'. If the identity field contains a SIP or SIPS URI,
the authentication service MUST extract the hostname portion of the URI in that header field,
identity field and compare this it to the domain(s) for which it is
responsible. If the identity field uses the TEL URI scheme, the
policy of the

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responsible for this identity. Some example policies are described in [TODO]. identity; see Section 12 for more information.
If the authentication service is not responsible for the identity in
question, it MAY handle the request as a normal proxy server; normally, but it MUST NOT add an
Identity header; see below for more information on authentication
service handling of an existing Identity header.

Second, the authentication service must needs to determine whether or not
the sender of the request is authorized to claim the identity given
in the identity field. In order to do so, the authentication service

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MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include:

For requests, challenging
o If the authentication service is instantiated by a SIP
intermediary (proxy or redirect server), it may challenge the
request with a 407 response code using the Digest authentication
scheme (or viewing a Proxy-
Authentication Proxy-Authentication header sent in the
request which was sent in anticipation of a challenge using cached
credentials, as described in RFC 3261 Section 22.3)

For requests and responses that are sent over 22.3).
o If the authentication service is instantiated by a persistent TLS
connection, relying SIP user agent,
a user agent can be said to authenticate its user on some prior authentication the grounds
that was
performed at the formation of user can provision the connection (most likely, user agent with the
authentication service previously challenged a REGISTER request
sent after private key of
the TLS connection was formed, domain, or possibly by preferably by providing a prior
challenged INVITE password that was sent over the TLS connection) unlocks
said private key.

Authorization of the assertion of a particular username in the From
header field of a SIP message is a matter of local policy for the
authorization service which depends greatly on the manner in which
authentication is performed. A RECOMMENDED policy is as follows: the
username asserted during Digest authentication MUST correspond
exactly to the username in the From header field of the SIP message.
However, there are many cases in which a user might manage multiple
accounts in the same administrative domain. Accordingly, provided
the authentication service is aware of the relationships between
these accounts, it might allow a user providing credentials for one
account to assert a username associated with another account
controlled by the user name. Furthermore, if the AoR asserted in the
From header field is anonymous (per RFC3323), RFC3323 [3]), then the proxy
should authenticate that the user is any a valid user in the domain and
insert the signature over the From header field as usual.

Third, the authentication service MUST form

Note that this check is performed on the identity signature,
as described addr-spec in Section 10, and the From header
field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the
display-name portion of the From header field (e.g., 'Alice
Atlanta'). Some SIP user agents that receive requests render the
display-name of the caller as the identity of the caller. However,
there are many environments in which legislating the display-name
isn't feasible, judging from experience with email, where users
frequent make slight textual changes to their display-names.
Ultimately, there is more value in focusing on the SIP address of the
sender (which has some meaning in the network and provides a chain of
accountability) than trying to constrain how the display-name is set.
As such, authentication services MAY check the display-name as well,
and compare it to a list of acceptable display-names that may be used
by the sender; if the display-name does not meet policy constraints,
the authentication service MUST return a 403 'Inappropriate
Display-Name' response code. However, in many environments this will
not make sense. For more information on rendering identity in a user

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interface, see Section 8.

Third, the authentication service MUST form the identity signature
and add an Identity header to the request containing this signature.
After the Identity header has been added to the request, the
authentication service MUST also add an Identity-
Info Identity-Info header. The
Identity-Info header contains a URI from which its certificate can be
acquired. Details are provided in section Section

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Finally, the authentication service MUST forward the message
normally.

7.1 UAs acting as an Authentication service

There are some instances in which a user agent may hold the private
key of the domain Certificate for its address-of-record. In these
cases, the UA MAY perform the services, and add the headers, that the
authentication service would normally add.

7. Verifying Identity

When a user agent or proxy server receives a SIP message containing
an Identity header, it can inspect the signature to verify the
identity of the sender of the message. If an Identity header is not
present in a request, and one is required by local policy (for
example, based on a global policy, a per-sending-domain policy, or a
per-sending-user policy), then a 428
Use Identity response is sent. If an 'Use Identity header is not present
in a response, and one is required by local policy, then the
recipient of the Header' response
MUST communicate this lapse to its user,
and MAY immediately terminate any created dialog or ignore
transactions, as policy dictates. be sent.

In order to verify the identity of the sender of a message, the user
agent or proxy server MUST first acquire the certificate for the
signing domain. Implementations supporting this specification should
have some means of retaining domain certificates (in accordance with
normal practices for certificate lifetimes and revocation) in order
to prevent themselves from needlessly downloading the same
certificate every time a request from the same domain is received.
Certificates retained in this manner should be indexed by the URI
given in the Identity-Info header field value.

Provided that the domain certificate used to sign this message is
unknown, not
previously known to the recipient, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header. header, unless they
have some more efficient implementation-specific way of acquiring
certificates for that domain. The client processes this certificate
in the usual ways ways, including checking that it has not expired, that
the chain is valid back to a trusted CA, and that it does not appear
on revocation lists. Once the certificate is acquired, it MUST be
validated.

Subsequently, the recipient MUST verify the signature in the Identity
header, and compare the identity of the signer (the subjectAltName of
the certificate) with the domain portion of the URI in the From
header field of the request as described in Section 11. 14.
Additionally, the Date, Contact and Call-ID headers MUST be analyzed
in the manner described in Section 11; 14; recipients that wish to verify
Identity signatures MUST support all of the operations described

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Identity signatures MUST support all of the operations described
there. Any discrepancies or violations MUST be reported to the user.

When the originating user agent of a request receives

If a response
containing an Authenticated Identity Body (AIB, see [4]), it SHOULD
compare verifier determines that the identity in signature on the From header field of message does not
correspond to the AIB text of the message, then a 428 'Invalid Identity
Header' response with the original value of the To header field in the
request. If these represent different identities, the user agent
SHOULD render MUST be returned.

Once the identity in the AIB of the response to its user.
Note that a discrepancy in these identity fields is not necessary an
indication sender of a security breach; normal retargeting may simply have
directed the request has been ascertained,
various policies MAY be used to a different final destination. Implementers
therefore may consider it unnecessary to alert make authorization decisions about
accepting communications and the user like. Such policies are outside the
scope of a security
violation in this case.

9. Proxy Server document.

8. User Agent Behavior

In most respects, a proxy server behaves normally when it receives a
SIP request or response containing an Identity header.

This mechanism is fully backwards-compatible with requires one important change to existing RFC3261 proxy
behavior. However, if a proxy intends user agent
requirements for sending requests: user agents using this mechanism
to send requests to act as an authentication service MUST support TLS.
Because this mechanism does not provide integrity protection for responses the
first hop to the authentication service, the UAC MUST send requests it receives, it must exhibit some
additional behavior
to ensure that retargeting requests are handled
properly. Essentially, a proxy server MUST NOT provide an Identity
header for authentication service only over a request that it retargets to TLS connection.

When a different administrative
domain. It is UAC sends a request, it MUST accurately populate the responsibility of header
field that administrative domain to
provide asserts its own identity assertion, if it can. However, proxying (for a SIP request, this is the
request to From
header field). In a remote domain where identity services may be provided
has its own problems - request it MUST set the originator URI portion of the request has no way its From
header to
know whether the request was legitimately retargeted, match a SIP, SIPS or if any
responses it receives from TEL URI AoR under which the new domain are spoofed or otherwise
illegitimate. It is thus much more secure for UAC can
register (including anonymous URIs, as described in RFC 3323 [3]).
In general, UACs SHOULD NOT use the proxy server to
redirect TEL URI form in cases where it might otherwise retarget.

If a proxy server intends to act as the From header
field (see Section 12).

The UAC MUST also be capable of sending requests, including mid-call
requests, through an 'outbound' proxy (the authentication service for a
response service).
The best way to a SIP request that it accomplish this is forwarding, it using pre-loaded Route headers and
loose routing. UAC implementations MUST do ALL provide a way of
the following:

Ascertain if it is responsible
provisioning pre-loaded Route headers in order for the domain indicated this mechanism to
work for mid-call requests in the
Request-URI field backwards direction of a dialog.

As a recipient of a request, a user agent that can verify signed
identities should also support an appropriate user interface to
render the request. If not, it MUST forward the
request normally. If so, it must then:

Determine the route set validity of targets identity to which this request might be
forwarded. a user. User agent
implementations SHOULD differentiate signed From that target list, header field values
from unsigned From header field values when rendering to an end user
the identity of the sender of a request.

9. Proxy Server Behavior

Domain policy may require proxy must determine which
contact addresses are associated with persistent TLS connections
that have been established servers to inspect and verify the
identity provided in SIP requests. A proxy server. It places all
such targets (if any) into a primary route set for server may wish to
ascertain the call, and
places remaining targets into a secondary route set for identity of the call.
It performs this operations irrespective sender of any qvalues associated the message to provide spam

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with the contact addresses.

The proxy then MUST follow normal administrative policies for
forwarding the request to any targets in the primary route set
(which may involve qvalue calculations

prevention or any other behaviors
described in RFC3261). Before the proxy forwards any responses to
this request upstream, the call control services. Even if a proxy server MUST does not
act as an authentication service (as described in Section 7), adding service, it MAY verify the existence of an
Identity before it makes a forwarding decision for a request. Proxy
servers MUST NOT remove or modify an existing Identity or
Identity-Info header in a request.

10. Header Syntax

This document specifies two new SIP headers: Identity and
Identity-Info. Each of these headers can appear only once in a SIP
message.

Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT

Identity-Info header.

If there are no appropriate responses = "Identity-Info" HCOLON ident-info
ident-info = LAQUOT absoluteURI RAQUOT

The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the primary route set
for
contents of the proxy server to forward upstream, it moves on to signed-identity-digest, the
secondary route set (essentially, following elements of a
SIP message MUST placed in a bit-exact string in the proxy server forks
sequentially, exploring the primary route set as one cluster, and
then moves on to the secondary set). The proxy server is unable
to act as an authentication service for those contact addresses.
Accordingly, the proxy server MUST NOT explore these route targets
itself; instead, it MUST redirect the request with order specified
here, separated by a 3xx class
response containing the contact addresses that constitute colon:
o The AoR of the
secondary route set.

In order to build UA sending the primary route set for message, or the call, 'identity field'.
For a request, this is the location
service associated with addr-spec from the domain From header field.
o The addr-spec component of the proxy server MUST implement
additional intelligence to determine To header field, which contact addresses are
associated with a persistent TLS connection - this is used to
determine when the server should act as a proxy and when it should
redirect. When AoR
to which the SIP registrar receives a REGISTER request over a
persistent TLS connection, it MUST compare any contact addresses
appearing in Contact is being sent.
o The callid from Call-Id header fields to field.
o The digit (1*DIGIT) and method (method) portions from CSeq header
field, separated by a single space (ABNF SP, or %x20). Note that
the topmost Via CSeq header field in
the REGISTER request. If the host portion of a contact address
matches allows LWS rather than SP to separate the hostname given in
digit and method portions, and thus the topmost Via CSeq header field, then that
contact address is said field may need
to be transformed in order to be "associated" canonicalized.
o The Date header field, with exactly one space each for each SP and
the persistent TLS
connection over which the REGISTER was received. Location services
must mark or flag these contact addresses accordingly, weekday and month items case set as shown in BNF in 3261. The
first letter is upper case and remember the identity rest of the letters are lower
case. All requests that use the user provided when they were authenticated
during registration. Only these contact addresses are added to the
primary route set by Identity mechanism MUST contain a proxy server that wishes to act as an
authentication service for responses.

Additionally, domain policy may require proxy servers to inspect and
verify the identity provided in SIP requests.
Date header.
o The proxy server may
wish to ascertain the identity addr-spec component of the sender of Contact header field value. If the message to
provide spam prevention or call control services. Even if a proxy
server
request does not act as an authentication service, it MAY verify the
signature present in an Identity header before it makes a forwarding
decision for contain a request. Proxy servers Contact header, this field MUST NOT remove or modify be
empty (i.e., there will be no whitespace between the
Identity or Identity-Info headers.

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10. Header Syntax

This document specifies two new SIP headers: Identity fourth and Identity-
Info. Each of these headers can appear only once
fifth colons in a SIP message.

Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT

Identity-Info = "Identity-Info" HCOLON ident-info
Ident-info = LAQUOT absoluteURI RAQUOT

To create the contents canonical string).
o The body content of the signed-identity-digest, the following
elements of a SIP message MUST placed in with the string bits exactly as they are
in the order
specified here, separated by a colon:

The AoR of Message (in the UA sending ABNF for SIP, the message, or message-body). Note that
the 'identity field'.
For a request, this is message-body does NOT include the addr-spec CRLF separating the SIP
headers from the From header field;
for responses, message-body, but does include everything that
follows that CRLF. If the addr-spec of message has no body, then message-body
will be empty, and the To header field. This needs
to be in lower case and to final colon will not be represented as a followed by any

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The callid from Call-Id header field.

The Date header field, with exactly one space each for each SP and
the weekday and month items case set as shown in BNF in 3261. The
first letter is upper case and the rest of the letters are lower
case.

The addr-spec component of the Contact header field value.

The body content of the message with the bits exactly as they are
in the message. Identity September 2004

additional characters.

For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see [4]. Section 14 and [5].
The precise formulation of this digest-string is, therefore: therefore
(following the ABNF [6] in RFC3261):

digest-string = addr-spec HCOLON ":" addr-spec ":" callid HCOLON ":" 1*DIGIT SP method ":"
SIP-Date
HCOLON ":" [ addr-spec HCOLON ] ":" message-body

Note again that the first addr-spec MUST be taken from the From
header field value, and the second addr-spec MUST be taken from the To
header field value, and the third addr-spec MUST be taken from the
Contact header field value. value, provided the Contact header is present in
the request.

After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain, as follows: compute the results of
signing this string with sha1WithRSAEncryption as described in RFC
3370 and base64 encode the results as specified in RFC 3548. Put the
result in the Identity header.

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Note on this choice: Assuming a 1024 bit RSA key, the raw signature
will result in about 170 octets of base64 encoded data. data (without
base64, as an aside, it would be about 130 bytes). For comparison's
sake, a typical HTTP Digest Authorization header (such as those used
in RFC3261) with no cnonce is around 180 octets. From a speed point
of view, a 2.8GHz Intel processor does somewhere in the range of 250
RSA 1024 bits signs per second or 1200 RSA 512 bits signs; verifies
are roughly 10 times faster. Hardware accelerator cards are
available that speed this up.

The Identity-Info header MUST contain either an HTTPS URI or a SIPS
URI. If it contains an HTTPS URI, the URI must dereference to a
resource that contains a single MIME body containing the certificate
of the authentication service. If it is a SIPS URI, then the
authentication service intends for a user agent that wishes to fetch
the certificate to form a TLS connection to that URI, acquire the
certificate during normal TLS negotiation, and close the connection.

This document adds the following entries to Table 2 of [1]:

Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity R a - o o - o o -

SUB NOT REF INF UPD PRA
--- --- --- --- --- ---

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o o o - - - o o o

Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- ---
Identity-Info R a - o o - o o -

SUB NOT REF INF UPD PRA
--- --- --- --- --- ---
o o o - - -

11. Security Considerations

This document describes a o o o

Note, in the table above, that this mechanism which provides a signature over does not protect the Contact, Date, Call-ID, and 'identity fields' (addr-spec of
REGISTER method or the
From header field for requests, CANCEL method. The CANCEL method cannot be
challenged, because it is hop-by-hop, and To header field accordingly authentication
service behavior for responses) of
SIP messages. While a signature over the identity field alone CANCEL would be sufficient significantly limited. The
REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to secure a URI alone, this mechanism. Accordingly, the additional headers provide
replay protection
Identity and reference integrity necessary to make sure that Identity-Info header MUST NOT appear in REGISTER or
CANCEL.

11. Compliance Tests and Examples

The examples in this section illustrate the use of the Identity
header will not be used in cut-and-paste attacks. In
general, the considerations related to the security context of a SIP transaction. Implementations MUST
verify their compliance with these examples, i.e.:
o Implementations of the authentication service role MUST generate
identical base64 identity strings to the ones shown in the
Identity headers
are in these examples when presented with the same as those given source
message and utilizing the appropriate supplied private key for the
domain in RFC3261 question.
o Implementations of the verifier role MUST correctly validate the
given messages containing the Identity header when utilizing the
supplied certificates (with the caveat about self-signed
certificates below).

Note that the following examples use self-signed certificates, rather
than certificates issued by a recognized certificate authority. The
use of self-signed certificates for including headers this mechanism is NOT
RECOMMENDED, and appear here only for illustrative purposes.
Therefore, in
tunneled 'message/sip' MIME bodies (see Section 23 compliance testing, implementations of verifiers SHOULD
generated appropriate warnings about the use of self-signed
certificates.

Bit-exact reference files for these messages and their various
transformations are supplied in particular). Appendix B.

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The identity field indicates the identity of the sender of

11.1 Identity-Info with a Singlepart MIME body

Consider the
message. The Date and Contact headers provide reference integrity following private key and replay protection, as described in RFC3261 Section 23.4.2.
Implementations certificate pair assigned to
'atlanta.example.com'.

A user of this specification MUST NOT consider valid a
request with atlanta.example.com, Alice, wants to send an outdated Date header field (the RECOMMENDED interval
is that INVITE to
bob@biloxi.example.org. She therefore creates the Date header must indicate a time within 3600 seconds of following INVITE
request, which she forwards to the receipt of a message). Implementations MUST also record Call-IDs
received in valid requests containing an atlanta.example.org proxy server
that instantiates the authentication service role:

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remember those Call-IDs for at least September 2004

v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

When the duration of authentication service receives the INVITE, in authenticates
Alice by sending a single Date
interval (i.e. 3600 seconds). Accordingly, if 407 response. As a result, Alice adds an Identity
Authorization header is
replayed within to her request, and resends to the Date interval, receivers will recognize
atlanta.example.com authentication service. Now that it the service is invalid because
sure of a Call-ID duplication; if Alice's identity, it calculates an Identity header is
replayed after for the Date interval, receivers
request. The canonical string over which the identity signature will recognize that it
be generated is
invalid because the Date is stale. following (note that the first line wraps because
of RFC editorial conventions):

sip:alice@atlanta.example.com:sip:bob@biloxi.example.org:a84b4c76e66710:314159 INVITE:Thu, 21 Feb 2002 13:02:03 GMT:alice@pc33.atlanta.example.com:v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

The Contact header field resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is
included to tie the Identity header to a particular device instance
that generated following:

CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=

Accordingly, the request. Were atlanta.example.com authentication service will
create an active attacker to intercept a
request Identity header containing that base64 signature string
(175 bytes). It will also add an Identity header, and cut-and-paste HTTPS URL where its certificate is
made available. With those two headers added, the message looks

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header field into their own request (reusing September 2004

like:

INVITE sip:bob@biloxi.exmple.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com>
Identity: CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=
Identity-Info: https://atlanta.example.com/cert
Content-Type: application/sdp
Content-Length: 147

v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

atlanta.example.com then forwards the identity fields,
Contact, Date and Call-ID fields that appear in request normally. When Bob
receives the original
message), they would request, if he does not be eligible to receive SIP requests from already know the
called user agent, since those requests are routed to certificate of
atlanta.example.com, he de-references the URI
identified in URL the Contact Identity-Info
header field.

This mechanism also provides a signature over to acquire the bodies certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP
requests. The most important reason for doing so is to protect SDP
bodies carried in SIP requests. There is little purpose
request. Using this canonical string, the signed digest in
establishing the identity of
Identity header, and the user agent that provided certificate discovered by de-referencing the
signaling if a man-in-the-middle
Identity-Info header, Bob can change verify that the SDP given set of headers
and direct media
to an alternate address. Note however that this is not perfect end-
to-end security. The authentication service itself, when
instantiated at a intermediary, can change the SDP (and SIP headers, message body have not been modified.

11.2 Identity for that matter) before providing a signature. Thus, while this
mechanism reduces the chance that a man-in-the-middle will interfere Request with sessions, it does not eliminate it entirely. Since it is
assumed that no MIME body or Contact

Consider the user trusts their local domain following private key and certificate pair assigned to vouch for their
security, they must also trust the service not
"biloxi.example.org".

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Bob (bob@biloxi.example.org) now wants to violate send a BYE request to Alice
at the
integrity end of their message bodies without good reason.

Users SHOULD NOT provide credentials the dialog initiated in the previous example. He
therefore creates the following BYE request which he forwards to an the
'biloxi.example.org' proxy server that instantiates the
authentication service to
which they cannot initiate a direct connection, preferably one
secured by TLS. If a user does not receive a certificate from role:

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When the authentication service over this TLS that corresponds to receives the expected
domain (especially when they receive a challenge via a mechanism such
as Digest), then BYE, it is possible that a rogue server is attempting to
pose as authenticates
Bob by sending a authentication service for 407 response. As a domain that it does not
control, possibly in result, Bob adds an attempt
Authorization header to collect shared secrets for that
domain. If a user cannot connect directly his request, and resends to the desired
biloxi.example.org authentication service, service. Now that the user SHOULD at least use a SIPS URI service is
sure of Bob's identity, it prepares to

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ensure header
for the request. Note that mutual TLS authentication this request does not have a Date header
field. Accordingly, the biloxi.example.org will be used add a Date header to reach
the
remote server.

Relying on an Identity request before calcuating the identity signature. If the
Content-Length header generated by a remote administrative
domain assumes that were not present, the issuing domain uses some trustworthy practice
to authenticate its users. However, authentication service
would add it as well. The baseline message is possible thus:

Also note that some domains this request contains no Contact header field.
Accordingly, biloxi.example.org will implement policies that effectively make users unaccountable
(such as accepting unauthenticated registrations from arbitrary
users). The place no value of an Identity header for such domains is
questionable.

Since a domain certificate is used by an authentication service
(rather than individual certificates for each identity), certain
problems can arise with name subordination. For example, if an
authentication service holds a common certificate in the canonical
string for the hostname
'sip.atlanta.com', can it legitimately sign a token containing an
identity addr-spec of 'sip:alice@atlanta.com'? It the Contact address. Also note that
there is difficult for no message body, and accordingly, the
recipient of a request to ascertain whether or not 'sip.atlanta.com' signature string will
terminate, in this case, with two colons. The canonical string over
which the identity signature will be generated is authoritative for the 'atlanta.com' domain unless following (note
that the recipient
has some foreknowledge first line wraps because of RFC editorial conventions):

sip:bob@biloxi.example.org:sip:alice@atlanta.example.com:a84b4c76e66710:231 BYE:Thu, 21 Feb 2002 14:19:51 GMT::

The resulting signature (sha1WithRsaEncryption) using the administration of 'atlanta.com'.
Therefore, it private RSA
key given above for biloxi.example.org, with base64 encoding, is RECOMMENDED that user agent recipients of the
following:

A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=

Accordingly, the biloxi.example.org authentication tokens notify end users if there service will
create an Identity header containing that base64 signature string.
It will also add an HTTPS URL where its certificate is ANY discrepancy
between made
available. With those two headers added, the subjectAltName of message looks like:

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BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Identity: A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=
Identity-Info: https://biloxi.example.org/cert
Content-Length: 0

biloxi.example.org then forwards the signers certificate request normally.

12. Identity and the
identity within the authentication token. Minor discrepancies MAY be
characterized TEL URI Scheme

Since many SIP applications provide a VoIP service, telephone numbers
are commonly used as such. Additionally, relying parties MAY follow the
procedures identities in RFC3264 to look up on SIP deployments. In the domain portion majority
of cases, this is not problematic for the identity mechanism
described in the From header field this document. Telephone numbers commonly appear in the DNS, and compare the
username portion of a SIP
services listed for that domain with URI (e.g.,
'sip:+17005551008@chicago.example.com'). That username conforms to
the subjectAltName syntax of the
certificate; TEL URI scheme (RFC2806bis [9]). For this can give sort of
SIP address-of-record, chicago.example.com is the relying party appropriate
signatory.

It is also possible for a better sense of TEL URI to appear in the
canonical SIP services To or From
header field outside the context of a SIP or SIPS URI (e.g.,
'tel:+17005551008'). In this case, it is much less clear which
signatory is appropriate for that domain.

Because the domain certificates that can be used by authentication
services need to assert only identity. Fortunately for the hostname
identity mechanism, this form of the authentication
service, existing certificate authorities can provide adequate
certificates TEL URI is more common for this mechanism. However, not all proxy servers and
user agents will be able support the root certificates of all
certificate authorities,
To header field and moreover there are some significant
differences Request-URI in SIP than in the policies by which certificate authorities issue
their certificates. This document makes From header field,
since the UAC has no recommendations for option but to provide a TEL URI alone when the
usage of particular certificate authorities, nor does
remote domain to which a request is sent is unknown. The local
domain, however, is usually known by the UAC, and accordingly it describe any
particular policies can
form a proper From header field containing a SIP URI with a username
in TEL URI form. Implementations that certificate authorities should follow, but
it intend to send their requests
through an authentication service MUST put telephone numbers in the
From header field into SIP or SIPS URIs, if possible.

If the local domain is anticipated that operational experience unknown to a UAC formulating a request, it
most likely will create de facto
standards not be able to locate an authentication service for
its request, and therefore the purposes question of providing identity in
these cases is somewhat moot. However, an authentication services. Some
federations of service providers, for example, might only trust
certificates that have been provided by MAY
sign a certificate authority
operated by request containing a TEL URI in the federation. From header field in
accordance with its local policies. Verifiers SHOULD NOT accept

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12. IANA Considerations

This document specifies two new SIP headers: Identity and Identity-
Info. Their syntax is given

signatures over From header TEL URIs in Section 10. This document requests the absence of some
pre-provisioned relationship with the signing domain that IANA add these headers authorizes
this usage of TEL URIs.

The guidance in the paragraph above is largely provided for forward
compatibility. In the longer-term, it is possible that ENUM [10] may
provide a way to determine which administrative domain is responsible
for a telephone number, and this may aid in the signing and
verification of SIP header registry. identities that contain telephone numbers. This document also defines
is a new subject for future work.

13. Privacy Considerations

The identity mechanism presented in this draft is compatible with the
standard SIP response code, 428 "Use
Identity", as practices for privacy described in Section 8.

Normative References

[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. RFC3323 [3]. A SIP
proxy server can act both as a privacy service and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[2] Bradner, S., "Key words for use in RFCs as an
authentication service. Since a user agent can provide any From
header field value which the authentication service is willing to indicate requirement
levels", RFC 2119, March 1997.

[3] Peterson, J., "A Privacy Mechanism for
authorize, there is no reason why private SIP URIs (e.g.,
sip:anonymous@example.com) cannot be signed by an authentication
service. The construction of the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.

[4] Identity header is the same for
private URIs as it is for any other sort of URIs.

Note, however, that an authentication service must possess a
certificate corresponding to the host portion of the addr-spec of the
From header field of any request that it signs; accordingly, using
domains like 'invalid.net' may not be possible for privacy services
that also act as authentication services. The assurance offered by
this combination service is "this is a known user in my domain that I
have authenticated, but I am keeping their identity private".

The "header" level of privacy described in RFC3323 requests that a
privacy service to alter the Contact header field value of a SIP
message. Since the Contact header field is protected by the
signature in an Identity header, privacy services cannot be applied
after authentication services without a resulting integrity
violation.

RFC3325 [8] defines the "id" priv-value token which is specific to
the P-Asserted-Identity header. The sort of assertion provided by
the P-Asserted-Identity header is very different from the Identity
header presented in this document. It contains additional
information about the sender of a message that may go beyond what
appears in the From header field; P-Asserted-Identity holds a
definitive identity for the sender which is somehow known to a closed
network of intermediaries that presumably the network will use this
identity for billing or security purposes. The danger of this
network-specific information leaking outside of the closed network

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motivated the "id" priv-value token. The "id" priv-value token has
no implications for the Identity header, and privacy services MUST
NOT remove the Identity header when a priv-value of "id" appears in a
Privacy header.

14. Security Considerations

This document describes a mechanism which provides a signature over
the Contact, Date, Call-ID, CSeq To, and From header fields of SIP
messages. While a signature over the From header field would be
sufficient to secure a URI alone, the additional headers provide
replay protection and reference integrity necessary to make sure that
the Identity header will not be used in cut-and-paste attacks. In
general, the considerations related to the security of these headers
are the same as those given in RFC3261 for including headers in
tunneled 'message/sip' MIME bodies (see Section 23 in particular).

The From header field indicates the identity of the sender of the
message, and the SIP address-of-record URI in the From header field
is the identity of a SIP user, for the purposes of this document.
The To header field provides the identity of the SIP user that this
request targets. Providing the To header field in the Identity
signature servers two purposes: first, it prevents replay attacks in
which an Identity header from legitimate request for one user is
cut-and-pasted into a request for a different user; second, it
preserves the starting URI scheme of the request, which helps prevent
downgrade attacks against the use of SIPS.

The Date and Contact headers provide reference integrity and replay
protection, as described in RFC3261 Section 23.4.2. Implementations
of this specification MUST NOT deem valid a request with an outdated
Date header field (the RECOMMENDED interval is that the Date header
must indicate a time within 3600 seconds of the receipt of a
message). Implementations MUST also record Call-IDs received in
valid requests containing an Identity header, and MUST remember those
Call-IDs for at least the duration of a single Date interval (i.e.
commonly 3600 seconds). Accordingly, if an Identity header is
replayed within the Date interval, receivers will recognize that it
is invalid because of a Call-ID duplication; if an Identity header is
replayed after the Date interval, receivers will recognize that it is
invalid because the Date is stale. The CSeq header field contains a
numbered identifier for the transaction, and the name of the method
of the request; without this information, an INVITE request could be
cut-and-pasted by an attacker and transformed into a BYE request
without changing any fields covered by the Identity header, and
moreover requests within a certain transaction could be replayed in
potentially confusing or malicious ways.

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The Contact header field is included to tie the Identity header to a
particular device instance that generated the request. Were an
active attacker to intercept a request containing an Identity header,
and cut-and-paste the Identity header field into their own request
(reusing the From, To, Contact, Date and Call-ID fields that appear
in the original message), they would not be eligible to receive SIP
requests from the called user agent, since those requests are routed
to the URI identified in the Contact header field. However, the
Contact header is only included in dialog-forming requests, so it
does not provide this protection in all cases.

It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert their own in a
cut-and-paste attack, which would cause all responses to the request
to be routed to a host of the attacker's choosing. However, a
signature over the topmost Via header does not prevent attacks of
this nature, since the attacker could leave the topmost Via intact
and merely insert a new Via header field directly after it, which
would cause responses to be routed to the attacker's host "on their
way" to the valid host, which has exactly the same end result.
Although it is possible that an intermediary-based authentication
service could guarantee that no Via hops are inserted between the
sending user agent and the authentication service, it could not
prevent an attacker from adding a Via hop after the authentication
service, and accordingly pre-empting responses. It is necessary for
the proper operation of SIP for subsequent intermediaries to be
capable of inserting such Via header fields, and thus it cannot be
prevented. As such, though it is desirable, securing Via is not
possible through the sort of identity mechanism described in this
document; the best known practice for securing Via is the use of
SIPS.

Note that this mechanism does not provide any protection for the
display-name portion of the From header field, and thus users are
free to use any display-name of their choosing, and attackers could
conceivably alter the display-names in a request with impunity. If
an administrative domain wants to control the display-names selected
by users, they could do so with policies outside the scope of this
document (for example, their authentication service could reject
requests from valid users that contain an improper display-name in
the From header field). While there are conceivably attacks that an
adversary could mount against SIP systems that rely too heavily on
the display-name in their user interface, this argues for intelligent
interface design, not changes to the protocol.

This mechanism also provides a signature over the bodies of SIP

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requests. The most important reason for doing so is to protect SDP
bodies carried in SIP requests. There is little purpose in
establishing the identity of the user agent that originated a SIP
request if a man-in-the-middle can change the SDP and direct media to
an different IP address. Note however that this is not perfect
end-to-end security. The authentication service itself, when
instantiated at a intermediary, could conceivably change the SDP (and
SIP headers, for that matter) before providing a signature. Thus,
while this mechanism reduces the chance that a man-in-the-middle will
interfere with sessions, it does not eliminate it entirely. Since it
is a foundational assumption of this mechanism that the user trusts
their local domain to vouch for their security, they must also trust
the service not to violate the integrity of their message without
good reason. Note that RFC3261 16.6 states that SIP proxy servers
"MUST NOT add to, modify, or remove the message body."

Users SHOULD NOT provide credentials to an authentication service to
which they cannot initiate a direct connection, preferably one
secured by TLS. If a user does not receive a certificate from the
authentication service over this TLS connection that corresponds to
the expected domain (especially when they receive a challenge via a
mechanism such as Digest), then it is possible that a rogue server is
attempting to pose as a authentication service for a domain that it
does not control, possibly in an attempt to collect shared secrets
for that domain. If a user cannot connect directly to the desired
authentication service, the user SHOULD at least use a SIPS URI to
ensure that mutual TLS authentication will be used to reach the
remote server.

Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the domain that issues the
assurance. Relying on an Identity header generated by a remote
administrative domain assumes that the issuing domain uses some
trustworthy practice to authenticate its users. However, it is
possible that some domains will implement policies that effectively
make users unaccountable (such as accepting unauthenticated
registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a
verifier to distinguish "good" from "bad" domains by inspecting a SIP
request, it is expected that further work in authorization practices
could be built on top of this identity solution; without such an
identity solution, many promising approaches to authorization policy
are impossible. That much said, it is RECOMMENDED that
authentication services based on proxy servers employ strong
authentication practices such as token-based identifiers.

Since a domain certificate is used by an authentication service
(rather than individual certificates for each identity), certain

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problems can arise with name subordination. For example, if an
authentication service holds a common certificate for the hostname
'sip.atlanta.example.com', can it legitimately sign a token
containing an identity of 'sip:alice@atlanta.example.com'? It is
difficult for the recipient of a request to ascertain whether or not
'sip.atlanta.example.com' is authoritative for the
'atlanta.example.com' domain unless the recipient has some
foreknowledge of the administration of 'atlanta.example.com'.
Therefore, it is RECOMMENDED that UASs receiving signed requests
notify end users if there is ANY discrepancy between the
subjectAltName of the signers certificate and the identity within the
authentication token. Minor discrepancies MAY be characterized as a
warning. Additionally, relying parties MAY follow the procedures in
RFC3263 [4] to look up in the DNS the domain portion of the identity
in the From header field, and compare the SIP services listed for
that domain with the subjectAltName of the certificate; this can give
the relying party a better sense of the canonical SIP services for
that domain.

Because the domain certificates that can be used by authentication
services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and
user agents will be able support the root certificates of all
certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but
it is anticipated that operational experience will create de facto
standards for authentication services. Some federations of service
providers, for example, might only trust certificates that have been
provided by a certificate authority operated by the federation.

Finally, the Identity and Identity-Info headers cannot protect
themselves. Any attacker could remove these headers from a SIP
request, and modify the request arbitrarily afterwards. Accordingly,
these headers are only truly efficacious if the would-be verifier
knows that they must be included in a request. In the long term,
some sort of identity mechanism along these lines must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected. In the
interim, however, identity reception policies at a domain level or an
address-book level should be used by verifiers to determine whether
or not identity is expected from a particular source of SIP requests.
Those authorization policies are outside the scope of this document.

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15. IANA Considerations

This document requests changes to the header and response-code
sub-registries of the SIP parameters IANA registry.

15.1 Header Field Names

This document specifies two new SIP headers: Identity and
Identity-Info. Their syntax is given in Section 10. These headers
are defined by the following information, which is to be added to the
header sub-registry under
http://www.iana.org/assignments/sip-parameters.
Header Name: Identity
Compact Form: y
Header Name: Identity-Info
Compact Form: (none)

15.2 Response Code

This document registers one new SIP response code which is described
in Section 7. This response codes is defined by the following
information, which is to be added to the method and response-code
sub-registry under http://www.iana.org/assignments/sip-parameters.
Response Code Number: 428
Default Reason Phrase: Use Identity Header

16. References

16.1 Normative References

[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.

[3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.

[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.

[5] Peterson, J., "SIP "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format",
draft-ietf-sip-authid-body-01 (work in progress), October 2002. RFC 3893, September 2004.

[6] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997.

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16.2 Informative References

[5]

[7] Kohl, J. and C. Neumann, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.

[6]

[8] Jennings, C., Peterson, J. and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002.

[9] Schulzrinne, H., "The TEL URI for Telephone Numbers",
draft-ietf-iptel-rfc2806bis-09 (work in progress), June 2004.

[10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS
Application", RFC 3761, April 2004.

Authors' Addresses

Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US

Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/

Cullen Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA

Phone: +1 408 902-3341
EMail: fluffy@cisco.com

Appendix A. Acknowledgments

The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
Johnston and Patrik Faltstrom for their comments. The bit-archive
presented in Appendix B follows the pioneering example of Robert
Sparks' torture-test draft.

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Appendix B. Bit-exact archive of example messages

The following text block is an encoded, gzip compressed TAR archive
of files that represent the transformations performed on the example
messages discussed in Section 11. It includes for each example:
o (foo).message: the original message
o (foo).canonical: the canonical string constructed from that
message
o (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
o (foo).signed: the RSA-signed SHA1 hash of the canonical string
(binary)
o (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
of the canonical string as it would appear in the request
o (foo).identity: the original message with the Identity
within Trusted Networks", RFC 3325, November 2002.

[7] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.

[8] Olson, S., "A Mechanism for Content Indirection in SIP
Messages", draft-ietf-sip-content-indirect-mech-01 (work and
Identity-Info headers added

Also included in
progress), August 2002.

[9] Freed, N., "Definition the archive are two public key/certificate pairs,
for atlanta.example.com and biloxi.example.org, respectively,
including:
o (foo).cert: the certificate of the URL MIME External-Body Access-
Type", RFC 2017, November 1996. domain
o (foo).privkey: the private key of the domain
o (foo).pubkey: the public key of the domain, extracted from the
cert file for convenience

To recover the compressed archive file intact, the text of this
document may be passed as input to the following Perl script (the
output should be redirected to a file or piped to "tar -xzvf -").

#!/usr/bin/perl
use strict;
my $bdata = "";
use MIME::Base64;
while(<>) {
if (/-- BEGIN MESSAGE ARCHIVE --/ .. /-- END MESSAGE ARCHIVE --/) {
if ( m/^\s*[^\s]+\s*$/) {
$bdata = $bdata . $_;
}
}
}
print decode_base64($bdata);

Alternatively, the base-64 encoded block can be edited by hand to
remove document structure lines and fed as input to any base-64
decoding utility.

B.1 Encoded Reference Files

-- BEGIN MESSAGE ARCHIVE --

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Authors' Addresses

Jon

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Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US

Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
URI: http://www.neustar.biz/

Cullen & Jennings
Cisco Systems
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA

Phone: +1 408 902-3341
EMail: fluffy@cisco.com Expires March 30, 2005 [Page 29]

Internet-Draft SIP Identity September 2004

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-- END MESSAGE ARCHIVE --

Appendix A. Acknowledgments

The authors would like C. Changelog

NOTE TO THE RFC-EDITOR: Please remove this section prior to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson and Patrik Faltstrom
publication as an RFC.

Changes from draft-ietf-sip-identity-02:
- Extracted text relating to providing identity in SIP responses;
this text will appear in a separate draft
- Added compliance testing/example section
- Added CSeq to the signature of the Identity header to prevent a
specific cut-and-paste attack; also added addr-spec of the To
header to the signature of the Identity header for similar reasons
- Added text about why neither Via headers nor display-names are
protected by this mechanism
- Added bit-exact reference files for
their comments.

Appendix B. Changelog compliance testing
- Added privacy considerations

Changes from draft-ietf-sip-identity-01:
- Completely changed underlying mechanism - instead of using an
AIB, the mechanism now recommends the use of the Identity header
and Identity-Info header
- Numerous other changes resulting from the above
- Various other editorial corrections

Changes from draft-peterson-sip-identity-01:
- Split off child draft-ietf-sip-authid-body-00 for defining of
the AIB
- Clarified scope in introduction

Peterson & Jennings Expires November 15, 2004 [Page 17]

Internet-Draft SIP Identity May 2004
- Removed a lot of text that was redundant with RFC3261
(especially about authentication practices)
- Added mention of content indirection mechanism for adding token
to requests and responses
- Improved Security Considerations (added piece about credential
strength)

Changes from draft-peterson-sip-identity-00:

Peterson & Jennings Expires March 30, 2005 [Page 30]

Internet-Draft SIP Identity September 2004

- Added a section on authenticated identities in responses
- Removed hostname convention for authentication services
- Added text about using 'message/sip' or 'message/sipfrag' in
authenticated identity bodies, also RECOMMENDED a few more headers
in sipfrags to increase reference integrity
- Various other editorial corrections

Peterson & Jennings Expires November 15, 2004 March 30, 2005 [Page 18] 31]

Internet-Draft SIP Identity May September 2004

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Peterson & Jennings Expires November 15, 2004 March 30, 2005 [Page 19] 32]

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