ietf-sip-identity-06.txt

Internet DRAFT - draft-ietf-sip-identity

draft-ietf-sip-identity

Last Version:	draft-ietf-sip-identity-06.txt	Tracker Entry
Date:	`27-Oct-2005`
Disposition:	RFC4474 (diff)
Previous Versions:	draft-ietf-sip-identity-05.txt (diff) - 07-May-2005
	draft-ietf-sip-identity-04.txt (diff) - 17-Feb-2005
	draft-ietf-sip-identity-03.txt (diff) - 30-Sep-2004
	draft-ietf-sip-identity-02.txt (diff) - 18-May-2004
	draft-ietf-sip-identity-01.txt (diff) - 16-Jan-2004
	draft-ietf-sip-identity-00.txt (diff) - 07-Mar-2003

SIP WG J. Peterson
Internet-Draft NeuStar
Expires: April 27, 2006 C. Jennings
Cisco Systems
October 24, 2005

Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)
draft-ietf-sip-identity-06

Status of this Memo

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This Internet-Draft will expire on April 27, 2006.

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, especially in an interdomain
context. This document defines a mechanism for securely identifying
originators of SIP messages. It does so by defining two new SIP
header fields, Identity, for conveying a signature used for

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validating the identity, and Identity-Info, for conveying a reference
to the certificate of the signer.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview of Operations . . . . . . . . . . . . . . . . . . . . 6
5. Authentication Service Behavior . . . . . . . . . . . . . . . 7
5.1. Identity within a Dialog and Retargeting . . . . . . . . . 9
6. Verifier Behavior . . . . . . . . . . . . . . . . . . . . . . 10
7. Considerations for User Agent . . . . . . . . . . . . . . . . 11
8. Considerations for Proxy Servers . . . . . . . . . . . . . . . 12
9. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 13
10. Compliance Tests and Examples . . . . . . . . . . . . . . . . 15
10.1. Identity-Info with a Singlepart MIME body . . . . . . . . 16
10.2. Identity for a Request with no MIME body or Contact . . . 19
11. Identity and the TEL URI Scheme . . . . . . . . . . . . . . . 22
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 23
13. Security Considerations . . . . . . . . . . . . . . . . . . . 24
13.1. Handling of digest-string Elements . . . . . . . . . . . . 24
13.2. Display Names and Identity . . . . . . . . . . . . . . . . 27
13.3. Securing the Connection to the Authentication Service . . 28
13.4. Domain Names and Subordination . . . . . . . . . . . . . . 28
13.5. Authorization and Transitional Strategies . . . . . . . . 30
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
14.1. Header Field Names . . . . . . . . . . . . . . . . . . . . 31
14.2. 428 'Use Identity Header' Response Code . . . . . . . . . 31
14.3. 436 'Bad Identity-Info' Response Code . . . . . . . . . . 31
14.4. 437 'Unsupported Certificate' Response Code . . . . . . . 32
14.5. 438 'Invalid Identity Header' Response Code . . . . . . . 32
14.6. Identity-Info Parameters . . . . . . . . . . . . . . . . . 32
14.7. Identity-Info Algorithm Parameter Values . . . . . . . . . 33
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 33
Appendix B. Bit-exact archive of example messages . . . . . . . . 33
B.1. Encoded Reference Files . . . . . . . . . . . . . . . . . 34
Appendix C. Original Requirements . . . . . . . . . . . . . . . . 36
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 37
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
15.1. Normative References . . . . . . . . . . . . . . . . . . . 39
15.2. Informative References . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 42

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1. Introduction

This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP, RFC 3261 [1]). An identity, for the purposes of this document,
is defined as a SIP URI, commonly a canonical address-of-record (AoR)
employed to reach a user (such as 'sip:alice@atlanta.example.com').

RFC3261 stipulates several places within a SIP request where a user
can express an identity for themselves, notably the 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
appropriately, 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 S/MIME, for example), 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 which 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. A
cryptographic approach, like the one described in this document, can
probably provide a much stronger and less-spoofable assurance of
identity than the telephone network provides today.

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

The usage of many SIP applications and services is governed by
authorization policies. These policies may be automated, or they may
be applied manually by humans. An example of the latter would be an
Internet telephone application which displays the "Caller-ID" of a
caller, which a human may review before answering a call. An example

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of the former would be a presence service that compares the identity
of potential subscribers to a whitelist before determining whether it
should accept or reject the subscription. In both of these cases,
attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of
a SIP request, the From header field, can be populated arbitrarily by
the controller of a user agent, impersonation is very simple today.
The mechanism described in this document aspires to provide a strong
identity system for SIP in which authorization policies cannot be
circumvented by impersonation.

All RFC3261 compliant user agents support Digest authentication,
which utilizes a shared secret, as a means for authenticating
themselves to a SIP registrar. Registration allows a user agent to
express that it is an appropriate entity to which requests should be
sent for a particular SIP AoR URI (e.g.,
'sip:alice@atlanta.example.com').

By the definition of identity used in this document, registration is
a proof of the identity of the user to a registrar. However, the
credentials with which a user agent proves its identity to a
registrar cannot be validated by just any user agent or proxy server
- these credentials are only shared between the user agent and their
domain administrator. So this shared secret does not immediately
help a user to authenticate to a wide range of recipients.
Recipients require a means of determining whether or not the 'return
address' identity of a non-REGISTER request (i.e., the From header
field value) has legitimately been asserted.

The AoR URI used for registration is also the URI with which a UA
commonly populates the From header field of requests in order to
provide a 'return address' identity to recipients. From an
authorization perspective, if you can prove you are eligible to
register in a domain under a particular AoR, you can prove you can
legitimately receive requests for that AoR, and accordingly, when you
place that AoR in the From header field of a SIP request other than a
registration (like an INVITE), you are providing a 'return address'
where you can legitimately be reached. In other words, if you are
authorized to receive requests for that 'return address', logically,
it follows that you are also authorized to assert that 'return
address' in your From header field. This is of course only one
manner in which a domain might determine how a particular user is
authorized to populate the From header field; as an aside, for other
sorts of URIs in the From (like anonymous URIs), other authorization
policies would apply.

Ideally, then, SIP user agents should have some way of proving to
recipients of SIP requests that their local domain has authenticated

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them and authorized the population of the From header field. This
document proposes 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 and their use of the From header field has been
authorized. 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 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.

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 [12]. However, this solution allows only hop-by-
hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict
mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment.

Accordingly, this document specifies a means of sharing a
cryptographic assurance of end-user SIP identity in an interdomain or
intradomain context which is based on the concept of an
'authentication service' and a new SIP header, the Identity header.
Note that the scope of this document is limited to providing this
identity assurance for SIP requests; solving this problem for SIP
responses is more complicated, and is a subject for future work.

This specification allows either a user agent or a proxy server to
provide identity services and to verify identities. To maximize end-
to-end security, it is obviously preferable for end users to acquire
their own certificates and corresponding private keys; if they do,
they can act as an authentication service. However, end-user
certificates may be neither practical nor affordable, given the
difficulties of establishing a PKI that extends to end users, and
moreover, given the potentially large number of SIP user agents

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(phones, PCs, laptops, PDAs, gaming devices) that may be employed by
a single user. In such environments, synchronizing keying material
across multiple devices may be very complex, and requires quite a
good deal of additional endpoint behavior. Managing several
certificates for the various devices is also quite problematic and
unpopular with users. Accordingly, in the initial use of this
mechanism, it is likely that intermediaries will instantiate the
authentication service role.

4. 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 assert the identity which is populated in the From header field.
This value 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 in the SIP message, the 'Identity'
header.

The proxy, as the holder of 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, Identity-Info, that tells Bob how
to acquire its certificate, if he doesn't already have it.

When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can validates that the
domain indicated by the host portion of the AoR in the From header
field authenticated the user, and permitted them to assert that From
header field value. This same validation operation may be performed
by Bob's UAS.

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5. Authentication Service Behavior

This document defines a new role for SIP entities called an
authentication service. The authentication service role can be
instantiated by a proxy server or a user agent. Any entity that
instantiates the authentication service role MUST possess the private
key of a domain certificate, and MUST be capable of authenticating
one or more SIP users that can register in that domain. Commonly,
this role will be instantiated by a proxy server, since these
entities are more likely to have a static hostname, hold a
corresponding certificate, and have access to SIP registrar
capabilities that allow them to authenticate users in their domain.
It is also possible that the authentication service role might be
instantiated by an entity that acts as a redirect server, but that is
left as a topic for future work.

SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present (see
Section 9 for information on how the Date header field assist
verifiers). Similarly, authentication services MUST add a Content-
Length header field to SIP requests if one is not already present;
this can help the verifier to double-check that they are hashing
exactly as many bytes of message-body as the authentication service
when they verify the message.

Entities instantiating the authentication service role performs the
following steps, in order, to generate an Identity header for a SIP
request:

Step 1: The authentication service MUST extract the identity of the
sender from the request. The authentication service takes this value
from the From header field; this AoR will be referred to here as the
'identity field'. If the identity field contains a SIP or SIPS URI,
the authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is
responsible (following the procedures in RFC3261 16.4 used by a proxy
server to determine the domain(s) for which it is responsible). If
the identity field uses the TEL URI scheme, the policy of the
authentication service determines whether or not it is responsible
for this identity; see Section 11 for more information. If the
authentication service is not responsible for the identity in
question, it SHOULD process and forward the request normally, but it
MUST NOT add an Identity header; see below for more information on
authentication service handling of an existing Identity header.

Step 2: The authentication service MUST 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:
If the authentication service is instantiated by a SIP
intermediary (proxy server), it may challenge the request with a
407 response code using the Digest authentication scheme (or
viewing a 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). Note that if that proxy
server is maintaining a TLS connection with the client over which
the client had previously authenticated itself using Digest
authentication, the identity value obtained from that previous
authentication step can be reused without an additional Digest
challenge.
If the authentication service is instantiated by a SIP user agent,
a user agent can be said to authenticate its user on the grounds
that the user can provision the user agent with the private key of
the domain, or preferably by providing a password that unlocks
said private key.

Authorization of the use of a particular username in the From header
field is a matter of local policy for the authentication service, one
which depends greatly on the manner in which authentication is
performed. For example, one policy might be as follows: the username
given in the 'username' parameter of the Proxy-Authorization header
MUST correspond exactly to the username in the From header field of
the SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use 'username' parameters in
Proxy-Authorization which do not correspond to the user-portion of
SIP From headers, or a user might manage multiple accounts in the
same administrative domain. In this latter case, a domain might
maintain a mapping between the values in the 'username' parameter of
Proxy-Authorization and a set of one or more SIP URIs which might
legitimately be asserted for that 'username'. For example, the
username can correspond to the 'private identity' as defined in 3GPP,
in which case the From header field can contain any one of the public
identities associated with this private identity. In this instance,
another policy might be as follows: the URI in the From header field
MUST correspond exactly to one of the mapped URIs associated with the
'username' given in the Proxy-Authorization header. Various
exceptions to such policies might arise for cases like anonymity; if
the AoR asserted in the From header field uses a form like
'sip:anonymous@example.com', then the 'example.com' proxy should
authenticate that the user is a valid user in the domain and insert
the signature over the From header field as usual.

Note that this check is performed on the addr-spec in the From header
field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the display-

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name portion of the From header field (e.g., 'Alice Atlanta').
Authentication services MAY check and validate 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 response
code with the reason phrase "Inappropriate Display-Name". However,
the display-name is not always present, and in many environments the
requisite operational procedures for display-name validation may not
exist. For more information, see Section 13.2.

Step 3: The authentication service SHOULD ensure that any pre-
existing Date header in the request is accurate. Local policy can
dictate precisely how accurate the Date must be, a RECOMMENDED
maximum discrepancy of ten minutes will ensure that the request is
unlikely to upset any verifiers. If the Date header contains a time
different by more than ten minutes from the current time noted by the
authentication service, the authentication service SHOULD reject the
request. This behavior is not mandatory because a user agent client
could only exploit the Date header in order to cause a request to
fail verification; the Identity header is not intended to provide a
source of non-repudiation or a perfect record of when messages are
processed. Finally, the authentication service MUST verify that the
Date header falls within the validity period of its certificate. For
more information on the security properties associated with the Date
header field value, see Section 9.

Step 4: 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 header. The
Identity-Info header contains a URI from which its certificate can be
acquired. Details on the generation of both of these headers are
provided in section Section 9.

Finally, the authentication service MUST forward the message
normally.

5.1. Identity within a Dialog and Retargeting

Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting supplants the
original target URI with one that corresponds to a different user, a
user that is not authorized to register under the original target
URI. By this definition, retargeting does not include translation of
the Request-URI to a contact address of an endpoint that has
registered under the original target URI, for example.

When a dialog-forming request is retargeted, this can cause a few

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wrinkles for the Identity mechanism when it is applied to requests
sent in the backwards direction within a dialog. This section
provides some non-normative considerations related to this case.

When a request is retargeted, it may reach a SIP endpoint whose user
is not identified by the URI designated in the To header field value.
The value in the To header field of a dialog-forming request is used
as the From header field of requests sent in the backwards direction
during the dialog, and is accordingly the header that would be signed
by an authentication service for requests sent in the backwards
direction. In retargeting cases, if the URI in the From header does
not identify the sender of the request in the backwards direction,
then clearly it would be inappropriate to provide an Identity
signature over that From header. As specified above, if the
authentication service is not responsible for the domain in the From
header field of the request, it MUST NOT add an Identity header to
the request, and should process/forward the request normally.

Any means of anticipating retargeting and so on is outside the scope
of this document, and likely to have equal applicability to response
identity as it does to requests in the backwards direction within a
dialog. Consequently, no special guidance is given for implementers
here regarding the 'connected party' problem; authentication service
behavior is unchanged if retargeting has occurred for a dialog-
forming request. Ultimately, the authentication service provides an
Identity header for requests in the backwards dialog when the user is
authorized to assert the identity given in the From header field, and
if they are not, an Identity header is not provided.

For further information on the problems of response identity and the
potential solution spaces, see [15].

6. Verifier Behavior

This document introduces a new logical role for SIP entities called a
'verifier', which may be instantiated by a user agent or proxy
server. When a verifier receives a SIP message containing an
Identity header, it may inspect the signature to verify the identity
of the sender of the message. Typically, the results of a
verification are provided as input to an authorization process which
is outside the scope of this document. If an Identity header is not
present in a request, and one is required by local policy (for
example, based on a per-sending-domain policy, or a per-sending-user
policy), then a 428 'Use Identity Header' response MUST be sent.

In order to verify the identity of the sender of a message, an entity
acting as a verifier MUST perform the following steps, in the order

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here specified.

Step 1: The verifier MUST 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 cached 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 not
previously known to the recipient, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header, unless they
have some more efficient implementation-specific way of acquiring
certificates for that domain. If the URI scheme in the Identity-Info
header cannot be dereferenced, then a 436 'Bad Identity-Info'
response MUST be returned. The client processes this certificate in
the usual 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 following the procedures in RFC3280 [9]. If the
certificate cannot be validated (it is self-signed and untrusted, or
signed by an untrusted or unknown certificate authority, expired, or
revoked), the verifier MUST send a 437 'Unsupported Certificate'
response.

Step 2: The verifier MUST follow the process described in
Section 13.4 to determine if the signer is authoritative for the URI
in the From header field.

Step 3: The verifier MUST verify the signature in the Identity header
field, following the procedures for generating the hashed digest-
string described in Section 9. If a verifier determines that the
signature on the message does not correspond to the reconstructed
digest-string, then a 438 'Invalid Identity Header' response MUST be
returned.

Step 4: The verifier MUST validate the Date, Contact and Call-ID
headers in the manner described in Section 13.1; recipients that wish
to verify Identity signatures MUST support all of the operations
described there. It must furthermore ensure that the value of the
Date header falls within the validity period of the certificate whose
corresponding private key was used to sign the Identity header.

7. Considerations for User Agent

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This mechanism can be applied opportunistically to existing SIP
deployments; accordingly, it requires no change to SIP user agent
behavior in order for it to be effective. However, because this
mechanism does not provide integrity protection between the UAC and
the authentication service, a UAC SHOULD implement some means of
providing this integrity. TLS would be one such mechanism, which is
attractive because it MUST be supported by SIP proxy servers, but is
potentially problematic because it is a hop-by-hop mechanism. See
Section 13.3 for more information about securing the channel between
the UAC and the authentication service.

When a UAC sends a request, it MUST accurately populate the From
header field with a value corresponding to an identity that it
believes it is authorized to claim. In a request it MUST set the URI
portion of its From header to match a SIP, SIPS or TEL URI AoR which
it is authorized to use in the domain (including anonymous URIs, as
described in RFC 3323 [3]). In general, UACs SHOULD NOT use the TEL
URI form in the From header field (see Section 11).

Note that this document defines a number of new 4xx response codes.
If user agents support these response codes, they will be able to
respond intelligently to Identity-based error conditions.

The UAC MUST also be capable of sending requests, including mid-call
requests, through an 'outbound' proxy (the authentication service).
The best way to accomplish this is using pre-loaded Route headers and
loose routing. For a given domain, if an entity that can instantiate
the authentication service role is not in the path of dialog-forming
requests, identity for mid-dialog requests in the backwards direction
cannot be provided.

As a recipient of a request, a user agent that can verify signed
identities should also support an appropriate user interface to
render the validity of identity to a user. User agent
implementations SHOULD differentiate signed From header field values
from unsigned From header field values when rendering to an end user
the identity of the sender of a request.

8. Considerations for Proxy Servers

Domain policy may require proxy servers to inspect and verify the
identity provided in SIP requests. A proxy server may wish to
ascertain the identity of the sender of the message to provide spam
prevention or call control services. Even if a proxy server does not
act as an authentication service, it MAY validate the Identity header
before it makes a forwarding decision for a request. Proxy servers
MUST NOT remove or modify an existing Identity or Identity-Info

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header in a request.

9. 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.
The grammar for these two headers is (following the ABNF [6] in s):
(following the ABNF [6] in RFC3261 [1]):

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

Identity-Info = "Identity-Info" HCOLON ident-info *( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-info-extension
ident-info-alg = "alg" EQUAL token
ident-info-extension = generic-param

The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a
SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character:
o The AoR of the UA sending the message, or addr-spec of the From
header field (referred to occasionally here as the 'identity
field').
o The addr-spec component of the To header field, which is the AoR
to which the request is being sent.
o The callid from Call-Id header 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 CSeq header field allows LWS rather than SP to separate the
digit and method portions, and thus the CSeq header field may need
to be transformed in order to be canonicalized. The
authentication service MUST strip leading zeros from the 'digit'
portion of the Cseq before generating the digest-string.
o 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. RFC
3261 specifies that the BNF for weekday and month are a choice
amongst a set of tokens. The RFC 2234 rules for the BNF specify
that tokens are case sensitive. However, when used to construct
the canonical string defined here, the first letter of each week
and month MUST be capitalized, and the remaining two letters must
be lowercase. This matches the capitalization provided in the
definition of each token. All requests that use the Identity
mechanism MUST contain a Date header.

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o The addr-spec component of the Contact header field value. If the
request does not contain a Contact header, this field MUST be
empty (i.e., there will be no whitespace between the fourth and
fifth "|" characters in the canonical string).
o The body content of the message with the bits exactly as they are
in the Message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that
the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that
follows that CRLF. If the message has no body, then message-body
will be empty, and the final "|" will not be followed by any
additional characters.

For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 13 and [5].
The precise formulation of this digest-string is, therefore
(following the ABNF [6] in RFC3261):

Note again that the first addr-spec MUST be taken from the From
header field value, 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, 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. The hashing and signing algorithm is
specified by the 'alg' parameter of the Identity-Info header (see
below for more information on Identity-Info header parameters). This
document defines only one value for the 'alg' parameter: 'rsa-sha1';
further values MUST be defined in a Standards Track RFC, see
Section 14.7 for more information. All implementations of this
specification MUST support 'rsa-sha1'. When the 'rsa-sha1' algorithm
is specified in the 'alg' parameter of Identity-Info, the hash and
signature MUST be generated as follows: compute the results of
signing this string with sha1WithRSAEncryption as described in RFC
3370 [7] and base64 encode the results as specified in RFC 3548 [8].
A 1024 bit or longer RSA key MUST be used. The result is placed in
the Identity header field. For detailed examples of the usage of
this algorithm, see Section 10.

The 'absoluteURI' portion of the Identity-Info header MUST contain
either an HTTP or HTTPS URI which dereferences to a resource that
contains a single MIME body containing the certificate of the
authentication service. These URIs MUST follow the conventions of
RFC2585 [10] and the indicated resource MUST be of the form

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'application/pkix-cert' described in that specification. Note that
this introduces key lifecycle management concerns; were a domain to
change the key available at the Identity-Info URI before a verifier
evaluates a request signed by an authentication service, this would
cause obvious verifier failures. When a rollover occurs,
authentication services SHOULD thus provide new Identity-Info URIs
for each new certificate, and SHOULD continue to make older key
acquisition URIs available for a duration longer than the plausible
lifetime of a SIP message (an hour would most likely suffice).

The Identity-Info header field MUST contain an 'alg' parameter. No
other parameters are defined for the Identity-Info header in this
document. Future Standards Track RFCs may define additional
Identity-Info header parameters.

This document adds the following entries to Table 2 of RFC 3261 [1]:

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

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

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

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

Note, in the table above, that this mechanism does not protect the
CANCEL method. The CANCEL method cannot be challenged, because it is
hop-by-hop, and accordingly authentication service behavior for
CANCEL would be significantly limited. Note as well that the
REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to this mechanism, and the use of
Identity with REGISTER is consequently a subject for future study,
although it is left as optional here for forward-compatibility
reasons. The Identity and Identity-Info header MUST NOT appear in
CANCEL.

10. Compliance Tests and Examples

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The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementers are advised
to verify their compliance with the specification against the
following criteria:
o Implementations of the authentication service role MUST generate
identical base64 identity strings to the ones shown in the
Identity headers in these examples when presented with the source
message and utilizing the appropriate supplied private key for the
domain in 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 this mechanism is NOT
RECOMMENDED, and it appears here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD
generate appropriate warnings about the use of self-signed
certificates. Also, the example certificates in this section have
placed their domain name subject in the subjectAltName field; in
practice, certificate authorities may place domain names in other
locations in the certificate (see Section 13.4 for more information).

Note that all examples in this section use the 'rsa-sha1' algorithm.

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

10.1. Identity-Info with a Singlepart MIME body

Consider the following private key and certificate pair assigned to
'atlanta.example.com' (rendered in OpenSSL format).

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A user of atlanta.example.com, Alice, wants to send an INVITE to
bob@biloxi.example.org. She therefore creates the following INVITE
request, which she forwards to the atlanta.example.org proxy server
that instantiates the authentication service role:

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INVITE sip:bob@biloxi.example.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>
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

When the authentication service receives the INVITE, it authenticates
Alice by sending a 407 response. As a result, Alice adds an
Authorization header to her request, and resends to the
atlanta.example.com authentication service. Now that the service is
sure of Alice's identity, it calculates an Identity header for the
request. The canonical string over which the identity signature will
be generated is the 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 resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is the following:

kjOP4YVZXmF0X3/4RUfAG6ffwbVQepNGRBz58b3dJq3prEV4h5GnS4F6udDRCI4/
rSK9cl+TFv45nu0Qu2d/0WPPOvvc3JWwuUmHrCwGwC+tW7fOWnC07QKgQn40uwg5
7WaXixQev5N0JfoLXnO3UDoum89JRhXPAIp2vffJbD4=

Accordingly, the atlanta.example.com authentication service will
create an Identity header containing that base64 signature string
(175 bytes). It will also add an HTTPS URL where its certificate is
made available. With those two headers added, the message looks

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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:"kjOP4YVZXmF0X3/4RUfAG6ffwbVQepNGRBz58b3dJq3prEV4h5GnS4F6udDRCI4/
rSK9cl+TFv45nu0Qu2d/0WPPOvvc3JWwuUmHrCwGwC+tW7fOWnC07QKgQn40uwg5
7WaXixQev5N0JfoLXnO3UDoum89JRhXPAIp2vffJbD4="
Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
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 request normally. When Bob
receives the request, if he does not already know the certificate of
atlanta.example.com, he de-references the URL in the Identity-Info
header to acquire the certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP
request. Using this canonical string, the signed digest in the
Identity header, and the certificate discovered by de-referencing the
Identity-Info header, Bob can verify that the given set of headers
and the message body have not been modified.

10.2. Identity for a Request with no MIME body or Contact

Consider the following private key and certificate pair assigned to
"biloxi.example.org".

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Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
at the end of the dialog initiated in the previous example. He
therefore creates the following BYE request which he forwards to the
'biloxi.example.org' proxy server that instantiates the
authentication service role:

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When the authentication service receives the BYE, it authenticates
Bob by sending a 407 response. As a result, Bob adds an
Authorization header to his request, and resends to the
biloxi.example.org authentication service. Now that the service is
sure of Bob's identity, it prepares to calculate an Identity header
for the request. Note that this request does not have a Date header
field. Accordingly, the biloxi.example.org will add a Date header to
the request before calculating the identity signature. If the
Content-Length header were not present, the authentication service
would add it as well. The baseline message is thus:

Also note that this request contains no Contact header field.
Accordingly, biloxi.example.org will place no value in the canonical
string for the addr-spec of the Contact address. Also note that
there is no message body, and accordingly, the signature string will
terminate, in this case, with two vertical bars. The canonical
string over which the identity signature will be generated is the
following (note that the 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 private RSA
key given above for biloxi.example.org, with base64 encoding, is the
following:

vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv
wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM
p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc=

Accordingly, the biloxi.example.org authentication service will
create an Identity header containing that base64 signature string.
It will also add an HTTPS URL where its certificate is made
available. With those two headers added, the 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: "vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv
wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM
p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc="
Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
Content-Length: 0

biloxi.example.org then forwards the request normally.

11. Identity and the TEL URI Scheme

Since many SIP applications provide a VoIP service, telephone numbers
are commonly used as identities in SIP deployments. In the majority
of cases, this is not problematic for the identity mechanism
described in this document. Telephone numbers commonly appear in the
username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com;user=phone'). That username
conforms to the syntax of the TEL URI scheme (RFC3966 [13]). For
this sort of SIP address-of-record, chicago.example.com is the
appropriate signatory.

It is also possible for a TEL URI to appear in the SIP 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 the identity. Fortunately for the
identity mechanism, this form of the TEL URI is more common for the
To header field and Request-URI in SIP than in the From header field,
since the UAC has no option but to provide a TEL URI alone when the
remote domain to which a request is sent is unknown. The local
domain, however, is usually known by the UAC, and accordingly it can
form a proper From header field containing a SIP URI with a username
in TEL URI form. Implementations that intend to send their requests
through an authentication service SHOULD put telephone numbers in the
From header field into SIP or SIPS URIs whenever possible.

If the local domain is unknown to a UAC formulating a request, it
most likely will not be able to locate an authentication service for
its request, and therefore the question of providing identity in
these cases is somewhat moot. However, an authentication service MAY
sign a request containing a TEL URI in the From header field. This

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is permitted in this specification strictly for forward compatibility
purposes. In the longer-term, it is possible that ENUM [14] 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 identities that contain telephone numbers. This
is a subject for future work.

12. Privacy Considerations

The identity mechanism presented in this draft is compatible with the
standard SIP practices for privacy described in RFC3323 [3]. A SIP
proxy server can act both as a privacy service and as an
authentication service. Since a user agent can provide any From
header field value which the authentication service is willing to
authorize, there is no reason why private SIP URIs which contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the 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 'anonymous.invalid' will not be possible for privacy
services that also act as authentication services. The assurance
offered by the usage of anonymous URIs with a valid domain portion is
"this is a known user in my domain that I have authenticated, but I
am keeping their identity private". The use of the domain
'anonymous.invalid' entails that no corresponding authority for the
domain can exist, and as a consequence, authentication service
functions are meaningless.

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 [12] 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

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identity for billing or security purposes. The danger of this
network-specific information leaking outside of the closed network
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.

Finally, note that unlike RFC3325, the mechanism described in this
specification adds no information to SIP requests that has privacy
implications.

13. Security Considerations

13.1. Handling of digest-string Elements

This document describes a mechanism which provides a signature over
the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
requests. 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 following section details the individual security properties
obtained by including each of these header fields within the
signature; collectively, this set of header fields provides the
necessary properties to prevent impersonation.

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 serves two purposes: first, it prevents cut-and-paste
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

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Call-IDs for at least the duration of a single Date interval (i.e.
commonly 3600 seconds). Because a SIP-compliant UA never generates
the same Call-ID twice, verifiers can use the Call-ID to recognize
cut-and-paste attacks; the Call-ID serves as a nonce. The result of
this is that if an Identity header is replayed within the Date
interval, verifiers will recognize that it is invalid because of a
Call-ID duplication; if an Identity header is replayed after the Date
interval, verifiers 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.

The Contact header field is included to tie the Identity header to a
particular user agent 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 thereby 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.

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This mechanism also provides a signature over the bodies of SIP
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 that originated a SIP request
if this assurance is not coupled with a comparable assurance over the
media descriptors. 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 replayer or man-in-the-middle
will modify SDP, 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."

In the end analysis, 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. However,
this mechanism is not intended to protect requests from men-in-the-
middle who interfere with SIP messages; it is intended only to
provide a way that SIP users can prove definitively that they are who
they claim to be. At best, by stripping identity information from a
request, a man-in-the-middle could make it impossible to distinguish
any illegitimate messages he would like to send from those messages
sent by an authorized user. However, it requires a considerably
greater amount of energy to mount such an attack than it does to
mount trivial impersonations by just copying someone else's From
header field. This mechanism provides a way that an authorized user
can provide a definitive assurance of their identity which an
unauthorized user, an impersonator, cannot.

One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. However, it is
important to note that preventing men-in-the-middle is not the
primary impetus for this mechanism. Moreover, changing the 'alg'
would at worst result in some sort of bid-down attack, and at best
cause a failure in the verifier. Note that only one valid 'alg'
parameter is defined in this document, and that thus there is
currently no weaker algorithm to which the mechanism can be bid-down.
'alg' has been incorporated into this mechanism for forward-
compatibility reasons in case the current algorithm exhibits
weaknesses, and requires swift replacement, in the future.

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13.2. Display Names and Identity

As a matter of interface design, SIP user agents might render the
display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email
user interfaces for this practice. As such, it might seem that the
lack of a signature over the display-name is a significant omission.

However, there are several important senses in which a signature over
the display-name does not prevent impersonation. In the first place,
a particular display-name, like "Jon Peterson", is not unique in the
world; many users in different administrative domains might
legitimately claim that name. Furthermore, enrollment practices for
SIP-based services might have a difficult time discerning the
legitimate display-name for a user; it is safe to assume that
impersonators will be capable of creating SIP accounts with
arbitrarily display-names. The same situation prevails in email
today. Note that an impersonator who attempted to replay a message
with an Identity header, changing only the display-name in the From
header field, would be detected by the other replay protection
mechanisms described in Section 13.1.

Of course, an authentication service can enforce policies about the
display-name even if the display-name is not signed. The exact
mechanics for creating and operationalizing such policies is outside
the scope of this document. The effect of this policy would not be
to prevent impersonation of a particular unique identifier like a SIP
URI (since display-names are not unique identifiers), but to allow a
domain to manage the claims made by its users. If such policies are
enforced, users would not be free to claim any display-name of their
choosing. In the absence of a signature, man-in-the-middle attackers
could conceivably alter the display-names in a request with impunity.
Note that the scope of this specification is impersonation attacks,
however, and that a man-in-the-middle might also strip the Identity
and Identity-Info headers from a message.

There are many environments in which policies regarding the display-
name aren't feasible. Distributing bit-exact and internationalizable
display-names to end users as part of the enrollment or registration
process would require mechanisms that are not explored in this
document. In the absence of policy enforcement regarding domain
names, there are conceivably attacks that an adversary could mount
against SIP systems that rely too heavily on the display-name in
their user interface, but this argues for intelligent interface
design, not changes to the mechanisms. Relying on a non-unique
identifier for identity would ultimately result in a weak mechanism.

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13.3. Securing the Connection to the Authentication Service

The assurance provided by this mechanism is strongest when a user
agent forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are
twofold:
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 an authentication service for a domain that
it does not control, possibly in an attempt to collect shared
secrets for that domain.
Without TLS, the various header field values and the body of the
request will not have integrity protection with the request
arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message
arbitrarily.

Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is possible that applications relying on
the presence of the Identity header could leverage this integrity
protection, especially body integrity, for services other than replay
protection.

Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS are not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association
with the authentication service is too weak.

13.4. Domain Names and Subordination

When a verifier processes a request containing an Identity-Info
header, it must compare the domain portion of the URI in the From
header field of the request with the domain name which is the subject
of the certificate acquired from the Identity-Info header. While it
might seem that this should be a straightforward process, it is

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complicated by two deployment realities. In the first place,
certificates have varying ways of describing their subjects, and may
indeed have multiple subjects, especially in 'virtual hosting' cases
where multiple domains are managed by a single application.
Secondly, some SIP services may delegate SIP functions to a
subordinate domain and utilize the procedures in RFC3263 [4] allow
requests for, say, 'example.com' to be routed to 'sip.example.com';
as a result, a user with the AoR 'sip:jon@example.com' may process
their requests through a host like 'sip.example.com', and it may be
that latter host which acts as an authentication service.

To meet the second of these problems, a domain that deploys an
authentication service on a subordinate host MUST be willing to
supply that host with the private keying material associated with a
certificate whose subject is a domain name that corresponds to the
domain portion of the AoRs that the domain distributes to users.
Note that this corresponds to the comparable case of routing inbound
SIP requests to a domain. When the NAPTR and SRV procedures of
RFC3263 are used to direct requests to a domain name other than the
domain in the original Request-URI (e.g., for 'sip:jon@example.com',
the corresponding SRV records point to the service
'sip1.example.org'), the client expects that the certificate passed
back in any TLS exchange with that host will correspond exactly with
the domain of the original Request-URI, not the domain name of the
host. Consequently, in order to make inbound routing to such SIP
services work, a domain administrator must similarly be willing to
share the domain's private key with the service. This design
decision was made to compensate for the insecurity of the DNS, and it
makes certain potential approaches to DNS-based 'virtual hosting'
unsecurable for SIP in environments where domain administrators are
unwilling to share keys with hosting services.

A verifier MUST evaluate the correspondence between the user's
identity and the signing certificate by following the procedures
defined in RFC 2818 [11] Section 3.1. While RFC2818 deals with the
use of HTTP in TLS, the procedures described are applicable to
verifying identity if one subtitutes the "hostname of the server" in
HTTP for the domain portion of the user's identity in the From header
field of a SIP request with an Identity header.

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 to 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

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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. It
is strongly RECOMMENDED that self-signed domain certificates should
not be trusted by verifiers, unless some previous key exchange has
justified such trust.

For further information on certificate security and practices see
RFC3280 [9]. The Security Considerations of RFC3280 are applicable
to this document.

13.5. Authorization and Transitional Strategies

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 used its
administrative practices to authenticate its users. However, it is
possible that some domains will implement policies that effectively
make users unaccountable (e.g., ones that accept 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.

One cannot expect the Identity and Identity-Info headers to be
supported by every SIP entity overnight. This leaves the verifier in
a compromising position; when it receives a request from a given SIP
user, how can it know whether or not the sender's domain supports
Identity? In the absence of ubiquitous support for identity, some
transitional strategies are necessary.
A verifier could remember when it receives a request from a domain
that uses Identity, and in the future, view messages received from
that domain without Identity headers with skepticism.
A verifier could query the domain through some sort of callback
system to determine whether or not it is running an authentication
service. There are a number of potential ways in which this could
be implemented; use of the SIP OPTIONS method is one possibility.
This is left as a subject for future work.

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In the long term, some sort of identity mechanism, either the one
documented in this specification or a successor, must become
mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected by verifiers.

Finally, it is worth noting that the presence or absence of the
Identity headers cannot be the sole factor in making an authorization
decision. Permissions might be granted to a message on the basis of
the specific verified Identity or really on any other aspect of a SIP
request. Authorization policies are outside the scope of this
specification, but this specification advises any future
authorization work not to assume that messages with valid Identity
headers are always good.

14. IANA Considerations

This document requests changes to the header and response-code sub-
registries of the SIP parameters IANA registry, and requests the
creation of two new registries for parameters for the Identity-Info
header.

14.1. Header Field Names

This document specifies two new SIP headers: Identity and Identity-
Info. Their syntax is given in Section 9. 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: n

14.2. 428 'Use Identity Header' Response Code

This document registers a new SIP response code which is described in
Section 6. It is sent when a verifier receives a SIP request that
lacks an Identity header in order to indicate that the request should
be re-sent with an Identity header. This response code 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

14.3. 436 'Bad Identity-Info' Response Code

This document registers a new SIP response code which is described in

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Section 6. It is used when the Identity-Info header contains a URI
that cannot be dereferenced by the verifier (either the URI scheme is
unsupported by the verifier, or the resource designated by the URI is
otherwise unavailable). This response code 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: 436
Default Reason Phrase: Bad Identity-Info

14.4. 437 'Unsupported Certificate' Response Code

This document registers a new SIP response code which is described in
Section 6. It is used when the verifier cannot validate the
certificate referenced by the URI of the Identity-Info header,
because, for example, the certificate is self-signed, or signed by a
root certificate authority for whom the verifier does not possess a
root certificate. This response code 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: 437
Default Reason Phrase: Unsupported Certificate

14.5. 438 'Invalid Identity Header' Response Code

This document registers a new SIP response code which is described in
Section 6. It is used when the verifier receives a message with an
Identity signature that does not correspond to the digest-string
calculated by the verifier. This response code 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: 438
Default Reason Phrase: Invalid Identity Header

14.6. Identity-Info Parameters

This document requests that the IANA create a new registry for
Identity-Info headers. This registry is to be prepopulated with a
single entry for a parameter called 'alg', which describes the
algorithm used to create the signature which appears in the Identity
header. Registry entries must contain the name of the parameter and
the specification in which the parameter is defined. New parameters
for the Identity-Info header may be defined only in Standards Track
RFCs.

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14.7. Identity-Info Algorithm Parameter Values

This document requests that the IANA create a new registry for
Identity-Info 'alg' parameter values. This registry is to be
prepopulated with a single entry for a value called 'rsa-sha1', which
describes the algorithm used to create the signature which appears in
the Identity header. Registry entries must contain the name of the
'alg' parameter value and the specification in which the value is
described. New values for the 'alg' parameter may be defined only in
Standards Track RFCs.

Appendix A. Acknowledgments

The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell,
Aki Niemi, and Jim Schaad for their comments. Jonathan Rosenberg
provided detailed fixes to innumerable sections of the document. The
bit-archive presented in Appendix B follows the pioneering example of
Robert Sparks' torture-test draft. Thanks to Hans Persson and Tao
Wan for thorough nit reviews.

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 10. 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 and
Identity-Info headers added

Also included in the archive are two public key/certificate pairs,
for atlanta.example.com and biloxi.example.org, respectively,
including:
o (foo).cer: the certificate of the 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

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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

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bHaXS6fwwAQsdMOsKrLE20oQeRrkrgmqqeHxbU8ywEfdIzYVvJiqs/HsUIjnZpoH

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zaQMe82hDO1kT7DbAAAAAAAAAAAAAAAAAAAAAAAAAAAAaMYfe7L1egB4AAA=
-- END MESSAGE ARCHIVE --

Appendix C. Original Requirements

The following requirements were crafted throughout the development of
the mechanism described in this document. They are preserved here
for historical reasons.
o The mechanism must allow a UAC or a proxy server to provide a
strong cryptographic identity assurance in a request that can be
verified by a proxy server or UAS.
o User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup.
o User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to 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 this mechanism in order for an
identity assurance to be added to a request in this fashion.
o The mechanism must prevent replay of the identity assurance by an
attacker.

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o In order to provide full replay protection, 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) which it is authorized to use within 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.

Appendix D. Changelog

NOTE TO THE RFC-EDITOR: Please remove this section prior to
publication as an RFC.

Changes from draft-ietf-sip-identity-06:
- Disambiguated 428 response code, added new 438 for invalid
Identity headers
- Used RFC2585 format for Identity-Info URIs
- Updated example certificates to comply with RFC3280
- Replaced certificate validation and mapping procedures with
reference to RFC2818
- Numerous editorial fixes

Changes from draft-ietf-sip-identity-05:
- Removed the requirements section
- Numerous editorial fixes

Changes from draft-ietf-sip-identity-04:
- Changed the delimiter of the digest-string from ":" to "|"
- Removed support for the SIPS URI scheme from the Identity-Info
header
- Made the Identity-Info header extensible; added an Identity-Info
header for algorithm with an initial defined value of 'rsa-sha1'
- Broke up the Security Considerations into smaller chunks for
organizational reasons; expanded discussion of most issues
- Added some guidelines for authentication service certificate
rollover and lifecycle management (also now based HTTP certificate
retrieval on RFC2585)

Changes from draft-ietf-sip-identity-03:
- Softened requirement for TLS and direct connections; now SHOULD-
strength, SIPS and Digest auth-int listed as alternatives.
- Added non-normative section about authentication service
behavior for backwards-direction requests within a dialog

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- Added support for CID URI in Identity Info
- Added new response codes (436 and 437) corresponding to error
cases for an unsupported URI scheme and an unsupported
certificate, respectively

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 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
- 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:
- 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

15. References

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15.1. 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., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004.

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

[7] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms",
RFC 3370, August 2002.

[8] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.

[9] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.

[10] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC 2585,
May 1999.

[11] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000.

15.2. Informative References

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

[13] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966,
December 2004.

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

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[15] Peterson, J., "Retargeting and Security in SIP: A Framework and
Requirements", draft-peterson-sipping-retarget-00 (work in
progress), February 2005.

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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

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