draft-ietf-sip-identity-04.txt   draft-ietf-sip-identity-05.txt 
SIP WG J. Peterson SIP WG J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Expires: August 17, 2005 C. Jennings Expires: September 2, 2005 C. Jennings
Cisco Systems Cisco Systems
February 16, 2005 March 2005
Enhancements for Authenticated Identity Management in the Session Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP) Initiation Protocol (SIP)
draft-ietf-sip-identity-04 draft-ietf-sip-identity-05
Status of this Memo Status of this Memo
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which he or she become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved. Copyright (C) The Internet Society (2005).
Abstract Abstract
The existing security mechanisms in the Session Initiation Protocol The existing security mechanisms in the Session Initiation Protocol
are inadequate for cryptographically assuring the identity of the end are inadequate for cryptographically assuring the identity of the end
users that originate SIP requests, especially in an interdomain users that originate SIP requests, especially in an interdomain
context. This document recommends practices and conventions for context. This document defines a mechanism for securely identifying
identifying end users in SIP messages, and proposes a way to originators of SIP messages. It does so by defining two new SIP
distribute cryptographically-secure authenticated identities. header fields, Identity, for conveying a signature used for
validating the identity, and Identity-Info, for conveying a reference
to the certificate of the signer.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6 5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
6. Authentication Service Behavior . . . . . . . . . . . . . . 7 6. Authentication Service Behavior . . . . . . . . . . . . . . 7
6.1 Identity within a Dialog and Retargeting . . . . . . . . . 9 6.1 Identity within a Dialog and Retargeting . . . . . . . . . 10
7. Verifying Identity . . . . . . . . . . . . . . . . . . . . . 11 7. Verifier Behavior . . . . . . . . . . . . . . . . . . . . . 10
8. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 12 8. Considerations for User Agent . . . . . . . . . . . . . . . 12
9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . 12 9. Considerations for Proxy Server . . . . . . . . . . . . . . 13
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 13 10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 13
11. Compliance Tests and Examples . . . . . . . . . . . . . . . 15 11. Compliance Tests and Examples . . . . . . . . . . . . . . . 16
11.1 Identity-Info with a Singlepart MIME body . . . . . . . 16 11.1 Identity-Info with a Singlepart MIME body . . . . . . . 16
11.2 Identity for a Request with no MIME body or Contact . . 18 11.2 Identity for a Request with no MIME body or Contact . . 19
12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 21 12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 22
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 23
14. Security Considerations . . . . . . . . . . . . . . . . . . 23 14. Security Considerations . . . . . . . . . . . . . . . . . . 24
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27 14.1 Handling of digest-string Elements . . . . . . . . . . . 24
15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 27 14.2 Display Names and Identity . . . . . . . . . . . . . . . 26
15.2 428 'Use Identity Header' Response Code . . . . . . . . 27 14.3 Securing the Connection to the Authentication Service . 27
15.3 436 'Bad Identity-Info' Response Code . . . . . . . . . 28 14.4 Domain Names and Subordination . . . . . . . . . . . . . 28
15.4 437 'Unsupported Certificate' Response Code . . . . . . 28 14.5 Authorization and Transitional Strategies . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 31
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 30 15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 31
B. Bit-exact archive of example messages . . . . . . . . . . . 30 15.2 428 'Use Identity Header' Response Code . . . . . . . . 31
B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 31 15.3 436 'Bad Identity-Info' Response Code . . . . . . . . . 31
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 15.4 437 'Unsupported Certificate' Response Code . . . . . . 32
16.1 Normative References . . . . . . . . . . . . . . . . . . . 28 15.5 Identity-Info Parameters . . . . . . . . . . . . . . . . 32
16.2 Informative References . . . . . . . . . . . . . . . . . . 29 15.6 Identity-Info Algorithm Parameter Values . . . . . . . . 32
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 34
Intellectual Property and Copyright Statements . . . . . . . 35 A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 34
B. Bit-exact archive of example messages . . . . . . . . . . . 34
B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 35
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
16.1 Normative References . . . . . . . . . . . . . . . . . . 33
16.2 Informative References . . . . . . . . . . . . . . . . . 33
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 37
Intellectual Property and Copyright Statements . . . . . . . 40
1. Introduction 1. Introduction
This document provides enhancements to the existing mechanisms for This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol authenticated identity management in the Session Initiation Protocol
(SIP [1]). An identity, for the purposes of this document, is (SIP [1]). An identity, for the purposes of this document, is
defined as a canonical SIP address-of-record URI employed to reach a defined as a SIP URI, commonly a canonical AoR employed to reach a
user (such as 'sip:alice@atlanta.example.com'). user (such as 'sip:alice@atlanta.example.com').
RFC3261 stipulates several places within a SIP request that a user RFC3261 stipulates several places within a SIP request where a user
can express an identity for themselves, notably the user-populated can express an identity for themselves, notably the user-populated
From header field. However, the recipient of a SIP request has no From header field. However, the recipient of a SIP request has no
way to verify that the From header field has been populated way to verify that the From header field has been populated
accurately, in the absence of some sort of cryptographic appropriately, in the absence of some sort of cryptographic
authentication mechanism. authentication mechanism.
RFC3261 specifies a number of security mechanisms that can be RFC3261 specifies a number of security mechanisms that can be
employed by SIP UAs, including Digest, TLS and S/MIME employed by SIP UAs, including Digest, TLS and S/MIME
(implementations may support other security schemes as well). (implementations may support other security schemes as well).
However, few SIP user agents today support the end-user certificates However, few SIP user agents today support the end-user certificates
necessary to authenticate themselves via TLS or S/MIME, and necessary to authenticate themselves (via S/MIME, for example), and
furthermore Digest authentication is limited by the fact that the furthermore Digest authentication is limited by the fact that the
originator and destination must share a pre-arranged secret. It is originator and destination must share a pre-arranged secret. It is
desirable for SIP user agents to be able to send requests to desirable for SIP user agents to be able to send requests to
destinations with which they have no previous association - just as destinations with which they have no previous association - just as
in the telephone network today, one can receive a call from someone in the telephone network today, one can receive a call from someone
with whom one has no previous association, and still have a with whom one has no previous association, and still have a
reasonable assurance that their displayed Caller-ID is accurate. reasonable assurance that their displayed Caller-ID is accurate.
2. Terminology 2. Terminology
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3. Background 3. Background
The usage of many SIP applications and services is governed by The usage of many SIP applications and services is governed by
authorization policies. These policies may be automated, or they may authorization policies. These policies may be automated, or they may
be applied manually by humans. An example of the latter would be an be applied manually by humans. An example of the latter would be an
Internet telephone application which displays the "Caller-ID" of a Internet telephone application which displays the "Caller-ID" of a
caller, which a human may review before answering a call. An example caller, which a human may review before answering a call. An example
of the former would be a presence service that compares the identity of the former would be a presence service that compares the identity
of potential subscribers to a whitelist before determining whether it of potential subscribers to a whitelist before determining whether it
should begin sending presence notifications. In both of these cases, should accept or reject the subscription. In both of these cases,
attackers might attempt to circumvent these authorization policies attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of through impersonation. Since the primary identifier of the sender of
a SIP request, the From header field, can be populated arbitrarily be a SIP request, the From header field, can be populated arbitrarily by
the controller of a user agent, impersonation is very simple today. the controller of a user agent, impersonation is very simple today.
The mechanism described in this document aspires to provide a strong The mechanism described in this document aspires to provide a strong
identity system for SIP in which authorization policies cannot be identity system for SIP in which authorization policies cannot be
circumvented by impersonation. circumvented by impersonation.
All RFC3261-compliant SIP user agents support a means of All RFC3261 compliant user agents support Digest authentication,
authenticating themselves to a SIP registrar, commonly with a shared which utilizes a shared secret, as a means for authenticating
secret; Digest authentication, which MUST be supported by SIP user themselves to a SIP registrar. Registration allows a user agent to
agents, is typically used for this purpose. Registration allows a express that it is an appropriate entity to which requests should be
user agent to express that it is an appropriate entity to which sent for a particular address-of-record SIP URI (e.g.,
requests should be sent for a particular address-of-record SIP URI 'sip:alice@atlanta.example.com').
(e.g., 'sip:alice@atlanta.example.com').
By the definition of identity used in this document, registration is By the definition of identity used in this document, registration is
a proof of the identity of the user to a registrar. However, the a proof of the identity of the user to a registrar. However, the
credentials with which a user agent proves their identity to a credentials with which a user agent proves their identity to a
registrar cannot be validated by just any user agent or proxy server registrar cannot be validated by just any user agent or proxy server
- these credentials are only shared between the user agent and their - these credentials are only shared between the user agent and their
domain administrator. So this shared secret does not immediately domain administrator. So this shared secret does not immediately
help a user to authenticate to a wide range of recipients. help a user to authenticate to a wide range of recipients.
Recipients require a means of determining whether or not the 'return Recipients require a means of determining whether or not the 'return
address' identity of a non-REGISTER request (i.e., the From header address' identity of a non-REGISTER request (i.e., the From header
field value) has legitimately been asserted. field value) has legitimately been asserted.
The address-of-record URI used for registration is also the URI with The address-of-record URI used for registration is also the URI with
which a UA commonly populates the From header of requests in order to which a UA commonly populates the From header field of requests in
provide a 'return address' identity to recipients. The identity order to provide a 'return address' identity to recipients. From an
mechanism specified in this document derives from the following authorization perspective, if you are can prove you are eligible to
principle: if you can prove you are eligible to register in a domain register in a domain under a particular address-of-record, you can
under a particular address-of-record, you are proving that you are prove you can legitimately receive requests for that address-of-
capable of legitimately receiving requests for that record, and accordingly, when you place that address-of-record in the
address-of-record, and accordingly, when you place that From header field of a SIP request other than a registration (like an
address-of-record in the From header field of a SIP request other INVITE), you are providing a 'return address' where you can
than a registration (like an INVITE), you are providing a 'return legitimately be reached. In other words, if you are authorized to
address' where you can legitimately be reached. In other words, if receive requests for that 'return address', logically, it follows
you are authorized to receive requests for that 'return address', that you are also authorized to assert that 'return address' in your
logically, it follows that you are also authorized to assert that From header field. This is of course only one manner in which a
'return address' in your From header field. 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 Ideally, then, SIP user agents should have some way of proving to
recipients of SIP requests that their local domain has authenticated recipients of SIP requests that their local domain has authenticated
them and authorized the population of the From header field. This them and authorized the population of the From header field. This
document proposes a mediated authentication architecture for SIP in document proposes a mediated authentication architecture for SIP in
which requests are sent to a server in the user's local domain, which which requests are sent to a server in the user's local domain, which
authenticates such requests (using the same practices by which the authenticates such requests (using the same practices by which the
domain would authenticate REGISTER requests). Once a message has domain would authenticate REGISTER requests). Once a message has
been authenticated, the local domain then needs some way to been authenticated, the local domain then needs some way to
communicate to other SIP entities that the sending user has been communicate to other SIP entities that the sending user has been
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this works well in some architectures, there are a few respects in this works well in some architectures, there are a few respects in
which this is impractical. For one, transitive trust is inherently which this is impractical. For one, transitive trust is inherently
weaker than an assertion that can be validated end-to-end. It is weaker than an assertion that can be validated end-to-end. It is
possible for SIP requests to cross multiple intermediaries in possible for SIP requests to cross multiple intermediaries in
separate administrative domains, in which case transitive trust separate administrative domains, in which case transitive trust
becomes even less compelling. becomes even less compelling.
One solution to this problem is to use 'trusted' SIP intermediaries One solution to this problem is to use 'trusted' SIP intermediaries
that assert an identity for users in the form of a privileged SIP 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. A mechanism for doing so (with the P-Asserted-Identity
header) is given in [8]. However, this solution allows only header) is given in [10]. However, this solution allows only hop-by-
hop-by-hop trust between intermediaries, not end-to-end cryptographic hop trust between intermediaries, not end-to-end cryptographic
authentication, and it assumes a managed network of nodes with strict authentication, and it assumes a managed network of nodes with strict
mutual trust relationships, an assumption that is incompatible with mutual trust relationships, an assumption that is incompatible with
widespread Internet deployment. widespread Internet deployment.
Accordingly, this document specifies a means of sharing a Accordingly, this document specifies a means of sharing a
cryptographic assurance of end-user SIP identity in an interdomain cryptographic assurance of end-user SIP identity in an interdomain or
context which is based on the concept of an 'authentication service' intradomain context which is based on the concept of an
and a new SIP header, the Identity header. Note that the scope of 'authentication service' and a new SIP header, the Identity header.
this document is limited to providing this identity assurance for SIP Note that the scope of this document is limited to providing this
requests; solving this problem for SIP responses is more complicated, identity assurance for SIP requests; solving this problem for SIP
and is a subject for future work. responses is more complicated, and is a subject for future work.
This specification allows either a user agent or a proxy server to This specification allows either a user agent or a proxy server to
act as an authentication service. To maximize end-to-end security, provide identity services and to verify identities. To maximize end-
it is obviously preferable for end users to hold their own to-end security, it is obviously preferable for end users to hold
certificates; if they do, they can act as an authentication service. their own certificates; if they do, they can act as an authentication
However, end-user certificates may be neither practical nor service. However, end-user certificates may be neither practical nor
affordable, given the difficulties of establishing a PKI that extends affordable, given the difficulties of establishing a PKI that extends
to end users, and moreover, given the potentially large number of SIP to end users, and moreover, given the potentially large number of SIP
user agents (phones, PCs, laptops, PDAs, gaming devices) that may be user agents (phones, PCs, laptops, PDAs, gaming devices) that may be
employed by a single user. In such environments, synchronizing employed by a single user. In such environments, synchronizing
certificates across multiple devices may be very complex, and certificates across multiple devices may be very complex, and
requires quite a good deal of additional endpoint behavior. Managing requires quite a good deal of additional endpoint behavior. Managing
several certificates for the various devices is also quite several certificates for the various devices is also quite
problematic and unpopular with users. Accordingly, in the initial problematic and unpopular with users. Accordingly, in the initial
use of this mechanism, it is likely that intermediaries will use of this mechanism, it is likely that intermediaries will
instantiate the authentication service role. instantiate the authentication service role.
4. Requirements 4. Requirements
This draft addresses the following requirements: This draft addresses the following requirements:
o The mechanism must allow a UAC to provide a strong cryptographic o The mechanism must allow a UAC or a proxy server to provide a
identity assurance in a request that can be verified by a proxy strong cryptographic identity assurance in a request that can be
server or UAS. verified by a proxy server or UAS.
o User agents that receive identity assurances must be able to o User agents that receive identity assurances must be able to
validate these assurances without performing any network lookup. validate these assurances without performing any network lookup.
o User agents that hold certificates on behalf of their user must be o User agents that hold certificates on behalf of their user must be
capable of adding this identity assurance to requests. capable of adding this identity assurance to requests.
o Proxy servers that hold certificates on behalf of their domain o Proxy servers that hold certificates on behalf of their domain
must be capable of adding this identity assurance to requests; a must be capable of adding this identity assurance to requests; a
UAC is not required to support this mechanism in order for an UAC is not required to support this mechanism in order for an
identity assurance to be added to a request in this fashion. identity assurance to be added to a request in this fashion.
o The mechanism must prevent replay of the identity assurance by an o The mechanism must prevent replay of the identity assurance by an
attacker. attacker.
o The mechanism must be capable of protecting the integrity of SIP o In order to provide full replay protection, the mechanism must be
message bodies (to ensure that media offers and answers are linked capable of protecting the integrity of SIP message bodies (to
to the signaling identity). 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. 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 accounts or aliases) which it is authorized to use within a
the UAC to assert one identity while authenticating itself as domain, and for the UAC to assert one identity while
another, related, identity, as permitted by the local policy of authenticating itself as another, related, identity, as permitted
the domain. 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
sent.
5. Overview of Operations 5. Overview of Operations
This section provides an informative (non-normative) high-level This section provides an informative (non-normative) high-level
overview of the mechanisms described in this document. overview of the mechanisms described in this document.
Imagine the case where Alice, who has the home proxy of example.com 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 and the address-of-record sip:alice@example.com, wants to communicate
with sip:bob@example.org. with sip:bob@example.org.
Alice generates an INVITE and places her identity in the From header 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 field of the request. She then sends an INVITE over TLS to an
authentication service proxy for her domain. authentication service proxy for her domain.
The authentication service authenticates Alice (possibly by sending a The authentication service authenticates Alice (possibly by sending a
Digest authentication challenge) and validates that she is authorized Digest authentication challenge) and validates that she is authorized
to populate the value of the From header field (which may be Alice's to assert the identity which is populated in the From header field.
AoR, or it may be some other value that the policy of the proxy This value may be Alice's AoR, or it may be some other value that the
server permits her to use). It then computes a hash over some policy of the proxy server permits her to use. It then computes a
particular headers, including the From header field and the bodies in hash over some particular headers, including the From header field
the message. This hash is signed with the certificate for the domain and the bodies in the message. This hash is signed with the
(example.com, in Alice's case) and inserted in a new header field in certificate for the domain (example.com, in Alice's case) and
the SIP message, the 'Identity' header. inserted in a new header field in the SIP message, the 'Identity'
header.
The proxy, as the holder the private key of its domain, is asserting The proxy, as the holder the private key of its domain, is asserting
that the originator of this request has been authenticated and that that the originator of this request has been authenticated and that
she is authorized to claim the identity (the SIP address-of-record) she is authorized to claim the identity (the SIP address-of-record)
which appears in the From header field. The proxy also inserts a which appears in the From header field. The proxy also inserts a
companion header field that tells Bob how to acquire its certificate, companion header field, Identity-Info, that tells Bob how to acquire
if he doesn't already have it. its certificate, if he doesn't already have it.
When Bob's domain receives the request, it verifies the signature When Bob's domain receives the request, it verifies the signature
provided in the Identity header, and thus can authenticate that the provided in the Identity header, and thus can validates that the
domain indicated by the host portion of the AoR in the From header domain indicated by the host portion of the AoR in the From header
field authenticated the user, and permitted them to assert that From field authenticated the user, and permitted them to assert that From
header field value. header field value. This same validation operation may be performed
by Bob's UAS.
6. Authentication Service Behavior 6. Authentication Service Behavior
This document defines a new role for SIP entities called an This document defines a new role for SIP entities called an
authentication service. The authentication service role can be authentication service. The authentication service role can be
instantiated by a proxy server or a user agent. Any entity that instantiated by a proxy server or a user agent. Any entity that
instantiates the authentication service role MUST possess the private instantiates the authentication service role MUST possess the private
key of a domain certificate, and MUST be capable of authenticating key of a domain certificate, and MUST be capable of authenticating
one or more SIP users that can register in that domain. Commonly, one or more SIP users that can register in that domain. Commonly,
this role will be instantiated by a proxy server, since these this role will be instantiated by a proxy server, since these
entities are more likely to have a static hostname, hold a entities are more likely to have a static hostname, hold a
corresponding certificate, and access to SIP registrar capabilities corresponding certificate, and have access to SIP registrar
that allow them to authenticate users in their domain. It is also capabilities that allow them to authenticate users in their domain.
possible that the authentication service role might be instantiated It is also possible that the authentication service role might be
by a redirect server, but that is left as a topic for future work. instantiated by an entity that acts redirect server, but that is left
as a topic for future work.
SIP entities that act as an authentication service MUST add a Date SIP entities that act as an authentication service MUST add a Date
header field to SIP requests if one is not already present. header field to SIP requests if one is not already present.
Similarly, authentication services MUST add a Content-Length header Similarly, authentication services MUST add a Content-Length header
field to SIP requests if one is not already present; this can help 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 the verifier to double-check that they are hashing exactly as many
bytes of message-body as the authentication service when they verify bytes of message-body as the authentication service when they verify
the message. the message.
The authentication service authenticates the identity of the message Entities instantiating the authentication service role performs the
sender and validates that the identity given in the message can following steps, in order, to generate an Identity header for a SIP
legitimately be asserted by the sender. Then it computes a signature request:
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 Step 1: The authentication service MUST extract the identity of the
sender from the request. The authentication service takes this value sender from the request. The authentication service takes this value
from the From header field; this AoR will be referred to here as the 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, 'identity field'. If the identity field contains a SIP or SIPS URI,
the authentication service MUST extract the hostname portion of the the authentication service MUST extract the hostname portion of the
identity field and compare it to the domain(s) for which it is identity field and compare it to the domain(s) for which it is
responsible. If the identity field uses the TEL URI scheme, the responsible. If the identity field uses the TEL URI scheme, the
policy of the authentication service determines whether or not it is policy of the authentication service determines whether or not it is
responsible for this identity; see Section 12 for more information. responsible for this identity; see Section 12 for more information.
If the authentication service is not responsible for the identity in If the authentication service is not responsible for the identity in
question, it SHOULD process and forward the request normally, but it question, it SHOULD process and forward the request normally, but it
MUST NOT add an Identity header; see below for more information on MUST NOT add an Identity header; see below for more information on
authentication service handling of an existing Identity header. authentication service handling of an existing Identity header.
Second, the authentication service needs to determine whether or not Step 2: The authentication service MUST determine whether or not the
the sender of the request is authorized to claim the identity given sender of the request is authorized to claim the identity given in
in the identity field. In order to do so, the authentication service the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include: which this authentication might be performed include:
o If the authentication service is instantiated by a SIP If the authentication service is instantiated by a SIP
intermediary (proxy server), it may challenge the request with a intermediary (proxy server), it may challenge the request with a
407 response code using the Digest authentication scheme (or 407 response code using the Digest authentication scheme (or
viewing a Proxy-Authentication header sent in the request which viewing a Proxy-Authentication header sent in the request which
was sent in anticipation of a challenge using cached credentials, was sent in anticipation of a challenge using cached credentials,
as described in RFC 3261 Section 22.3). as described in RFC 3261 Section 22.3). Note that if that proxy
o If the authentication service is instantiated by a SIP user agent, 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 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 that the user can provision the user agent with the private key of
the domain, or by preferably by providing a password that unlocks the domain, or by preferably by providing a password that unlocks
said private key. said private key.
Authorization of the use of a particular username in the From header Authorization of the use of a particular username in the From header
field is a matter of local policy for the authorization service, one field is a matter of local policy for the authentication service, one
which depends greatly on the manner in which authentication is which depends greatly on the manner in which authentication is
performed. For example, one policy might be as follows: the username performed. For example, one policy might be as follows: the username
given in the 'username' parameter of the Proxy-Authorization header given in the 'username' parameter of the Proxy-Authorization header
MUST correspond exactly to the username in the From header field of 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 the SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use 'username' parameters in limiting or inappropriate; a realm might use 'username' parameters in
Proxy-Authorization which do not correspond to the user-portion of Proxy-Authorization which do not correspond to the user-portion of
SIP From headers, or a user might manage multiple accounts in the SIP From headers, or a user might manage multiple accounts in the
same administrative domain. In this latter case, a domain might same administrative domain. In this latter case, a domain might
maintain a mapping between the values in the 'username' parameter of maintain a mapping between the values in the 'username' parameter of
Proxy-Authorization and a set of one or more SIP URIs which might Proxy-Authorization and a set of one or more SIP URIs which might
legitimately be asserted for that 'username'. In this instance, 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 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 MUST correspond exactly to one of the mapped URIs associated with the
'username' given in the Proxy-Authorization header. Various 'username' given in the Proxy-Authorization header. Various
exceptions to such policies might arise for cases like anonymity; if exceptions to such policies might arise for cases like anonymity; if
the AoR asserted in the From header field is anonymous (per RFC3323 the AoR asserted in the From header field uses a form like
[3]), then the proxy should authenticate that the user is a valid 'sip:anonymous@example.com', then the 'example.com' proxy should
user in the domain and insert the signature over the From header authenticate that the user is a valid user in the domain and insert
field as usual. the signature over the From header field as usual.
Note that this check is performed on the addr-spec in the From header Note that this check is performed on the addr-spec in the From header
field (e.g., the URI of the sender, like field (e.g., the URI of the sender, like
'sip:alice@atlanta.example.com'); it does not convert the 'sip:alice@atlanta.example.com'); it does not convert the display-
display-name portion of the From header field (e.g., 'Alice name portion of the From header field (e.g., 'Alice Atlanta').
Atlanta'). Some SIP user agents that receive requests render the Authentication services MAY check and validate the display-name as
display-name of the caller as the identity of the caller. However, well, and compare it to a list of acceptable display-names that may
there are many environments in which legislating the display-name be used by the sender; if the display-name does not meet policy
isn't feasible, judging from experience with email, where users constraints, the authentication service MUST return a 403 response
frequent make slight textual changes to their display-names. code with the reason phrase "Inappropriate Display-Name". However,
Ultimately, there is more value in focusing on the SIP address of the the display-name is not always present, and in many environments the
sender (which has some meaning in the network and provides a chain of requisite operational procedures for display-name validation may not
accountability) than trying to constrain how the display-name is set. exist. For more information, see Section 14.2.
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
interface, see Section 8.
Third, the authentication service MUST form the identity signature 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.
Step 4: The authentication service MUST form the identity signature
and add an Identity header to the request containing this signature. and add an Identity header to the request containing this signature.
After the Identity header has been added to the request, the After the Identity header has been added to the request, the
authentication service MUST also add an Identity-Info header. The authentication service MUST also add an Identity-Info header. The
Identity-Info header contains a URI from which its certificate can be Identity-Info header contains a URI from which its certificate can be
acquired. Details are provided in section Section 10. acquired. Details on the generation of both of these headers are
provided in section Section 10.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
6.1 Identity within a Dialog and Retargeting 6.1 Identity within a Dialog and Retargeting
Retargeting, the alteration by intermediaries of the Request-URI of a Retargeting is defined as the alteration of the Request-URI by
SIP request, can cause a few wrinkles for the Identity mechanism when intermediaries in order to point to another URI that corresponds to a
it is applied to requests sent in the backwards direction within a user that cannot authenticate itself with the identity originally
dialog. This section provides some non-normative considerations present in the Request-URI. By this definition, retargeting excludes
related to this case. translation of the Request-URI to a registered contact of an endpoint
that has authenticated itself as that user.
When a dialog-forming request is retargeted, this can cause a few
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 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. 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 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 as the From header field of requests sent in the backwards direction
during the dialog, and is accordingly the header that would be signed during the dialog, and is accordingly the header that would be signed
by an authentication service for requests sent in the backwards by an authentication service for requests sent in the backwards
direction. In retargeting cases, if the URI in the From header does direction. In retargeting cases, if the URI in the From header does
not identify the sender of the request in the backwards direction, not identify the sender of the request in the backwards direction,
then clearly it would be inappropriate to provide an Identity then clearly it would be inappropriate to provide an Identity
signature over that From header. As specified above, if the signature over that From header. As specified above, if the
authentication service is not responsible for the domain in the From authentication service is not responsible for the domain in the From
header field of the request, it must not add an Identity header to header field of the request, it must not add an Identity header to
the request, and should process/forward the request normally. the request, and should process/forward the request normally.
If there were a means in backwards-direction requests to signify a Any means of anticipating retargeting and so on is outside the scope
'connected party', an identity of the unanticipated user whose SIP of this document, and likely to have equal applicability to response
endpoint was reached by the dialog-forming request, it isn't clear identity as it does to requests in the backwards direction within a
that it would actually be beneficial to provide a corresponding dialog. Consequently, no special guidance is given for implementers
Identity header signature over that information. The Identity header here regarding the 'connected party' problem; authentication service
is designed to prevent impersonation-based attacks, and it is very behavior is unchanged if retargeting has occurred for a dialog-
unclear how and why an attacker might attempt to impersonate an forming request. Ultimately, the authentication service provides an
unanticipated third party in a backwards-direction request within an Identity header for requests in the backwards dialog when the user is
existing dialog. That is, it's unclear how the caller's potential authorized to assert the identity given in the From header field, and
authorization policies would be any more successful at thwarting if they are not, an Identity header is not provided.
impersonation if new requests in the backwards direction came from an
assured unanticipated third-party instead of an unassured
unanticipated third-party. Thwarting impersonation is, ultimately,
the purpose of this Identity mechanism, and it must be left to other
mechanisms to solve other security problems for SIP.
The mechanism in this draft cannot aid in determining whether or not
the unanticipated party is an appropriate target of this request and,
accordingly, solving this problem is outside the scope of this draft.
If, however, it were possible for the sender of the dialog-forming
request to anticipate that retargeting had occurred, and to gain some
kind of assurance of the new target of the request before any
requests in the backwards direction were received, this would open up
some new approaches to authorization policy.
Any such means of anticipating retargeting and so on is outside the For further information on the problems of response identity and the
scope of this document, and likely to have equal applicability to potential solution spaces, see [14].
response identity as it does to requests in the backwards direction
within a dialog. Consequently, no special guidandance 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.
7. Verifying Identity 7. Verifier Behavior
When a user agent or proxy server receives a SIP message containing This document introduces a new logical role for SIP entities called a
an Identity header, it may inspect the signature to verify the 'verifier', which may be instantiated by a user agent or proxy
identity of the sender of the message. If an Identity header is not 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 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 example, based on a per-sending-domain policy, or a per-sending-user
per-sending-user policy), then a 428 'Use Identity Header' response policy), then a 428 'Use Identity Header' response MUST be sent.
MUST be sent.
In order to verify the identity of the sender of a message, the user In order to verify the identity of the sender of a message, an entity
agent or proxy server MUST first acquire the certificate for the acting as a verifier MUST perform the following steps, in the order
signing domain. Implementations supporting this specification should here specified.
have some means of retaining domain certificates (in accordance with
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 normal practices for certificate lifetimes and revocation) in order
to prevent themselves from needlessly downloading the same to prevent themselves from needlessly downloading the same
certificate every time a request from the same domain is received. certificate every time a request from the same domain is received.
Certificates retained in this manner should be indexed by the URI Certificates cached in this manner should be indexed by the URI given
given in the Identity-Info header field value. in the Identity-Info header field value.
Provided that the domain certificate used to sign this message is not Provided that the domain certificate used to sign this message is not
previously known to the recipient, SIP entities SHOULD discover this previously known to the recipient, SIP entities SHOULD discover this
certificate by dereferencing the Identity-Info header, unless they certificate by dereferencing the Identity-Info header, unless they
have some more efficient implementation-specific way of acquiring have some more efficient implementation-specific way of acquiring
certificates for that domain. If the URI scheme in the Identity-Info certificates for that domain. If the URI scheme in the Identity-Info
header cannot be dereferenced, then a 436 'Bad Identity-Info' header cannot be dereferenced, then a 436 'Bad Identity-Info'
response MUST be returned. The client processes this certificate in response MUST be returned. The client processes this certificate in
the usual ways, including checking that it has not expired, that the 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 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 revocation lists. Once the certificate is acquired, it MUST be
validated. If the certificate cannot be validated (it is self-signed validated. If the certificate cannot be validated (it is self-signed
and untrusted, or signed by an untrusted or unknown certificate and untrusted, or signed by an untrusted or unknown certificate
authority), the verifier MUST send a 437 'Unsupported Certificate' authority, expired, or revoked), the verifier MUST send a 437
response. 'Unsupported Certificate' response.
Subsequently, the recipient MUST verify the signature in the Identity Step 2: The verifier MUST compare the identity of the signer with the
header, and compare the identity of the signer (the subjectAltName of domain portion of the URI in the From header field. The verifier
the certificate) with the domain portion of the URI in the From MUST follow the process described in Section 14.4 to determine if the
header field of the request as described in Section 14. signer is authoritative for the URI in the From header field.
Additionally, the Date, Contact and Call-ID headers MUST be analyzed
in the manner described in Section 14; recipients that wish to verify
Identity signatures MUST support all of the operations described
there.
If a verifier determines that the signature on the message does not Step 3: The verifier MUST verify the signature in the Identity header
correspond to the text of the message, then a 428 'Invalid Identity field, following the procedures for generating the hashed digest-
Header' response MUST be returned. string described in Section 10. If a verifier determines that the
signature on the message does not correspond to the reconstructed
digest-string, then a 428 'Invalid Identity Header' response MUST be
returned.
Once the identity of the sender of a request has been ascertained, Step 4: The verifier MUST validate the Date, Contact and Call-ID
various policies MAY be used to make authorization decisions about headers the manner described in Section 14.1; recipients that wish to
accepting communications and the like. Such policies are outside the verify Identity signatures MUST support all of the operations
scope of this document. described there.
8. User Agent Behavior 8. Considerations for User Agent
This mechanism can be applied opportunistically to existing SIP This mechanism can be applied opportunistically to existing SIP
deployments; accordingly, it requires no change to SIP user agent deployments; accordingly, it requires no change to SIP user agent
behavior in order for it to be effective. However, because this behavior in order for it to be effective. However, because this
mechanism does not provide integrity protection between the UAC and mechanism does not provide integrity protection between the UAC and
the authentication service, a UAC SHOULD implement some means of the authentication service, a UAC SHOULD implement some means of
providing this integrity. TLS would be one such mechanism, which is providing this integrity. TLS would be one such mechanism, which is
attractive because it MUST be supported by SIP proxy servers, but is attractive because it MUST be supported by SIP proxy servers, but is
potentially problematic because it is a hop-by-hop mechanism. See potentially problematic because it is a hop-by-hop mechanism. See
Section 14 for more information about securing the channel between Section 14.3 for more information about securing the channel between
the UAC and the authentication service. the UAC and the authentication service.
When a UAC sends a request, it MUST accurately populate the header When a UAC sends a request, it MUST accurately populate the From
field that asserts its identity (for a SIP request, this is the From header field with a value corresponding to an identity that it
header field). In a request it MUST set the URI portion of its From believes it is authorized to claim. In a request it MUST set the URI
header to match a SIP, SIPS or TEL URI AoR under which the UAC can portion of its From header to match a SIP, SIPS or TEL URI AoR which
register (including anonymous URIs, as described in RFC 3323 [3]). it is authorized to use in the domain (including anonymous URIs, as
In general, UACs SHOULD NOT use the TEL URI form in the From header described in RFC 3323 [3]). In general, UACs SHOULD NOT use the TEL
field (see Section 12). URI form in the From header field (see Section 12).
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 The UAC MUST also be capable of sending requests, including mid-call
requests, through an 'outbound' proxy (the authentication service). requests, through an 'outbound' proxy (the authentication service).
The best way to accomplish this is using pre-loaded Route headers and The best way to accomplish this is using pre-loaded Route headers and
loose routing. UAC implementations MUST provide a way of loose routing. For a given domain, if an entity that can instantiate
provisioning pre-loaded Route headers in order for this mechanism to the authentication service role is not in the path of dialog-forming
work for mid-call requests in the backwards direction of a dialog. 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 As a recipient of a request, a user agent that can verify signed
identities should also support an appropriate user interface to identities should also support an appropriate user interface to
render the validity of identity to a user. User agent render the validity of identity to a user. User agent
implementations SHOULD differentiate signed From header field values implementations SHOULD differentiate signed From header field values
from unsigned From header field values when rendering to an end user from unsigned From header field values when rendering to an end user
the identity of the sender of a request. the identity of the sender of a request.
9. Proxy Server Behavior 9. Considerations for Proxy Server
Domain policy may require proxy servers to inspect and verify the Domain policy may require proxy servers to inspect and verify the
identity provided in SIP requests. A proxy server may wish to identity provided in SIP requests. A proxy server may wish to
ascertain the identity of the sender of the message to provide spam ascertain the identity of the sender of the message to provide spam
prevention or call control services. Even if a proxy server does not prevention or call control services. Even if a proxy server does not
act as an authentication service, it MAY verify the existence of an act as an authentication service, it MAY validate the Identity header
Identity before it makes a forwarding decision for a request. Proxy before it makes a forwarding decision for a request. Proxy servers
servers MUST NOT remove or modify an existing Identity or MUST NOT remove or modify an existing Identity or Identity-Info
Identity-Info header in a request. header in a request.
For the purposes of identifying mid-dialog requests, proxy servers
that instantiate the authentication service role MUST Record-Route
themselves in dialog-forming requests.
10. Header Syntax 10. Header Syntax
This document specifies two new SIP headers: Identity and This document specifies two new SIP headers: Identity and Identity-
Identity-Info. Each of these headers can appear only once in a SIP Info. Each of these headers can appear only once in a SIP message.
message. The grammar for these two headers is:
Identity = "Identity" HCOLON signed-identity-digest Identity = "Identity" HCOLON signed-identity-digest
signed-identity-digest = LDQUOT 32LHEX RDQUOT signed-identity-digest = LDQUOT 32LHEX RDQUOT
Identity-Info = "Identity-Info" HCOLON ident-info Identity-Info = "Identity-Info" HCOLON ident-info (* SEMI identi-info-params )
ident-info = LAQUOT absoluteURI RAQUOT 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 The signed-identity-digest is a signed hash of a canonical string
generated from certain components of a SIP request. To create the generated from certain components of a SIP request. To create the
contents of the signed-identity-digest, the following elements of a contents of the signed-identity-digest, the following elements of a
SIP message MUST placed in a bit-exact string in the order specified SIP message MUST placed in a bit-exact string in the order specified
here, separated by a colon: here, separated by a vertical line, "|" or %x7C, character:
o The AoR of the UA sending the message, or the 'identity field'. o The AoR of the UA sending the message, or addr-spec of the From
For a request, this is the addr-spec from the From header field. header field (referred to occasionally here as the 'identity
field').
o The addr-spec component of the To header field, which is the AoR o The addr-spec component of the To header field, which is the AoR
to which the request is being sent. to which the request is being sent.
o The callid from Call-Id header field. o The callid from Call-Id header field.
o The digit (1*DIGIT) and method (method) portions from CSeq header o The digit (1*DIGIT) and method (method) portions from CSeq header
field, separated by a single space (ABNF SP, or %x20). Note that field, separated by a single space (ABNF SP, or %x20). Note that
the CSeq header field allows LWS rather than SP to separate the the CSeq header field allows LWS rather than SP to separate the
digit and method portions, and thus the CSeq header field may need digit and method portions, and thus the CSeq header field may need
to be transformed in order to be canonicalized. The to be transformed in order to be canonicalized. The
authentication service MUST strip leading zeros from the 'digit' authentication service MUST strip leading zeros from the 'digit'
portion of the Cseq before generating the digest-string. portion of the Cseq before generating the digest-string.
o The Date header field, with exactly one space each for each SP and 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. The the weekday and month items case set as shown in BNF in 3261. RFC
first letter is upper case and the rest of the letters are lower 3261 specifies that the BNF for weekday and month are a choice
case. All requests that use the Identity mechanism MUST contain a amongst a set of tokens. The RFC 2234 rules for the BNF specify
Date header. 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 letter 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.
o The addr-spec component of the Contact header field value. If the o The addr-spec component of the Contact header field value. If the
request does not contain a Contact header, this field MUST be request does not contain a Contact header, this field MUST be
empty (i.e., there will be no whitespace between the fourth and empty (i.e., there will be no whitespace between the fourth and
fifth colons in the canonical string). fifth "|" characters in the canonical string).
o The body content of the message with the bits exactly as they are 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). Note that 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 the message-body does NOT include the CRLF separating the SIP
headers from the message-body, but does include everything that headers from the message-body, but does include everything that
follows that CRLF. If the message has no body, then message-body follows that CRLF. If the message has no body, then message-body
will be empty, and the final colon will not be followed by any will be empty, and the final "|" will not be followed by any
additional characters. additional characters.
For more information on the security properties of these headers, and For more information on the security properties of these headers, and
why their inclusion mitigates replay attacks, see Section 14 and [5]. why their inclusion mitigates replay attacks, see Section 14 and [5].
The precise formulation of this digest-string is, therefore The precise formulation of this digest-string is, therefore
(following the ABNF [6] in RFC3261): (following the ABNF [6] in RFC3261):
digest-string = addr-spec ":" addr-spec ":" callid ":" 1*DIGIT SP method ":" digest-string = addr-spec "|" addr-spec "|" callid "|" 1*DIGIT SP method "|"
SIP-Date ":" [ addr-spec ] ":" message-body SIP-Date "|" [ addr-spec ] "|" message-body
Note again that the first addr-spec MUST be taken from the From 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, the second addr-spec MUST be taken from the To
header field value, and the third addr-spec MUST be taken from the header field value, and the third addr-spec MUST be taken from the
Contact header field value, provided the Contact header is present in Contact header field value, provided the Contact header is present in
the request. the request.
After the digest-string is formed, it MUST be hashed and signed with After the digest-string is formed, it MUST be hashed and signed with
the certificate for the domain, as follows: compute the results of the certificate for the domain. The hashing and signing algorithm is
signing this string with sha1WithRSAEncryption as described in RFC specified by the 'alg' parameter of the Identity-Info header (see
3370 and base64 encode the results as specified in RFC 3548. Put the below for more information on Identity-Info header parameters). This
result in the Identity header. document defines only one value for the 'alg' parameter: 'rsa-sha1';
further values MUST be defined in a Standards Track RFC, see
Section 15.6 for more information. It is MANDATORY for all
implementations of this specification to 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 in placed int the Identity header field. For
detailed examples of the usage of this algorithm, see Section 11.
Note on this choice: Assuming a 1024 bit RSA key, the raw signature Note on the use of 'rsa-sha1': The raw signature will result in about
will result in about 170 octets of base64 encoded data (without 170 octets of base64 encoded data (without base64, as an aside, it
base64, as an aside, it would be about 130 bytes). For comparison's would be about 130 bytes). For comparison's sake, a typical HTTP
sake, a typical HTTP Digest Authorization header (such as those used Digest Authorization header (such as those used in RFC3261) with no
in RFC3261) with no cnonce is around 180 octets. From a speed point cnonce is around 180 octets. From a speed point of view, a 2.8GHz
of view, a 2.8GHz Intel processor does somewhere in the range of 250 Intel processor does somewhere in the range of 250 RSA 1024 bits
RSA 1024 bits signs per second or 1200 RSA 512 bits signs; verifies signs per second or 1200 RSA 512 bits signs; verifies are roughly 10
are roughly 10 times faster. Hardware accelerator cards are times faster. Hardware accelerator cards are available that speed
available that speed this up. this up.
The Identity-Info header MUST contain either an HTTPS URI or a SIPS The 'absoluteURI' portion of the Identity-Info header MUST contain
URI. If it contains an HTTPS URI, the URI must dereference to a either an HTTP or HTTPS URI which dereferences to a resource that
resource that contains a single MIME body containing the certificate contains a single MIME body containing the certificate of the
of the authentication service. If it is a SIPS URI, then the authentication service. These URIs MUST follow the conventions of
authentication service intends for a user agent that wishes to fetch RFC2585 [11] and the indicated resource MUST be of the form
the certificate to form a TLS connection to that URI, acquire the 'application/pkix-cert' described in that specification. Note that
certificate during normal TLS negotiation, and close the connection. 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 [1]: This document adds the following entries to Table 2 of [1]:
Header field where proxy ACK BYE CAN INV OPT REG Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- --- ------------ ----- ----- --- --- --- --- --- ---
Identity R a o o - o o - Identity R a o o - o o o
SUB NOT REF INF UPD PRA SUB NOT REF INF UPD PRA
--- --- --- --- --- --- --- --- --- --- --- ---
o o o o o o o o o o o o
Header field where proxy ACK BYE CAN INV OPT REG Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- --- ------------ ----- ----- --- --- --- --- --- ---
Identity-Info R a o o - o o - Identity-Info R a o o - o o o
SUB NOT REF INF UPD PRA SUB NOT REF INF UPD PRA
--- --- --- --- --- --- --- --- --- --- --- ---
o o o o o o o o o o o o
Note, in the table above, that this mechanism does not protect the Note, in the table above, that this mechanism does not protect the
REGISTER method or the CANCEL method. The CANCEL method cannot be CANCEL method. The CANCEL method cannot be challenged, because it is
challenged, because it is hop-by-hop, and accordingly authentication hop-by-hop, and accordingly authentication service behavior for
service behavior for CANCEL would be significantly limited. The CANCEL would be significantly limited. Note as well that the
REGISTER method uses Contact header fields in very unusual ways that REGISTER method uses Contact header fields in very unusual ways that
complicate its applicability to this mechanism. Accordingly, the complicate its applicability to this mechanism, and the use of
Identity and Identity-Info header MUST NOT appear in REGISTER or 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. CANCEL.
11. Compliance Tests and Examples 11. Compliance Tests and Examples
The examples in this section illustrate the use of the Identity The examples in this section illustrate the use of the Identity
header in the context of a SIP transaction. Implementations MUST header in the context of a SIP transaction. Implementers are advised
verify their compliance with these examples, i.e.: to verify their compliance with the specification against the
following criteria:
o Implementations of the authentication service role MUST generate o Implementations of the authentication service role MUST generate
identical base64 identity strings to the ones shown in the identical base64 identity strings to the ones shown in the
Identity headers in these examples when presented with the source Identity headers in these examples when presented with the source
message and utilizing the appropriate supplied private key for the message and utilizing the appropriate supplied private key for the
domain in question. domain in question.
o Implementations of the verifier role MUST correctly validate the o Implementations of the verifier role MUST correctly validate the
given messages containing the Identity header when utilizing the given messages containing the Identity header when utilizing the
supplied certificates (with the caveat about self-signed supplied certificates (with the caveat about self-signed
certificates below). certificates below).
Note that the following examples use self-signed certificates, rather Note that the following examples use self-signed certificates, rather
than certificates issued by a recognized certificate authority. The than certificates issued by a recognized certificate authority. The
use of self-signed certificates for this mechanism is NOT use of self-signed certificates for this mechanism is NOT
RECOMMENDED, and appear here only for illustrative purposes. RECOMMENDED, and it appears here only for illustrative purposes.
Therefore, in compliance testing, implementations of verifiers SHOULD Therefore, in compliance testing, implementations of verifiers SHOULD
generated appropriate warnings about the use of self-signed generated appropriate warnings about the use of self-signed
certificates. 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 14.4 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 Bit-exact reference files for these messages and their various
transformations are supplied in Appendix B. transformations are supplied in Appendix B.
11.1 Identity-Info with a Singlepart MIME body 11.1 Identity-Info with a Singlepart MIME body
Consider the following private key and certificate pair assigned to Consider the following private key and certificate pair assigned to
'atlanta.example.com'. 'atlanta.example.com'.
-----BEGIN RSA PRIVATE KEY----- -----BEGIN RSA PRIVATE KEY-----
skipping to change at page 17, line 34 skipping to change at page 18, line 34
When the authentication service receives the INVITE, in authenticates When the authentication service receives the INVITE, in authenticates
Alice by sending a 407 response. As a result, Alice adds an Alice by sending a 407 response. As a result, Alice adds an
Authorization header to her request, and resends to the Authorization header to her request, and resends to the
atlanta.example.com authentication service. Now that the service is atlanta.example.com authentication service. Now that the service is
sure of Alice's identity, it calculates an Identity header for the sure of Alice's identity, it calculates an Identity header for the
request. The canonical string over which the identity signature will request. The canonical string over which the identity signature will
be generated is the following (note that the first line wraps because be generated is the following (note that the first line wraps because
of RFC editorial conventions): 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 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 o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP s=Session SDP
c=IN IP4 pc33.atlanta.example.com c=IN IP4 pc33.atlanta.example.com
t=0 0 t=0 0
m=audio 49172 RTP/AVP 0 m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
The resulting signature (sha1WithRsaEncryption) using the private RSA The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above, with base64 encoding, is the following: key given above, with base64 encoding, is the following:
CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN NJguAbpmYXjnlxFmlOkumMI+MZXjB2iV/NW5xsFQqzD/p4yiovrJBqhd3TZkegns
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl moHryzk9gTBH7Gj/erixEFIf82o3Anmb+CIbrgdl03gGaD6ICvkpVqoMXZZjdvSp
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI= ycyHOhh1cmUx3b9Vr3pZuEh+cB01pbMQ8B1ch++iMjw=
Accordingly, the atlanta.example.com authentication service will Accordingly, the atlanta.example.com authentication service will
create an Identity header containing that base64 signature string create an Identity header containing that base64 signature string
(175 bytes). It will also add an HTTPS URL where its certificate is (175 bytes). It will also add an HTTPS URL where its certificate is
made available. With those two headers added, the message looks made available. With those two headers added, the message looks
like: like:
INVITE sip:bob@biloxi.exmple.org SIP/2.0 INVITE sip:bob@biloxi.exmple.org SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Bob <sip:bob@biloxi.example.org> To: Bob <sip:bob@biloxi.example.org>
From: Alice <sip:alice@atlanta.example.com>;tag=1928301774 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 314159 INVITE CSeq: 314159 INVITE
Max-Forwards: 70 Max-Forwards: 70
Date: Thu, 21 Feb 2002 13:02:03 GMT Date: Thu, 21 Feb 2002 13:02:03 GMT
Contact: <sip:alice@pc33.atlanta.example.com> Contact: <sip:alice@pc33.atlanta.example.com>
Identity: "CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2s Identity:"NJguAbpmYXjnlxFmlOkumMI+MZXjB2iV/NW5xsFQqzD/p4yiovrJBqhd3TZkegn
N3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGC smoHryzk9gTBH7Gj/erixEFIf82o3Anmb+CIbrgdl03gGaD6ICvkpVqoMXZZjdvS
lsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=" pycyHOhh1cmUx3b9Vr3pZuEh+cB01pbMQ8B1ch++iMjw="
Identity-Info: https://atlanta.example.com/cert Identity-Info: <https://atlanta.example.com/cert>;alg=rsa-sha1
Content-Type: application/sdp Content-Type: application/sdp
Content-Length: 147 Content-Length: 147
v=0 v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP s=Session SDP
c=IN IP4 pc33.atlanta.example.com c=IN IP4 pc33.atlanta.example.com
t=0 0 t=0 0
m=audio 49172 RTP/AVP 0 m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000 a=rtpmap:0 PCMU/8000
skipping to change at page 20, line 11 skipping to change at page 21, line 11
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 231 BYE CSeq: 231 BYE
Content-Length: 0 Content-Length: 0
When the authentication service receives the BYE, it authenticates When the authentication service receives the BYE, it authenticates
Bob by sending a 407 response. As a result, Bob adds an Bob by sending a 407 response. As a result, Bob adds an
Authorization header to his request, and resends to the Authorization header to his request, and resends to the
biloxi.example.org authentication service. Now that the service is biloxi.example.org authentication service. Now that the service is
sure of Bob's identity, it prepares to calculate an Identity header 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 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 field. Accordingly, the biloxi.example.org will add a Date header to
the request before calcuating the identity signature. If the the request before calculating the identity signature. If the
Content-Length header were not present, the authentication service Content-Length header were not present, the authentication service
would add it as well. The baseline message is thus: would add it as well. The baseline message is thus:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0 BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10 Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70 Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774 To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
skipping to change at page 20, line 33 skipping to change at page 21, line 33
Content-Length: 0 Content-Length: 0
Also note that this request contains no Contact header field. Also note that this request contains no Contact header field.
Accordingly, biloxi.example.org will place no value in the canonical Accordingly, biloxi.example.org will place no value in the canonical
string for the addr-spec of the Contact address. Also note that string for the addr-spec of the Contact address. Also note that
there is no message body, and accordingly, the signature string will there is no message body, and accordingly, the signature string will
terminate, in this case, with two colons. The canonical string over terminate, in this case, with two colons. The canonical string over
which the identity signature will be generated is the following (note which the identity signature will be generated is the following (note
that the first line wraps because of RFC editorial conventions): 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:: 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 The resulting signature (sha1WithRsaEncryption) using the private RSA
key given above for biloxi.example.org, with base64 encoding, is the key given above for biloxi.example.org, with base64 encoding, is the
following: following:
A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ kJl0ILrbzGtQX4zW4GlPo5DELq1hYXgfvI77xeQ1H7mXblNJBf6cLE0JAnRiDMp+
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B tbwSi9tj7JoknqeZAXtj5czqAKskj7axdYfe40basFy34HhNVc3WH2c3TwAlqbrm
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM= kspEbEWUnBnIRXjnihQ3Pi5rHwUVkKPdogI26IqRgQE=
Accordingly, the biloxi.example.org authentication service will Accordingly, the biloxi.example.org authentication service will
create an Identity header containing that base64 signature string. create an Identity header containing that base64 signature string.
It will also add an HTTPS URL where its certificate is made It will also add an HTTPS URL where its certificate is made
available. With those two headers added, the message looks like: available. With those two headers added, the message looks like:
BYE sip:alice@pc33.atlanta.example.com SIP/2.0 BYE sip:alice@pc33.atlanta.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10 Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70 Max-Forwards: 70
From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
To: Alice <sip:alice@atlanta.example.com>;tag=1928301774 To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
Date: Thu, 21 Feb 2002 14:19:51 GMT Date: Thu, 21 Feb 2002 14:19:51 GMT
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 231 BYE CSeq: 231 BYE
Identity: "A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlr Identity: "kJl0ILrbzGtQX4zW4GlPo5DELq1hYXgfvI77xeQ1H7mXblNJBf6cLE0JAnRiDMp
ZOuwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9 +tbwSi9tj7JoknqeZAXtj5czqAKskj7axdYfe40basFy34HhNVc3WH2c3TwAlqbr
BfxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=" mkspEbEWUnBnIRXjnihQ3Pi5rHwUVkKPdogI26IqRgQE="
Identity-Info: https://biloxi.example.org/cert Identity-Info: <https://biloxi.example.org/cert>;alg=rsa-sha1
Content-Length: 0 Content-Length: 0
biloxi.example.org then forwards the request normally. biloxi.example.org then forwards the request normally.
12. Identity and the TEL URI Scheme 12. Identity and the TEL URI Scheme
Since many SIP applications provide a VoIP service, telephone numbers Since many SIP applications provide a VoIP service, telephone numbers
are commonly used as identities in SIP deployments. In the majority are commonly used as identities in SIP deployments. In the majority
of cases, this is not problematic for the identity mechanism of cases, this is not problematic for the identity mechanism
described in this document. Telephone numbers commonly appear in the described in this document. Telephone numbers commonly appear in the
username portion of a SIP URI (e.g., username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com'). That username conforms to 'sip:+17005551008@chicago.example.com;user=phone'). That username
the syntax of the TEL URI scheme (RFC2806bis [9]). For this sort of conforms to the syntax of the TEL URI scheme (RFC3966 [12]). For
SIP address-of-record, chicago.example.com is the appropriate this sort of SIP address-of-record, chicago.example.com is the
signatory. appropriate signatory.
It is also possible for a TEL URI to appear in the SIP To or From 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., header field outside the context of a SIP or SIPS URI (e.g.,
'tel:+17005551008'). In this case, it is much less clear which 'tel:+17005551008'). In this case, it is much less clear which
signatory is appropriate for the identity. Fortunately for the signatory is appropriate for the identity. Fortunately for the
identity mechanism, this form of the TEL URI is more common 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, 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 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 remote domain to which a request is sent is unknown. The local
domain, however, is usually known by the UAC, and accordingly it can 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 form a proper From header field containing a SIP URI with a username
in TEL URI form. Implementations that intend to send their requests in TEL URI form. Implementations that intend to send their requests
through an authentication service MUST put telephone numbers in the through an authentication service SHOULD put telephone numbers in the
From header field into SIP or SIPS URIs, if possible. From header field into SIP or SIPS URIs whenever possible.
If the local domain is unknown to a UAC formulating a request, it 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 most likely will not be able to locate an authentication service for
its request, and therefore the question of providing identity in its request, and therefore the question of providing identity in
these cases is somewhat moot. However, an authentication service MAY these cases is somewhat moot. However, an authentication service MAY
sign a request containing a TEL URI in the From header field in sign a request containing a TEL URI in the From header field. This
accordance with its local policies. Verifiers SHOULD NOT accept is permitted in this specification strictly for forward compatibility
signatures over From header TEL URIs in the absence of some purposes. In the longer-term, it is possible that ENUM [13] may
pre-provisioned relationship with the signing domain that 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 provide a way to determine which administrative domain is responsible
for a telephone number, and this may aid in the signing and for a telephone number, and this may aid in the signing and
verification of SIP identities that contain telephone numbers. This verification of SIP identities that contain telephone numbers. This
is a subject for future work. is a subject for future work.
13. Privacy Considerations 13. Privacy Considerations
The identity mechanism presented in this draft is compatible with the The identity mechanism presented in this draft is compatible with the
standard SIP practices for privacy described in RFC3323 [3]. A SIP standard SIP practices for privacy described in RFC3323 [3]. A SIP
proxy server can act both as a privacy service and as an proxy server can act both as a privacy service and as an
authentication service. Since a user agent can provide any From authentication service. Since a user agent can provide any From
header field value which the authentication service is willing to header field value which the authentication service is willing to
authorize, there is no reason why private SIP URIs (e.g., authorize, there is no reason why private SIP URIs which contain
sip:anonymous@example.com) cannot be signed by an authentication legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
service. The construction of the Identity header is the same for by an authentication service. The construction of the Identity
private URIs as it is for any other sort of URIs. 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 Note, however, that an authentication service must possess a
certificate corresponding to the host portion of the addr-spec of the certificate corresponding to the host portion of the addr-spec of the
From header field of any request that it signs; accordingly, using From header field of any request that it signs; accordingly, using
domains like 'invalid.net' may not be possible for privacy services domains like 'invalid.net' will not be possible for privacy services
that also act as authentication services. The assurance offered by that also act as authentication services. The assurance offered by
this combination service is "this is a known user in my domain that I the usage of anonymous URIs with a valid domain portion is "this is a
have authenticated, but I am keeping their identity private". known user in my domain that I have authenticated, but I am keeping
their identity private". The use of the domain 'invalid.net' implies
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 The "header" level of privacy described in RFC3323 requests that a
privacy service to alter the Contact header field value of a SIP privacy service to alter the Contact header field value of a SIP
message. Since the Contact header field is protected by the message. Since the Contact header field is protected by the
signature in an Identity header, privacy services cannot be applied signature in an Identity header, privacy services cannot be applied
after authentication services without a resulting integrity after authentication services without a resulting integrity
violation. violation.
RFC3325 [8] defines the "id" priv-value token which is specific to RFC3325 [10] 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. The sort of assertion provided by
the P-Asserted-Identity header is very different from the Identity the P-Asserted-Identity header is very different from the Identity
header presented in this document. It contains additional header presented in this document. It contains additional
information about the sender of a message that may go beyond what information about the sender of a message that may go beyond what
appears in the From header field; P-Asserted-Identity holds a appears in the From header field; P-Asserted-Identity holds a
definitive identity for the sender which is somehow known to a closed definitive identity for the sender which is somehow known to a closed
network of intermediaries that presumably the network will use this network of intermediaries that presumably the network will use this
identity for billing or security purposes. The danger of this identity for billing or security purposes. The danger of this
network-specific information leaking outside of the closed network network-specific information leaking outside of the closed network
motivated the "id" priv-value token. The "id" priv-value token has motivated the "id" priv-value token. The "id" priv-value token has
no implications for the Identity header, and privacy services MUST no implications for the Identity header, and privacy services MUST
NOT remove the Identity header when a priv-value of "id" appears in a NOT remove the Identity header when a priv-value of "id" appears in a
Privacy header. Privacy header.
Finally, note that unlike RFC3325, the mechanism described in this
specification adds no information to SIP requests that has privacy
implications.
14. Security Considerations 14. Security Considerations
14.1 Handling of digest-string Elements
This document describes a mechanism which provides a signature over This document describes a mechanism which provides a signature over
the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
messages. While a signature over the From header field would be requests. While a signature over the From header field would be
sufficient to secure a URI alone, the additional headers provide sufficient to secure a URI alone, the additional headers provide
replay protection and reference integrity necessary to make sure that replay protection and reference integrity necessary to make sure that
the Identity header will not be used in cut-and-paste attacks. In the Identity header will not be used in cut-and-paste attacks. In
general, the considerations related to the security of these headers general, the considerations related to the security of these headers
are the same as those given in RFC3261 for including headers in are the same as those given in RFC3261 for including headers in
tunneled 'message/sip' MIME bodies (see Section 23 in particular). tunneled 'message/sip' MIME bodies (see Section 23 in particular).
The From header field indicates the identity of the sender of the 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 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. 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 The To header field provides the identity of the SIP user that this
request targets. Providing the To header field in the Identity request targets. Providing the To header field in the Identity
signature servers two purposes: first, it prevents replay attacks in signature serves two purposes: first, it prevents replay attacks in
which an Identity header from legitimate request for one user is which an Identity header from legitimate request for one user is cut-
cut-and-pasted into a request for a different user; second, it and-pasted into a request for a different user; second, it preserves
preserves the starting URI scheme of the request, which helps prevent the starting URI scheme of the request, which helps prevent downgrade
downgrade attacks against the use of SIPS. attacks against the use of SIPS.
The Date and Contact headers provide reference integrity and replay The Date and Contact headers provide reference integrity and replay
protection, as described in RFC3261 Section 23.4.2. Implementations protection, as described in RFC3261 Section 23.4.2. Implementations
of this specification MUST NOT deem valid a request with an outdated of this specification MUST NOT deem valid a request with an outdated
Date header field (the RECOMMENDED interval is that the Date header Date header field (the RECOMMENDED interval is that the Date header
must indicate a time within 3600 seconds of the receipt of a must indicate a time within 3600 seconds of the receipt of a
message). Implementations MUST also record Call-IDs received in message). Implementations MUST also record Call-IDs received in
valid requests containing an Identity header, and MUST remember those valid requests containing an Identity header, and MUST remember those
Call-IDs for at least the duration of a single Date interval (i.e. Call-IDs for at least the duration of a single Date interval (i.e.
commonly 3600 seconds). Accordingly, if an Identity header is commonly 3600 seconds). This result of this is that if an Identity
replayed within the Date interval, receivers will recognize that it header is replayed within the Date interval, verifiers will recognize
is invalid because of a Call-ID duplication; if an Identity header is that it is invalid because of a Call-ID duplication; if an Identity
replayed after the Date interval, receivers will recognize that it is header is replayed after the Date interval, verifiers will recognize
invalid because the Date is stale. The CSeq header field contains a that it is invalid because the Date is stale. The CSeq header field
numbered identifier for the transaction, and the name of the method contains a numbered identifier for the transaction, and the name of
of the request; without this information, an INVITE request could be the method of the request; without this information, an INVITE
cut-and-pasted by an attacker and transformed into a BYE request request could be cut-and-pasted by an attacker and transformed into a
without changing any fields covered by the Identity header, and BYE request without changing any fields covered by the Identity
moreover requests within a certain transaction could be replayed in header, and moreover requests within a certain transaction could be
potentially confusing or malicious ways. replayed in potentially confusing or malicious ways.
The Contact header field is included to tie the Identity header to a The Contact header field is included to tie the Identity header to a
particular device instance that generated the request. Were an particular user agent instance that generated the request. Were an
active attacker to intercept a request containing an Identity header, active attacker to intercept a request containing an Identity header,
and cut-and-paste the Identity header field into their own request and cut-and-paste the Identity header field into their own request
(reusing the From, To, Contact, Date and Call-ID fields that appear (reusing the From, To, Contact, Date and Call-ID fields that appear
in the original message), they would not be eligible to receive SIP in the original message), they would not be eligible to receive SIP
requests from the called user agent, since those requests are routed requests from the called user agent, since those requests are routed
to the URI identified in the Contact header field. However, the to the URI identified in the Contact header field. However, the
Contact header is only included in dialog-forming requests, so it Contact header is only included in dialog-forming requests, so it
does not provide this protection in all cases. does not provide this protection in all cases.
It might seem attractive to provide a signature over some of the It might seem attractive to provide a signature over some of the
skipping to change at page 24, line 31 skipping to change at page 25, line 35
to be routed to a host of the attacker's choosing. However, a to be routed to a host of the attacker's choosing. However, a
signature over the topmost Via header does not prevent attacks of signature over the topmost Via header does not prevent attacks of
this nature, since the attacker could leave the topmost Via intact this nature, since the attacker could leave the topmost Via intact
and merely insert a new Via header field directly after it, which 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 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. way" to the valid host, which has exactly the same end result.
Although it is possible that an intermediary-based authentication Although it is possible that an intermediary-based authentication
service could guarantee that no Via hops are inserted between the service could guarantee that no Via hops are inserted between the
sending user agent and the authentication service, it could not sending user agent and the authentication service, it could not
prevent an attacker from adding a Via hop after the authentication prevent an attacker from adding a Via hop after the authentication
service, and thereby pre-empting responses. It is necessary for the
service, and accordingly pre-empting responses. It is necessary for proper operation of SIP for subsequent intermediaries to be capable
the proper operation of SIP for subsequent intermediaries to be of inserting such Via header fields, and thus it cannot be prevented.
capable of inserting such Via header fields, and thus it cannot be As such, though it is desirable, securing Via is not possible through
prevented. As such, though it is desirable, securing Via is not the sort of identity mechanism described in this document; the best
possible through the sort of identity mechanism described in this known practice for securing Via is the use of SIPS.
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 This mechanism also provides a signature over the bodies of SIP
requests. The most important reason for doing so is to protect SDP requests. The most important reason for doing so is to protect SDP
bodies carried in SIP requests. There is little purpose in bodies carried in SIP requests. There is little purpose in
establishing the identity of the user agent that originated a SIP establishing the identity of the user that originated a SIP request
request if a man-in-the-middle can change the SDP and direct media to if this assurance is not coupled a comparable assurance over the
an different IP address. Note however that this is not perfect media descriptors. Note however that this is not perfect end-to-end
end-to-end security. The authentication service itself, when security. The authentication service itself, when instantiated at a
instantiated at a intermediary, could conceivably change the SDP (and intermediary, could conceivably change the SDP (and SIP headers, for
SIP headers, for that matter) before providing a signature. Thus, that matter) before providing a signature. Thus, while this
while this mechanism reduces the chance that a man-in-the-middle will mechanism reduces the chance that a replayer or man-in-the-middle
interfere with sessions, it does not eliminate it entirely. Since it will modify SDP, it does not eliminate it entirely. Since it is a
is a foundational assumption of this mechanism that the user trusts foundational assumption of this mechanism that the user trusts their
their local domain to vouch for their security, they must also trust local domain to vouch for their security, they must also trust the
the service not to violate the integrity of their message without service not to violate the integrity of their message without good
good reason. Note that RFC3261 16.6 states that SIP proxy servers reason. Note that RFC3261 16.6 states that SIP proxy servers "MUST
"MUST NOT add to, modify, or remove the message body." 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 merely result in a failure at the verifier. Numerous changes
that a man-in-the-middle might make would have the same effect. As
such, 'alg' does not seem to introduce any new security
considerations for this mechanism.
14.2 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 term 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 14.1.
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.
14.3 Securing the Connection to the Authentication Service
The assurance provided by this mechanism is strongest when a user The assurance provided by this mechanism is strongest when a user
agent forms a direct connection, preferably one secured by TLS, to an agent forms a direct connection, preferably one secured by TLS, to an
intermediary-based authentication service. The reasons for this are intermediary-based authentication service. The reasons for this are
twofold: twofold:
If a user does not receive a certificate from the authentication If a user does not receive a certificate from the authentication
service over this TLS connection that corresponds to the expected service over this TLS connection that corresponds to the expected
domain (especially when they receive a challenge via a mechanism domain (especially when they receive a challenge via a mechanism
such as Digest), then it is possible that a rogue server is such as Digest), then it is possible that a rogue server is
attempting to pose as a authentication service for a domain that attempting to pose as a authentication service for a domain that
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Without TLS, the various header field values and the body of the Without TLS, the various header field values and the body of the
request will not have integrity protection into the request request will not have integrity protection into the request
arrives at an authentication service. Accordingly, a prior arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message legitimate or illegitimate intermediary could modify the message
arbitrarily. arbitrarily.
Of these two concerns, the first is most material to the intended Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is likely that applications building on replay attacks. However, it is possible that applications relying on
the Identity header could leverage this integrity protection, the presence of the Identity header could leverage this integrity
especially body integrity, to provide further security services. protection, especially body integrity, for services other than replay
protection.
Accordingly, direct TLS connections SHOULD be used between the UAC Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly, UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS is not possible, a UAC should use when a direct connection and TLS is not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course, domain request lies with the authentication service, of course; domain
policy must identify those cases where the UAC's security association policy must identify those cases where the UAC's security association
with the authentication service is too weak. with the authentication service is too weak.
Ultimately, the worth of an assurance provided by an Identity header 14.4 Domain Names and Subordination
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 When a verifier processes a request containing an Identity-Info
(rather than individual certificates for each identity), certain header, it must compare the domain portion of the URI in the From
problems can arise with name subordination. For example, if an header field of the request with the domain name which is the subject
authentication service holds a common certificate for the hostname of the certificate acquired from the Identity-Info header. While it
'sip.atlanta.example.com', can it legitimately sign a token might seem that this should be a straightforward process, it is
containing an identity of 'sip:alice@atlanta.example.com'? It is complicated by two deployment realities. In the first place,
difficult for the recipient of a request to ascertain whether or not certificates have varying ways of describing their subjects, and may
'sip.atlanta.example.com' is authoritative for the indeed have multiple subjects, especially in 'virtual hosting' cases
'atlanta.example.com' domain unless the recipient has some where multiple domains are managed by a single application.
foreknowledge of the administration of 'atlanta.example.com'. Secondly, some SIP services may delegate SIP functions to a
Therefore, it is RECOMMENDED that UASs receiving signed requests subordinate domain and utilize the procedures in RFC3263 [4] allow
notify end users if there is ANY discrepancy between the requests for, say, 'example.com' to be routed to 'sip.example.com';
subjectAltName of the signer's certificate and the host portion of as a result, a user with the AoR 'sip:jon@example.com' may process
the identity within the From header field. If the domain name in the their requests through a host like 'sip.example.com', and it may be
subject of the certificate is subordinate to the domain name in the that latter host which acts as an authentication service.
identity URI, then verifiers may consider this a minor discrepancy.
Additionally, there are ways that a verifier might leverage the To meet the second of these problems, a domain that deploys an
information about canonical SIP servers within a domain stored in the authentication service on a subordinate host MUST be willing to
DNS (see RFC3263 [4]) to determine whether or not a particular supply that host with the private keying material associated with a
authentication service is authoritative for a domain; however, this certificate whose subject is a domain name that corresponds to the
is a subject for future work. 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 by 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 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 as follows:
First, a verifier must acquire a list of one or more domain names
which constitute the subject(s) of the certificate. A verifier MUST
extract the subject CN field from the certificate. If the CN
contains a domain name, it is added to a list we will call the
'subject list'. A verifier MUST also extract all subjectAltName
fields from the certificate. If any subjectAltName fields contain
domain names, these domain names should also be added to the subject
list.
Once it accumulates the subject list, the verifier MUST compare each
name in the subject list to the domain portion of the URI in the From
header field of the request. If the domain portion of that URI
matches any domain in the subject list, the verifier should consider
the certificate to match the URI in the From header field for the
purpose of verification.
If no member of the subject list matches the domain portion of the
URI in the From header field, then the verifier should consider the
certificate ineligible to sign the request.
Because the domain certificates that can be used by authentication Because the domain certificates that can be used by authentication
services need to assert only the hostname of the authentication services need to assert only the hostname of the authentication
service, existing certificate authorities can provide adequate service, existing certificate authorities can provide adequate
certificates for this mechanism. However, not all proxy servers and certificates for this mechanism. However, not all proxy servers and
user agents will be able support the root certificates of all user agents will be able support the root certificates of all
certificate authorities, and moreover there are some significant certificate authorities, and moreover there are some significant
differences in the policies by which certificate authorities issue differences in the policies by which certificate authorities issue
their certificates. This document makes no recommendations for the their certificates. This document makes no recommendations for the
usage of particular certificate authorities, nor does it describe any usage of particular certificate authorities, nor does it describe any
particular policies that certificate authorities should follow, but particular policies that certificate authorities should follow, but
it is anticipated that operational experience will create de facto it is anticipated that operational experience will create de facto
standards for authentication services. Some federations of service standards for authentication services. Some federations of service
providers, for example, might only trust certificates that have been providers, for example, might only trust certificates that have been
provided by a certificate authority operated by the federation. It provided by a certificate authority operated by the federation. It
is STRONGLY RECOMMENDED that self-signed domain certificates should is strongly RECOMMENDED that self-signed domain certificates should
not be trusted by verifiers, unless some pre-existing key exchange not be trusted by verifiers, unless some pre-existing key exchange
has justified such trust. has justified such trust.
Finally, the Identity and Identity-Info headers cannot protect 14.5 Authorization and Transitional Strategies
themselves. Any attacker could remove these headers from a SIP
request, and modify the request arbitrarily afterwards. Accordingly, Ultimately, the worth of an assurance provided by an Identity header
these headers are only truly efficacious if the would-be verifier is limited by the security practices of the domain that issues the
knows that they must be included in a request. In the long term, assurance. Relying on an Identity header generated by a remote
some sort of identity mechanism along these lines must become 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.
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 mandatory-to-use for the SIP protocol; that is the only way to
guarantee that this protection can always be expected. In the guarantee that this protection can always be expected by verifiers.
interim, however, identity reception policies at a domain level or an
address-book level should be used by verifiers to determine whether Finally, it is worth noting that the presence or absence of the
or not identity is expected from a particular source of SIP requests. Identity headers cannot be sole factor in making an authorization
Those authorization policies are outside the scope of this document. 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.
15. IANA Considerations 15. IANA Considerations
This document requests changes to the header and response-code This document requests changes to the header and response-code sub-
sub-registries of the SIP parameters IANA registry. registries of the SIP parameters IANA registry, and requests the
creation of two new registries for parameters for the Identity-Info
header.
15.1 Header Field Names 15.1 Header Field Names
This document specifies two new SIP headers: Identity and This document specifies two new SIP headers: Identity and Identity-
Identity-Info. Their syntax is given in Section 10. These headers Info. Their syntax is given in Section 10. These headers are
are defined by the following information, which is to be added to the defined by the following information, which is to be added to the
header sub-registry under header sub-registry under
http://www.iana.org/assignments/sip-parameters. http://www.iana.org/assignments/sip-parameters.
Header Name: Identity Header Name: Identity
Compact Form: y Compact Form: y
Header Name: Identity-Info Header Name: Identity-Info
Compact Form: (none) Compact Form: n
15.2 428 'Use Identity Header' Response Code 15.2 428 'Use Identity Header' Response Code
This document registers a new SIP response code which is described in This document registers a new SIP response code which is described in
Section 7. It is used when a verifier received a SIP request that Section 7. It is sent when a verifier receives a SIP request that
lacks an Identity header as a response indicating that the request lacks an Identity header in order to indicate that the request should
should be re-sent with an Identity header. This response code is be re-sent with an Identity header. This response code is defined by
defined by the following information, which is to be added to the the following information, which is to be added to the method and
method and response-code sub-registry under response-code sub-registry under
http://www.iana.org/assignments/sip-parameters. http://www.iana.org/assignments/sip-parameters.
Response Code Number: 428 Response Code Number: 428
Default Reason Phrase: Use Identity Header Default Reason Phrase: Use Identity Header
15.3 436 'Bad Identity-Info' Response Code 15.3 436 'Bad Identity-Info' Response Code
This document registers a new SIP response code which is described in This document registers a new SIP response code which is described in
Section 7. It is used when the Identity-Info header contains a URI Section 7. It is used when the Identity-Info header contains a URI
that cannot be dereferenced by the verifier (either the URI scheme is that cannot be dereferenced by the verifier (either the URI scheme is
unsupported by the verifier, or the resource designated by the URI is unsupported by the verifier, or the resource designated by the URI is
skipping to change at page 28, line 39 skipping to change at page 32, line 25
Section 7. It is used when the verifier cannot validate the Section 7. It is used when the verifier cannot validate the
certificate referenced by the URI of the Identity-Info header, certificate referenced by the URI of the Identity-Info header,
because, for example, the certificate is self-signed, or signed by a 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 authority for whom the verifier does not possess a
root certificate. This response code is defined by the following root certificate. This response code is defined by the following
information, which is to be added to the method and response-code information, which is to be added to the method and response-code
sub-registry under http://www.iana.org/assignments/sip-parameters. sub-registry under http://www.iana.org/assignments/sip-parameters.
Response Code Number: 437 Response Code Number: 437
Default Reason Phrase: Unsupported Certificate Default Reason Phrase: Unsupported Certificate
15.5 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.
15.6 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.
16. References 16. References
16.1 Normative References 16.1 Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement [2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997. levels", RFC 2119, March 1997.
[3] Peterson, J., "A Privacy Mechanism for the Session Initiation [3] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002. Protocol (SIP)", RFC 3323, November 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol [4] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002. (SIP): Locating SIP Servers", RFC 3263, June 2002.
[5] Peterson, J., "Session Initiation Protocol (SIP) Authenticated [5] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004. Identity Body (AIB) Format", RFC 3893, September 2004.
[6] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF", [6] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997. 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.
16.2 Informative References 16.2 Informative References
[7] Kohl, J. and C. Neumann, "The Kerberos Network Authentication [9] Kohl, J. and C. Neumann, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993. Service (V5)", RFC 1510, September 1993.
[8] Jennings, C., Peterson, J. and M. Watson, "Private Extensions [10] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002. within Trusted Networks", RFC 3325, November 2002.
[9] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966, [11] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC 2585,
May 1999.
[12] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966,
December 2004. December 2004.
[10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS [13] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS
Application", RFC 3761, April 2004. Application", RFC 3761, April 2004.
[14] Peterson, J., "Retargeting and Security in SIP: A Framework and
Requirements", draft-peterson-sipping-retarget-00 (work in
progress), February 2005.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
NeuStar, Inc. NeuStar, Inc.
1800 Sutter St 1800 Sutter St
Suite 570 Suite 570
Concord, CA 94520 Concord, CA 94520
US US
Phone: +1 925/363-8720 Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz Email: jon.peterson@neustar.biz
URI: http://www.neustar.biz/ URI: http://www.neustar.biz/
Cullen Jennings Cullen Jennings
Cisco Systems Cisco Systems
170 West Tasman Drive 170 West Tasman Drive
MS: SJC-21/2 MS: SJC-21/2
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone: +1 408 902-3341 Phone: +1 408 902-3341
EMail: fluffy@cisco.com Email: fluffy@cisco.com
Appendix A. Acknowledgments Appendix A. Acknowledgments
The authors would like to thank Eric Rescorla, Rohan Mahy, Robert The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell, Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell,
and Aki Niemi for their comments. The bit-archive presented in Aki Niemi, and Jim Schaad for their comments. Jonathan Rosenberg
Appendix B follows the pioneering example of Robert Sparks' provided detailed fixed to innumerable sections of the document. The
torture-test draft. bit-archive presented in Appendix B follows the pioneering example of
Robert Sparks' torture-test draft.
Appendix B. Bit-exact archive of example messages Appendix B. Bit-exact archive of example messages
The following text block is an encoded, gzip compressed TAR archive The following text block is an encoded, gzip compressed TAR archive
of files that represent the transformations performed on the example of files that represent the transformations performed on the example
messages discussed in Section 11. It includes for each example: messages discussed in Section 11. It includes for each example:
o (foo).message: the original message o (foo).message: the original message
o (foo).canonical: the canonical string constructed from that o (foo).canonical: the canonical string constructed from that
message message
o (foo).sha1: the SHA1 hash of the canonical string (hexadecimal) o (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
skipping to change at page 31, line 13 skipping to change at page 35, line 43
} }
print decode_base64($bdata); print decode_base64($bdata);
Alternatively, the base-64 encoded block can be edited by hand to Alternatively, the base-64 encoded block can be edited by hand to
remove document structure lines and fed as input to any base-64 remove document structure lines and fed as input to any base-64
decoding utility. decoding utility.
B.1 Encoded Reference Files B.1 Encoded Reference Files
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-- END MESSAGE ARCHIVE -- -- END MESSAGE ARCHIVE --
Appendix C. Changelog Appendix C. Changelog
NOTE TO THE RFC-EDITOR: Please remove this section prior to NOTE TO THE RFC-EDITOR: Please remove this section prior to
publication as an RFC. publication as an RFC.
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: Changes from draft-ietf-sip-identity-03:
- Softened requirement for TLS and direct connections; now - Softened requirement for TLS and direct connections; now SHOULD-
SHOULD-strength, SIPS and Digest auth-int listed as alternatives. strength, SIPS and Digest auth-int listed as alternatives.
- Added non-normative section about authentication service - Added non-normative section about authentication service
behavior for backwards-direction requests within a dialog behavior for backwards-direction requests within a dialog
- Added support for CID URI in Identity Info - Added support for CID URI in Identity Info
- Added new response codes (436 and 437) corresponding to error - Added new response codes (436 and 437) corresponding to error
cases for an unsupported URI scheme and an unsupported cases for an unsupported URI scheme and an unsupported
certificate, respectively certificate, respectively
Changes from draft-ietf-sip-identity-02: Changes from draft-ietf-sip-identity-02:
- Extracted text relating to providing identity in SIP responses; - Extracted text relating to providing identity in SIP responses;
this text will appear in a separate draft this text will appear in a separate draft
 End of changes. 

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