draft-ietf-sip-identity-03.txt   draft-ietf-sip-identity-04.txt 
SIP WG J. Peterson SIP WG J. Peterson
Internet-Draft NeuStar Internet-Draft NeuStar
Expires: March 30, 2005 C. Jennings Expires: August 17, 2005 C. Jennings
Cisco Systems Cisco Systems
September 29, 2004 February 16, 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-03 draft-ietf-sip-identity-04
Status of this Memo Status of this Memo
By submitting this Internet-Draft, I certify that any applicable By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed, patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with and any of which I become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 30, 2005. This Internet-Draft will expire on August 17, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2005). All Rights Reserved.
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 recommends practices and conventions for
identifying end users in SIP messages, and proposes a way to identifying end users in SIP messages, and proposes a way to
distribute cryptographically-secure authenticated identities. distribute cryptographically-secure authenticated identities.
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 . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6 5. Overview of Operations . . . . . . . . . . . . . . . . . . . 6
6. Authentication Service Behavior . . . . . . . . . . . . . . 7 6. Authentication Service Behavior . . . . . . . . . . . . . . 7
7. Verifying Identity . . . . . . . . . . . . . . . . . . . . . 9 6.1 Identity within a Dialog and Retargeting . . . . . . . . . 9
8. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 10 7. Verifying Identity . . . . . . . . . . . . . . . . . . . . . 11
9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . 10 8. User Agent Behavior . . . . . . . . . . . . . . . . . . . . 12
10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 11 9. Proxy Server Behavior . . . . . . . . . . . . . . . . . . . 12
11. Compliance Tests and Examples . . . . . . . . . . . . . . . 13 10. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . 13
11.1 Identity-Info with a Singlepart MIME body . . . . . . . 14 11. Compliance Tests and Examples . . . . . . . . . . . . . . . 15
11.2 Identity for a Request with no MIME body or Contact . . 16 11.1 Identity-Info with a Singlepart MIME body . . . . . . . 16
12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 19 11.2 Identity for a Request with no MIME body or Contact . . 18
13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20 12. Identity and the TEL URI Scheme . . . . . . . . . . . . . . 21
14. Security Considerations . . . . . . . . . . . . . . . . . . 21 13. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25 14. Security Considerations . . . . . . . . . . . . . . . . . . 23
15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 25 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . 27
15.2 Response Code . . . . . . . . . . . . . . . . . . . . . 25 15.1 Header Field Names . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26 15.2 428 'Use Identity Header' Response Code . . . . . . . . 27
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 26 15.3 436 'Bad Identity-Info' Response Code . . . . . . . . . 28
B. Bit-exact archive of example messages . . . . . . . . . . . 27 15.4 437 'Unsupported Certificate' Response Code . . . . . . 28
B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 27 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 30
16.1 Normative References . . . . . . . . . . . . . . . . . . . 25 B. Bit-exact archive of example messages . . . . . . . . . . . 30
16.2 Informative References . . . . . . . . . . . . . . . . . . 26 B.1 Encoded Reference Files . . . . . . . . . . . . . . . . . 31
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 30 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . 32 16.1 Normative References . . . . . . . . . . . . . . . . . . . 28
16.2 Informative References . . . . . . . . . . . . . . . . . . 29
C. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . 35
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 canonical SIP address-of-record URI employed to reach a
user (such as 'sip:alice@atlanta.example.com'). user (such as 'sip:alice@atlanta.example.com').
RFC3261 enumerates a number of places within a SIP request that a RFC3261 stipulates several places within a SIP request that a user
user can express an identity for themselves, notably the can express an identity for themselves, notably the user-populated
user-populated From header field. However, the recipient of a SIP From header field. However, the recipient of a SIP request has no
request has no way to verify that the From header field has been way to verify that the From header field has been populated
populated accurately, in the absence of some sort of cryptographic accurately, 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 TLS or S/MIME, 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 they have no previous association - just as in the destinations with which they have no previous association - just as
telephone network today, one can receive a call from someone with in the telephone network today, one can receive a call from someone
whom one has no previous association, and still have a reasonable with whom one has no previous association, and still have a
assurance that their displayed Caller-ID is accurate. reasonable assurance that their displayed Caller-ID is accurate.
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC2119 [2] and indicate requirement levels for described in RFC2119 [2] and indicate requirement levels for
compliant SIP implementations. compliant SIP implementations.
3. Background 3. Background
The usage of many SIP applications and services is governed by
authorization policies. These policies may be automated, or they may
be applied manually by humans. An example of the latter would be an
Internet telephone application which displays the "Caller-ID" of a
caller, which a human may review before answering a call. An example
of the former would be a presence service that compares the identity
of potential subscribers to a whitelist before determining whether it
should begin sending presence notifications. In both of these cases,
attackers might attempt to circumvent these authorization policies
through impersonation. Since the primary identifier of the sender of
a SIP request, the From header field, can be populated arbitrarily be
the controller of a user agent, impersonation is very simple today.
The mechanism described in this document aspires to provide a strong
identity system for SIP in which authorization policies cannot be
circumvented by impersonation.
All RFC3261-compliant SIP user agents support a means of All RFC3261-compliant SIP user agents support a means of
authenticating themselves to a SIP registrar, commonly with a shared authenticating themselves to a SIP registrar, commonly with a shared
secret; Digest authentication, which MUST be supported by SIP user secret; Digest authentication, which MUST be supported by SIP user
agents, is typically used for this purpose. Registration allows a agents, is typically used for this purpose. Registration allows a
user agent to express that it is the proper entity to which requests user agent to express that it is an appropriate entity to which
should be 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
a proof of the identity of the user to a registrar. However, the
credentials with which a user agent proves their identity to a
registrar cannot be validated by just any user agent or proxy server
- these credentials are only shared between the user agent and their
domain administrator. So this shared secret does not immediately
help a user to authenticate to a wide range of recipients.
Recipients require a means of determining whether or not the 'return
address' identity of a non-REGISTER request (i.e., the From header
field value) has legitimately been asserted.
The 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 of requests in order to
provide their 'return address' identity to recipients. If you can provide a 'return address' identity to recipients. The identity
prove you are eligible to register in a domain under a particular mechanism specified in this document derives from the following
address-of-record, you are proving that you are capable of principle: if you can prove you are eligible to register in a domain
legitimately receiving requests for that address-of-record, and under a particular address-of-record, you are proving that you are
accordingly, when you place that address-of-record in the From header capable of legitimately receiving requests for that
field of a SIP request other than a registration (like an INVITE), address-of-record, and accordingly, when you place that
you are providing a 'return address' where you can legitimately be address-of-record in the From header field of a SIP request other
reached. In other words, if you are authorized to receive requests than a registration (like an INVITE), you are providing a 'return
for that 'return address', you are also authorized to assert that address' where you can legitimately be reached. In other words, if
you are authorized to receive requests for that 'return address',
logically, it follows that you are also authorized to assert that
'return address' in your From header field. 'return address' in your From header field.
In the context of registration, users already have a means of proving
their identity to a registrar. However, the credentials with which a
user agent proves their identity to a registrar cannot be validated
by just any user agent or proxy server - these credentials are only
shared between the user agent and their domain administrator. For
the purposes of determining whether or not the 'return address' of a
request can legitimately be asserted in the From header field of a
request, SIP entities that are not operated by the domain
administrator require an assurance that the sender of a message is
capable of authenticating themselves to a registrar in their own
domain.
Ideally, then, SIP user agents should have some way of proving to 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. In the absence of end-user certificates in user agents, it is them and authorized the population of the From header field. This
possible to implement a mediated authentication architecture for SIP document proposes a mediated authentication architecture for SIP in
in which requests are sent to a server in the user's local domain which requests are sent to a server in the user's local domain, which
which authenticates such requests (using the same practices by which authenticates such requests (using the same practices by which the
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
authenticated. This draft addresses how that imprimatur of authenticated and their use of the From header field has been
authorized. This draft addresses how that imprimatur of
authentication can be shared. authentication can be shared.
RFC3261 already describes an architecture very similar to this in RFC3261 already describes an architecture very similar to this in
Section 26.3.2.2, in which a user agent authenticates itself to a Section 26.3.2.2, in which a user agent authenticates itself to a
local proxy server which in turn authenticates itself to a remote local proxy server which in turn authenticates itself to a remote
proxy server via mutual TLS, creating a two-link chain of transitive proxy server via mutual TLS, creating a two-link chain of transitive
authentication between the originator and the remote domain. While authentication between the originator and the remote domain. While
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. It also requires intermediaries to act becomes even less compelling.
as proxies, rather than redirecting requests to their destinations
(redirection lightens loads on SIP intermediaries).
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 [8]. However, this solution allows only
hop-by-hop trust between intermediaries, not end-to-end cryptographic hop-by-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
context based on the concept of an 'authentication service' and a new context which is based on the concept of an 'authentication service'
SIP header, the Identity header. Note that the scope of this and a new SIP header, the Identity header. Note that the scope of
document is limited to providing this identity assurance for SIP this document is limited to providing this identity assurance for SIP
requests; solving this problem for SIP responses is more complicated, requests; solving this problem for SIP responses is more complicated,
and is a subject for future work. 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, act as an authentication service. To maximize end-to-end security,
it is obviously preferable for end users to hold their own it is obviously preferable for end users to hold their own
certificates; if they do, they can act as an authentication service. certificates; if they do, they can act as an authentication service.
However, end-user certificates may be neither practical nor 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
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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 to provide a strong cryptographic
identity assurance in a request that can be verified by a proxy identity assurance in a request that can be verified by a proxy
server or UAS. 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 the Identity header in order for UAC is not required to support this mechanism in order for an
identity 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 The mechanism must be capable of protecting the integrity of SIP
message bodies (to ensure that media offers and answers are linked message bodies (to ensure that media offers and answers are linked
to the signaling identity). 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) under which it is known at a domain, and for
the UAC to assert one identity while authenticating itself as the UAC to assert one identity while authenticating itself as
another, related, identity, as permitted by the local policy of another, related, identity, as permitted by the local policy of
the domain. the domain.
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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 authenticate 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.
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, redirect server or a user agent. Any instantiated by a proxy server or a user agent. Any entity that
entity that instantiates the authentication service role MUST possess instantiates the authentication service role MUST possess the private
the private key of a domain certificate, and MUST be capable of key of a domain certificate, and MUST be capable of authenticating
authenticating one or more SIP users that can register in that one or more SIP users that can register in that domain. Commonly,
domain. Commonly, this role will be instantiated by a proxy server this role will be instantiated by a proxy server, since these
or redirect server, since these entities are more likely to have a entities are more likely to have a static hostname, hold a
static hostname, hold a corresponding certificate, and access to SIP corresponding certificate, and access to SIP registrar capabilities
registrar capabilities that allow them to authenticate users in their that allow them to authenticate users in their domain. It is also
domain. possible that the authentication service role might be instantiated
by a redirect server, but that is left as a topic for future work.
SIP entities that act as an authentication service MUST add a Date 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 The authentication service authenticates the identity of the message
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First, an authentication service MUST extract the identity of the First, an 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 MAY handle the request normally, but it MUST NOT add an question, it SHOULD process and forward the request normally, but it
Identity header; see below for more information on authentication MUST NOT add an Identity header; see below for more information on
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 Second, the authentication service needs to determine whether or not
the sender of the request is authorized to claim the identity given the sender of the request is authorized to claim the identity given
in the identity field. In order to do so, the authentication service in 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 o If the authentication service is instantiated by a SIP
intermediary (proxy or redirect server), it may challenge the intermediary (proxy server), it may challenge the request with a
request with a 407 response code using the Digest authentication 407 response code using the Digest authentication scheme (or
scheme (or viewing a Proxy-Authentication header sent in the viewing a Proxy-Authentication header sent in the request which
request which was sent in anticipation of a challenge using cached was sent in anticipation of a challenge using cached credentials,
credentials, as described in RFC 3261 Section 22.3). as described in RFC 3261 Section 22.3).
o If the authentication service is instantiated by a SIP user agent, o 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 assertion of a particular username in the From Authorization of the use of a particular username in the From header
header field of a SIP message is a matter of local policy for the field is a matter of local policy for the authorization service, one
authorization service which depends greatly on the manner in which which depends greatly on the manner in which authentication is
authentication is performed. A RECOMMENDED policy is as follows: the performed. For example, one policy might be as follows: the username
username asserted during Digest authentication MUST correspond given in the 'username' parameter of the Proxy-Authorization header
exactly to the username in the From header field of the SIP message. MUST correspond exactly to the username in the From header field of
However, there are many cases in which a user might manage multiple the SIP message. However, there are many cases in which this is too
accounts in the same administrative domain. Accordingly, provided limiting or inappropriate; a realm might use 'username' parameters in
the authentication service is aware of the relationships between Proxy-Authorization which do not correspond to the user-portion of
these accounts, it might allow a user providing credentials for one SIP From headers, or a user might manage multiple accounts in the
account to assert a username associated with another account same administrative domain. In this latter case, a domain might
controlled by the user name. Furthermore, if the AoR asserted in the maintain a mapping between the values in the 'username' parameter of
From header field is anonymous (per RFC3323 [3]), then the proxy Proxy-Authorization and a set of one or more SIP URIs which might
should authenticate that the user is a valid user in the domain and legitimately be asserted for that 'username'. In this instance,
insert the signature over the From header field as usual. another policy might be as follows: the URI in the From header field
MUST correspond exactly to one of the mapped URIs associated with the
'username' given in the Proxy-Authorization header. Various
exceptions to such policies might arise for cases like anonymity; if
the AoR asserted in the From header field is anonymous (per RFC3323
[3]), then the proxy should authenticate that the user is a valid
user in the domain and insert the signature over the From header
field as usual.
Note that this check is performed on the addr-spec in the From header 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-name portion of the From header field (e.g., 'Alice display-name portion of the From header field (e.g., 'Alice
Atlanta'). Some SIP user agents that receive requests render the Atlanta'). Some SIP user agents that receive requests render the
display-name of the caller as the identity of the caller. However, display-name of the caller as the identity of the caller. However,
there are many environments in which legislating the display-name there are many environments in which legislating the display-name
isn't feasible, judging from experience with email, where users isn't feasible, judging from experience with email, where users
frequent make slight textual changes to their display-names. frequent make slight textual changes to their display-names.
skipping to change at page 9, line 16 skipping to change at page 9, line 40
Third, the authentication service MUST form the identity signature Third, 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 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
Retargeting, the alteration by intermediaries of the Request-URI of a
SIP request, 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
is not identified by the URI designated in the To header field value.
The value in the To header field of a dialog-forming request is used
as the From header field of requests sent in the backwards direction
during the dialog, and is accordingly the header that would be signed
by an authentication service for requests sent in the backwards
direction. In retargeting cases, if the URI in the From header does
not identify the sender of the request in the backwards direction,
then clearly it would be inappropriate to provide an Identity
signature over that From header. As specified above, if the
authentication service is not responsible for the domain in the From
header field of the request, it must not add an Identity header to
the request, and should process/forward the request normally.
If there were a means in backwards-direction requests to signify a
'connected party', an identity of the unanticipated user whose SIP
endpoint was reached by the dialog-forming request, it isn't clear
that it would actually be beneficial to provide a corresponding
Identity header signature over that information. The Identity header
is designed to prevent impersonation-based attacks, and it is very
unclear how and why an attacker might attempt to impersonate an
unanticipated third party in a backwards-direction request within an
existing dialog. That is, it's unclear how the caller's potential
authorization policies would be any more successful at thwarting
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
scope of this document, and likely to have equal applicability to
response identity as it does to requests in the backwards direction
within a dialog. Consequently, no special 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. Verifying Identity
When a user agent or proxy server receives a SIP message containing When a user agent or proxy server receives a SIP message containing
an Identity header, it can inspect the signature to verify the an Identity header, it may inspect the signature to verify the
identity of the sender of the message. If an Identity header is not identity of the sender of the message. If an Identity header is not
present in a request, and one is required by local policy (for 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 global policy, a per-sending-domain policy, or a
per-sending-user policy), then a 428 'Use Identity Header' response per-sending-user 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, the user
agent or proxy server MUST first acquire the certificate for the agent or proxy server MUST first acquire the certificate for the
signing domain. Implementations supporting this specification should signing domain. Implementations supporting this specification should
have some means of retaining domain certificates (in accordance with 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 retained in this manner should be indexed by the URI
given in the Identity-Info header field value. given 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. The client processes this certificate certificates for that domain. If the URI scheme in the Identity-Info
in the usual ways, including checking that it has not expired, that header cannot be dereferenced, then a 436 'Bad Identity-Info'
the chain is valid back to a trusted CA, and that it does not appear response MUST be returned. The client processes this certificate in
on revocation lists. Once the certificate is acquired, it MUST be the usual ways, including checking that it has not expired, that the
validated. chain is valid back to a trusted CA, and that it does not appear on
revocation lists. Once the certificate is acquired, it MUST be
validated. If the certificate cannot be validated (it is self-signed
and untrusted, or signed by an untrusted or unknown certificate
authority), the verifier MUST send a 437 'Unsupported Certificate'
response.
Subsequently, the recipient MUST verify the signature in the Identity Subsequently, the recipient MUST verify the signature in the Identity
header, and compare the identity of the signer (the subjectAltName of header, and compare the identity of the signer (the subjectAltName of
the certificate) with the domain portion of the URI in the From the certificate) with the domain portion of the URI in the From
header field of the request as described in Section 14. header field of the request as described in Section 14.
Additionally, the Date, Contact and Call-ID headers MUST be analyzed Additionally, the Date, Contact and Call-ID headers MUST be analyzed
in the manner described in Section 14; recipients that wish to verify in the manner described in Section 14; recipients that wish to verify
Identity signatures MUST support all of the operations described Identity signatures MUST support all of the operations described
there. Any discrepancies or violations MUST be reported to the user. there.
If a verifier determines that the signature on the message does not If a verifier determines that the signature on the message does not
correspond to the text of the message, then a 428 'Invalid Identity correspond to the text of the message, then a 428 'Invalid Identity
Header' response MUST be returned. Header' response MUST be returned.
Once the identity of the sender of a request has been ascertained, Once the identity of the sender of a request has been ascertained,
various policies MAY be used to make authorization decisions about various policies MAY be used to make authorization decisions about
accepting communications and the like. Such policies are outside the accepting communications and the like. Such policies are outside the
scope of this document. scope of this document.
8. User Agent Behavior 8. User Agent Behavior
This mechanism requires one important change to existing user agent This mechanism can be applied opportunistically to existing SIP
requirements for sending requests: user agents using this mechanism deployments; accordingly, it requires no change to SIP user agent
to send requests to an authentication service MUST support TLS. behavior in order for it to be effective. However, because this
Because this mechanism does not provide integrity protection for the mechanism does not provide integrity protection between the UAC and
first hop to the authentication service, the UAC MUST send requests the authentication service, a UAC SHOULD implement some means of
to an authentication service only over a TLS connection. providing this integrity. TLS would be one such mechanism, which is
attractive because it MUST be supported by SIP proxy servers, but is
potentially problematic because it is a hop-by-hop mechanism. See
Section 14 for more information about securing the channel between
the UAC and the authentication service.
When a UAC sends a request, it MUST accurately populate the header When a UAC sends a request, it MUST accurately populate the header
field that asserts its identity (for a SIP request, this is the From field that asserts its identity (for a SIP request, this is the From
header field). In a request it MUST set the URI portion of its From header field). In a request it MUST set the URI portion of its From
header to match a SIP, SIPS or TEL URI AoR under which the UAC can header to match a SIP, SIPS or TEL URI AoR under which the UAC can
register (including anonymous URIs, as described in RFC 3323 [3]). register (including anonymous URIs, as described in RFC 3323 [3]).
In general, UACs SHOULD NOT use the TEL URI form in the From header In general, UACs SHOULD NOT use the TEL URI form in the From header
field (see Section 12). field (see Section 12).
The UAC MUST also be capable of sending requests, including mid-call The UAC MUST also be capable of sending requests, including mid-call
skipping to change at page 11, line 10 skipping to change at page 13, line 7
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 verify the existence of an
Identity before it makes a forwarding decision for a request. Proxy Identity before it makes a forwarding decision for a request. Proxy
servers MUST NOT remove or modify an existing Identity or servers MUST NOT remove or modify an existing Identity or
Identity-Info header in a request. Identity-Info 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-Info. Each of these headers can appear only once in a SIP Identity-Info. Each of these headers can appear only once in a SIP
message. message.
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
skipping to change at page 11, line 36 skipping to change at page 13, line 37
here, separated by a colon: here, separated by a colon:
o The AoR of the UA sending the message, or the 'identity field'. o The AoR of the UA sending the message, or the 'identity field'.
For a request, this is the addr-spec from the From header field. For a request, this is the addr-spec from the From header 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. to be transformed in order to be canonicalized. The
authentication service MUST strip leading zeros from the 'digit'
portion of the Cseq before generating the digest-string.
o The Date header field, with exactly one space each for each SP and 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. The
first letter is upper case and the rest of the letters are lower first letter is upper case and the rest of the letters are lower
case. All requests that use the Identity mechanism MUST contain a case. All requests that use the Identity mechanism MUST contain a
Date header. 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 colons 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
skipping to change at page 16, line 15 skipping to change at page 18, line 15
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+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN Identity: "CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2s
3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl N3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGC
sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI= lsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI="
Identity-Info: https://atlanta.example.com/cert Identity-Info: https://atlanta.example.com/cert
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
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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+lP8eoiXlrZ Identity: "A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlr
Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B ZOuwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9
fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM= BfxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM="
Identity-Info: https://biloxi.example.org/cert Identity-Info: https://biloxi.example.org/cert
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
skipping to change at page 21, line 12 skipping to change at page 23, line 12
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.
14. Security Considerations 14. Security Considerations
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 messages. 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
skipping to change at page 23, line 20 skipping to change at page 25, line 20
instantiated at a intermediary, could conceivably change the SDP (and instantiated at a intermediary, could conceivably change the SDP (and
SIP headers, for that matter) before providing a signature. Thus, SIP headers, for that matter) before providing a signature. Thus,
while this mechanism reduces the chance that a man-in-the-middle will while this mechanism reduces the chance that a man-in-the-middle will
interfere with sessions, it does not eliminate it entirely. Since it interfere with sessions, it does not eliminate it entirely. Since it
is a foundational assumption of this mechanism that the user trusts is a foundational assumption of this mechanism that the user trusts
their local domain to vouch for their security, they must also trust their local domain to vouch for their security, they must also trust
the service not to violate the integrity of their message without the service not to violate the integrity of their message without
good reason. Note that RFC3261 16.6 states that SIP proxy servers good reason. Note that RFC3261 16.6 states that SIP proxy servers
"MUST NOT add to, modify, or remove the message body." "MUST NOT add to, modify, or remove the message body."
Users SHOULD NOT provide credentials to an authentication service to The assurance provided by this mechanism is strongest when a user
which they cannot initiate a direct connection, preferably one agent forms a direct connection, preferably one secured by TLS, to an
secured by TLS. If a user does not receive a certificate from the intermediary-based authentication service. The reasons for this are
authentication service over this TLS connection that corresponds to twofold:
the expected domain (especially when they receive a challenge via a If a user does not receive a certificate from the authentication
mechanism such as Digest), then it is possible that a rogue server is service over this TLS connection that corresponds to the expected
attempting to pose as a authentication service for a domain that it domain (especially when they receive a challenge via a mechanism
does not control, possibly in an attempt to collect shared secrets such as Digest), then it is possible that a rogue server is
for that domain. If a user cannot connect directly to the desired attempting to pose as a authentication service for a domain that
authentication service, the user SHOULD at least use a SIPS URI to it does not control, possibly in an attempt to collect shared
ensure that mutual TLS authentication will be used to reach the secrets for that domain.
remote server. Without TLS, the various header field values and the body of the
request will not have integrity protection into the request
arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message
arbitrarily.
Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over
the header and bodies is provided by this mechanism only to prevent
replay attacks. However, it is likely that applications building on
the Identity header could leverage this integrity protection,
especially body integrity, to provide further security services.
Accordingly, direct TLS connections SHOULD be used between the UAC
and the authentication service whenever possible. The opportunistic
nature of this mechanism, however, makes it very difficult to
constrain UAC behavior, and moreover there will be some deployment
architectures where a direct connection is simply infeasible and the
UAC cannot act as an authentication service itself. Accordingly,
when a direct connection and TLS is not possible, a UAC should use
the SIPS mechanism, Digest 'auth-int' for body integrity, or both
when it can. The ultimate decision to add an Identity header to a
request lies with the authentication service, of course, domain
policy must identify those cases where the UAC's security association
with the authentication service is too weak.
Ultimately, the worth of an assurance provided by an Identity header Ultimately, the worth of an assurance provided by an Identity header
is limited by the security practices of the domain that issues the is limited by the security practices of the domain that issues the
assurance. Relying on an Identity header generated by a remote assurance. Relying on an Identity header generated by a remote
administrative domain assumes that the issuing domain uses some administrative domain assumes that the issuing domain uses some
trustworthy practice to authenticate its users. However, it is trustworthy practice to authenticate its users. However, it is
possible that some domains will implement policies that effectively possible that some domains will implement policies that effectively
make users unaccountable (such as accepting unauthenticated make users unaccountable (such as accepting unauthenticated
registrations from arbitrary users). The value of an Identity header registrations from arbitrary users). The value of an Identity header
from such domains is questionable. While there is no magic way for a from such domains is questionable. While there is no magic way for a
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problems can arise with name subordination. For example, if an problems can arise with name subordination. For example, if an
authentication service holds a common certificate for the hostname authentication service holds a common certificate for the hostname
'sip.atlanta.example.com', can it legitimately sign a token 'sip.atlanta.example.com', can it legitimately sign a token
containing an identity of 'sip:alice@atlanta.example.com'? It is containing an identity of 'sip:alice@atlanta.example.com'? It is
difficult for the recipient of a request to ascertain whether or not difficult for the recipient of a request to ascertain whether or not
'sip.atlanta.example.com' is authoritative for the 'sip.atlanta.example.com' is authoritative for the
'atlanta.example.com' domain unless the recipient has some 'atlanta.example.com' domain unless the recipient has some
foreknowledge of the administration of 'atlanta.example.com'. foreknowledge of the administration of 'atlanta.example.com'.
Therefore, it is RECOMMENDED that UASs receiving signed requests Therefore, it is RECOMMENDED that UASs receiving signed requests
notify end users if there is ANY discrepancy between the notify end users if there is ANY discrepancy between the
subjectAltName of the signers certificate and the identity within the subjectAltName of the signer's certificate and the host portion of
authentication token. Minor discrepancies MAY be characterized as a the identity within the From header field. If the domain name in the
warning. Additionally, relying parties MAY follow the procedures in subject of the certificate is subordinate to the domain name in the
RFC3263 [4] to look up in the DNS the domain portion of the identity identity URI, then verifiers may consider this a minor discrepancy.
in the From header field, and compare the SIP services listed for Additionally, there are ways that a verifier might leverage the
that domain with the subjectAltName of the certificate; this can give information about canonical SIP servers within a domain stored in the
the relying party a better sense of the canonical SIP services for DNS (see RFC3263 [4]) to determine whether or not a particular
that domain. authentication service is authoritative for a domain; however, this
is a subject for future work.
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. provided by a certificate authority operated by the federation. It
is STRONGLY RECOMMENDED that self-signed domain certificates should
not be trusted by verifiers, unless some pre-existing key exchange
has justified such trust.
Finally, the Identity and Identity-Info headers cannot protect Finally, the Identity and Identity-Info headers cannot protect
themselves. Any attacker could remove these headers from a SIP themselves. Any attacker could remove these headers from a SIP
request, and modify the request arbitrarily afterwards. Accordingly, request, and modify the request arbitrarily afterwards. Accordingly,
these headers are only truly efficacious if the would-be verifier these headers are only truly efficacious if the would-be verifier
knows that they must be included in a request. In the long term, knows that they must be included in a request. In the long term,
some sort of identity mechanism along these lines must become some sort of identity mechanism along these lines 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. In the
interim, however, identity reception policies at a domain level or an interim, however, identity reception policies at a domain level or an
skipping to change at page 25, line 22 skipping to change at page 27, line 49
This document specifies two new SIP headers: Identity and This document specifies two new SIP headers: Identity and
Identity-Info. Their syntax is given in Section 10. These headers Identity-Info. Their syntax is given in Section 10. These headers
are defined by the following information, which is to be added to the are 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: (none)
15.2 Response Code 15.2 428 'Use Identity Header' Response Code
This document registers one new SIP response code which is described This document registers a new SIP response code which is described in
in Section 7. This response codes is defined by the following Section 7. It is used when a verifier received a SIP request that
information, which is to be added to the method and response-code lacks an Identity header as a response indicating that the request
sub-registry under http://www.iana.org/assignments/sip-parameters. should be re-sent with an Identity header. This response code is
defined by the following information, which is to be added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.
Response Code Number: 428 Response Code Number: 428
Default Reason Phrase: Use Identity Header Default Reason Phrase: Use Identity Header
15.3 436 'Bad Identity-Info' Response Code
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
that cannot be dereferenced by the verifier (either the URI scheme is
unsupported by the verifier, or the resource designated by the URI is
otherwise unavailable). This response code is defined by the
following information, which is to be added to the method and
response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.
Response Code Number: 436
Default Reason Phrase: Bad Identity-Info
15.4 437 'Unsupported Certificate' Response Code
This document registers a new SIP response code which is described in
Section 7. It is used when the verifier cannot validate the
certificate referenced by the URI of the Identity-Info header,
because, for example, the certificate is self-signed, or signed by a
root certificate authority for whom the verifier does not possess a
root certificate. This response code is defined by the following
information, which is to be added to the method and response-code
sub-registry under http://www.iana.org/assignments/sip-parameters.
Response Code Number: 437
Default Reason Phrase: Unsupported Certificate
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.
skipping to change at page 26, line 14 skipping to change at page 29, line 23
16.2 Informative References 16.2 Informative References
[7] Kohl, J. and C. Neumann, "The Kerberos Network Authentication [7] 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 [8] 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", [9] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966,
draft-ietf-iptel-rfc2806bis-09 (work in progress), June 2004. December 2004.
[10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS [10] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS
Application", RFC 3761, April 2004. Application", RFC 3761, April 2004.
Authors' Addresses Authors' Addresses
Jon Peterson Jon Peterson
NeuStar, Inc. NeuStar, Inc.
1800 Sutter St 1800 Sutter St
Suite 570 Suite 570
skipping to change at page 26, line 47 skipping to change at page 30, line 9
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 and Patrik Faltstrom for their comments. The bit-archive Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell,
presented in Appendix B follows the pioneering example of Robert and Aki Niemi for their comments. The bit-archive presented in
Sparks' torture-test draft. 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 28, line 4 skipping to change at page 31, line 13
} }
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-03:
- Softened requirement for TLS and direct connections; now
SHOULD-strength, SIPS and Digest auth-int listed as alternatives.
- Added non-normative section about authentication service
behavior for backwards-direction requests within a dialog
- Added support for CID URI in Identity Info
- Added new response codes (436 and 437) corresponding to error
cases for an unsupported URI scheme and an unsupported
certificate, respectively
Changes from draft-ietf-sip-identity-02: 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
- Added compliance testing/example section - Added compliance testing/example section
- Added CSeq to the signature of the Identity header to prevent a - Added CSeq to the signature of the Identity header to prevent a
specific cut-and-paste attack; also added addr-spec of the To specific cut-and-paste attack; also added addr-spec of the To
header to the signature of the Identity header for similar reasons header to the signature of the Identity header for similar reasons
- Added text about why neither Via headers nor display-names are - Added text about why neither Via headers nor display-names are
protected by this mechanism protected by this mechanism
- Added bit-exact reference files for compliance testing - Added bit-exact reference files for compliance testing
skipping to change at page 32, line 41 skipping to change at page 35, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
 End of changes. 

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