draft-ietf-sip-identity-06.txt   rfc4474.txt 
SIP WG J. Peterson Network Working Group J. Peterson
Internet-Draft NeuStar Request for Comments: 4474 NeuStar
Expires: April 27, 2006 C. Jennings Category: Standards Track C. Jennings
Cisco Systems Cisco Systems
October 24, 2005 August 2006
Enhancements for Authenticated Identity Management in the Session
Initiation Protocol (SIP)
draft-ietf-sip-identity-06
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
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 (SIP) are inadequate for cryptographically assuring the identity of
users that originate SIP requests, especially in an interdomain the end users that originate SIP requests, especially in an
context. This document defines a mechanism for securely identifying interdomain context. This document defines a mechanism for securely
originators of SIP messages. It does so by defining two new SIP identifying originators of SIP messages. It does so by defining two
header fields, Identity, for conveying a signature used for new SIP header fields, Identity, for conveying a signature used for
validating the identity, and Identity-Info, for conveying a reference validating the identity, and Identity-Info, for conveying a reference
to the certificate of the signer. 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. Overview of Operations . . . . . . . . . . . . . . . . . . . . 6 4. Overview of Operations ..........................................6
5. Authentication Service Behavior . . . . . . . . . . . . . . . 7 5. Authentication Service Behavior .................................7
5.1. Identity within a Dialog and Retargeting . . . . . . . . . 9 5.1. Identity within a Dialog and Retargeting ..................10
6. Verifier Behavior . . . . . . . . . . . . . . . . . . . . . . 10 6. Verifier Behavior ..............................................11
7. Considerations for User Agent . . . . . . . . . . . . . . . . 11 7. Considerations for User Agent ..................................12
8. Considerations for Proxy Servers . . . . . . . . . . . . . . . 12 8. Considerations for Proxy Servers ...............................13
9. Header Syntax . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Header Syntax ..................................................13
10. Compliance Tests and Examples . . . . . . . . . . . . . . . . 15 10. Compliance Tests and Examples .................................16
10.1. Identity-Info with a Singlepart MIME body . . . . . . . . 16 10.1. Identity-Info with a Singlepart MIME body ................17
10.2. Identity for a Request with no MIME body or Contact . . . 19 10.2. Identity for a Request with No MIME Body or Contact ......20
11. Identity and the TEL URI Scheme . . . . . . . . . . . . . . . 22 11. Identity and the TEL URI Scheme ...............................22
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 23 12. Privacy Considerations ........................................23
13. Security Considerations . . . . . . . . . . . . . . . . . . . 24 13. Security Considerations .......................................24
13.1. Handling of digest-string Elements . . . . . . . . . . . . 24 13.1. Handling of digest-string Elements .......................24
13.2. Display Names and Identity . . . . . . . . . . . . . . . . 27 13.2. Display-Names and Identity ...............................27
13.3. Securing the Connection to the Authentication Service . . 28 13.3. Securing the Connection to the Authentication Service ....28
13.4. Domain Names and Subordination . . . . . . . . . . . . . . 28 13.4. Domain Names and Subordination ...........................29
13.5. Authorization and Transitional Strategies . . . . . . . . 30 13.5. Authorization and Transitional Strategies ................30
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 14. IANA Considerations ...........................................31
14.1. Header Field Names . . . . . . . . . . . . . . . . . . . . 31 14.1. Header Field Names .......................................31
14.2. 428 'Use Identity Header' Response Code . . . . . . . . . 31 14.2. 428 'Use Identity Header' Response Code ..................32
14.3. 436 'Bad Identity-Info' Response Code . . . . . . . . . . 31 14.3. 436 'Bad Identity-Info' Response Code ....................32
14.4. 437 'Unsupported Certificate' Response Code . . . . . . . 32 14.4. 437 'Unsupported Certificate' Response Code ..............32
14.5. 438 'Invalid Identity Header' Response Code . . . . . . . 32 14.5. 438 'Invalid Identity Header' Response Code ..............33
14.6. Identity-Info Parameters . . . . . . . . . . . . . . . . . 32 14.6. Identity-Info Parameters .................................33
14.7. Identity-Info Algorithm Parameter Values . . . . . . . . . 33 14.7. Identity-Info Algorithm Parameter Values .................33
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 33 Appendix A. Acknowledgements ......................................34
Appendix B. Bit-exact archive of example messages . . . . . . . . 33 Appendix B. Bit-Exact Archive of Examples of Messages .............34
B.1. Encoded Reference Files . . . . . . . . . . . . . . . . . 34 B.1. Encoded Reference Files ...................................35
Appendix C. Original Requirements . . . . . . . . . . . . . . . . 36 Appendix C. Original Requirements .................................38
Appendix D. Changelog . . . . . . . . . . . . . . . . . . . . . . 37 References ........................................................39
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Normative References ...........................................39
15.1. Normative References . . . . . . . . . . . . . . . . . . . 39 Informative References .........................................39
15.2. Informative References . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 42
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, RFC 3261 [1]). An identity, for the purposes of this document, (SIP, RFC 3261 [1]). An identity, for the purposes of this document,
is defined as a SIP URI, commonly a canonical address-of-record (AoR) is defined as a SIP URI, commonly a canonical address-of-record (AoR)
employed to reach a user (such as 'sip:alice@atlanta.example.com'). employed to reach a user (such as 'sip:alice@atlanta.example.com').
RFC3261 stipulates several places within a SIP request where 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
appropriately, 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 user agents (UAs), including Digest, Transport Layer
(implementations may support other security schemes as well). Security (TLS), and S/MIME (implementations may support other
However, few SIP user agents today support the end-user certificates security schemes as well). However, few SIP user agents today
necessary to authenticate themselves (via S/MIME, for example), and support the end-user certificates necessary to authenticate
furthermore Digest authentication is limited by the fact that the themselves (via S/MIME, for example), and furthermore Digest
originator and destination must share a pre-arranged secret. It is authentication is limited by the fact that the originator and
desirable for SIP user agents to be able to send requests to destination must share a prearranged secret. It is desirable for SIP
destinations with which they have no previous association - just as user agents to be able to send requests to destinations with which
in the telephone network today, one can receive a call from someone they have no previous association -- just as in the telephone network
with whom one has no previous association, and still have a today, one can receive a call from someone with whom one has no
reasonable assurance that their displayed Caller-ID is accurate. A previous association, and still have a reasonable assurance that the
cryptographic approach, like the one described in this document, can person's displayed Caller-ID is accurate. A cryptographic approach,
probably provide a much stronger and less-spoofable assurance of like the one described in this document, can probably provide a much
identity than the telephone network provides today. stronger and less-spoofable assurance of identity than the telephone
network provides today.
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 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 that 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 accept or reject the subscription. 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 by 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 user agents support Digest authentication, All RFC 3261-compliant user agents support Digest authentication,
which utilizes a shared secret, as a means for authenticating which utilizes a shared secret, as a means for authenticating
themselves to a SIP registrar. Registration allows a user agent to themselves to a SIP registrar. Registration allows a user agent to
express that it is an appropriate entity to which requests should be express that it is an appropriate entity to which requests should be
sent for a particular SIP AoR URI (e.g., sent for a particular SIP AoR URI (e.g.,
'sip:alice@atlanta.example.com'). '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 its identity to a credentials with which a user agent proves its 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 AoR URI used for registration is also the URI with which a UA The AoR URI used for registration is also the URI with which a UA
commonly populates the From header field of requests in order to commonly populates the From header field of requests in order to
provide a 'return address' identity to recipients. From an provide a 'return address' identity to recipients. From an
authorization perspective, if you can prove you are eligible to authorization perspective, if you can prove you are eligible to
skipping to change at page 5, line 11 skipping to change at page 5, line 10
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
authenticated and their use of the From header field has been authenticated and its use of the From header field has been
authorized. This draft addresses how that imprimatur of authorized. This document 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. 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 [12]. However, this solution allows only hop-by- header) is given in [12]. However, this solution allows only hop-
hop trust between intermediaries, not end-to-end cryptographic 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 or cryptographic assurance of end-user SIP identity in an interdomain or
intradomain context which is based on the concept of an intradomain context that is based on the concept of an
'authentication service' and a new SIP header, the Identity header. 'authentication service' and a new SIP header, the Identity header.
Note that the scope of this document is limited to providing this Note that the scope of this document is limited to providing this
identity assurance for SIP requests; solving this problem for SIP identity assurance for SIP requests; solving this problem for SIP
responses is more complicated, 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
provide identity services and to verify identities. To maximize end- provide identity services and to verify identities. To maximize
to-end security, it is obviously preferable for end users to acquire end-to-end security, it is obviously preferable for end-users to
their own certificates and corresponding private keys; if they do, acquire their own certificates and corresponding private keys; if
they can act as an authentication service. However, end-user they do, they can act as an authentication service. However, end-
certificates may be neither practical nor affordable, given the user certificates may be neither practical nor affordable, given the
difficulties of establishing a PKI that extends to end users, and difficulties of establishing a Public Key Infrastructure (PKI) that
moreover, given the potentially large number of SIP user agents extends to end-users, and moreover, given the potentially large
(phones, PCs, laptops, PDAs, gaming devices) that may be employed by number of SIP user agents (phones, PCs, laptops, PDAs, gaming
a single user. In such environments, synchronizing keying material devices) that may be employed by a single user. In such
across multiple devices may be very complex, and requires quite a environments, synchronizing keying material across multiple devices
good deal of additional endpoint behavior. Managing several may be very complex and requires quite a good deal of additional
certificates for the various devices is also quite problematic and endpoint behavior. Managing several certificates for the various
unpopular with users. Accordingly, in the initial use of this devices is also quite problematic and unpopular with users.
mechanism, it is likely that intermediaries will instantiate the Accordingly, in the initial use of this mechanism, it is likely that
authentication service role. intermediaries will instantiate the authentication service role.
4. Overview of Operations 4. 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 assert the identity which is populated in the From header field. to assert the identity that is populated in the From header field.
This value may be Alice's AoR, or it may be some other value that the This value may be Alice's AoR, or it may be some other value that the
policy of the proxy server permits her to use. It then computes a policy of the proxy server permits her to use. It then computes a
hash over some particular headers, including the From header field hash over some particular headers, including the From header field
and the bodies in the message. This hash is signed with the and the bodies in the message. This hash is signed with the
certificate for the domain (example.com, in Alice's case) and certificate for the domain (example.com, in Alice's case) and
inserted in a new header field in the SIP message, the 'Identity' inserted in a new header field in the SIP message, the 'Identity'
header. header.
The proxy, as the holder of the private key of its domain, is The proxy, as the holder of the private key of its domain, is
asserting that the originator of this request has been authenticated asserting that the originator of this request has been authenticated
and that she is authorized to claim the identity (the SIP address-of- and that she is authorized to claim the identity (the SIP address-
record) which appears in the From header field. The proxy also of-record) that appears in the From header field. The proxy also
inserts a companion header field, Identity-Info, that tells Bob how inserts a companion header field, Identity-Info, that tells Bob how
to acquire its certificate, if he doesn't already have it. to acquire 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 validates that the provided in the Identity header, and thus can validate 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 the user to assert that
header field value. This same validation operation may be performed From header field value. This same validation operation may be
by Bob's UAS. performed by Bob's user agent server (UAS).
5. Authentication Service Behavior 5. 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. Intermediaries that instantiate this
one or more SIP users that can register in that domain. Commonly, role MUST be capable of authenticating one or more SIP users that can
this role will be instantiated by a proxy server, since these register in that domain. Commonly, this role will be instantiated by
entities are more likely to have a static hostname, hold a a proxy server, since these entities are more likely to have a static
corresponding certificate, and have access to SIP registrar hostname, hold a corresponding certificate, and have access to SIP
capabilities that allow them to authenticate users in their domain. registrar capabilities that allow them to authenticate users in their
It is also possible that the authentication service role might be domain. It is also possible that the authentication service role
instantiated by an entity that acts as a redirect server, but that is might be instantiated by an entity that acts as a redirect server,
left as a topic for future work. 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 (see header field to SIP requests if one is not already present (see
Section 9 for information on how the Date header field assist Section 9 for information on how the Date header field assists
verifiers). Similarly, authentication services MUST add a Content- verifiers). Similarly, authentication services MUST add a Content-
Length header field to SIP requests if one is not already present; Length header field to SIP requests if one is not already present;
this can help the verifier to double-check that they are hashing this can help verifiers to double-check that they are hashing exactly
exactly as many bytes of message-body as the authentication service as many bytes of message-body as the authentication service when they
when they verify the message. verify the message.
Entities instantiating the authentication service role performs the Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header for a SIP following steps, in order, to generate an Identity header for a SIP
request: request:
Step 1: The authentication service MUST extract the identity of the Step 1:
sender from the request. The authentication service takes this value
from the From header field; this AoR will be referred to here as the The authentication service MUST extract the identity of the sender
'identity field'. If the identity field contains a SIP or SIPS URI, from the request. The authentication service takes this value from
the authentication service MUST extract the hostname portion of the the From header field; this AoR will be referred to here as the
identity field and compare it to the domain(s) for which it is 'identity field'. If the identity field contains a SIP or SIP Secure
responsible (following the procedures in RFC3261 16.4 used by a proxy (SIPS) URI, the authentication service MUST extract the hostname
server to determine the domain(s) for which it is responsible). If portion of the identity field and compare it to the domain(s) for
the identity field uses the TEL URI scheme, the policy of the which it is responsible (following the procedures in RFC 3261,
authentication service determines whether or not it is responsible Section 16.4, used by a proxy server to determine the domain(s) for
for this identity; see Section 11 for more information. If the which it is responsible). If the identity field uses the TEL URI
authentication service is not responsible for the identity in scheme, the policy of the authentication service determines whether
question, it SHOULD process and forward the request normally, but it or not it is responsible for this identity; see Section 11 for more
MUST NOT add an Identity header; see below for more information on information. If the authentication service is not responsible for
authentication service handling of an existing Identity header. the identity in question, it SHOULD process and forward the request
normally, but it MUST NOT add an Identity header; see below for more
information on authentication service handling of an existing
Identity header.
Step 2:
The authentication service MUST determine whether or not the sender
of the request is authorized to claim the identity given in the
identity field. In order to do so, the authentication service MUST
authenticate the sender of the message. Some possible ways in which
this authentication might be performed include:
Step 2: The authentication service MUST determine whether or not the
sender of the request is authorized to claim the identity given in
the identity field. In order to do so, the authentication service
MUST authenticate the sender of the message. Some possible ways in
which this authentication might be performed include:
If the authentication service is instantiated by a SIP 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
407 response code using the Digest authentication scheme (or a 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,
was sent in anticipation of a challenge using cached credentials, which was sent in anticipation of a challenge using cached
as described in RFC 3261 Section 22.3). Note that if that proxy credentials, as described in RFC 3261, Section 22.3). Note
server is maintaining a TLS connection with the client over which that if that proxy server is maintaining a TLS connection with
the client had previously authenticated itself using Digest the client over which the client had previously authenticated
authentication, the identity value obtained from that previous itself using Digest authentication, the identity value obtained
authentication step can be reused without an additional Digest from that previous authentication step can be reused without an
challenge. 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 If the authentication service is instantiated by a SIP user
that the user can provision the user agent with the private key of agent, a user agent can be said to authenticate its user on the
the domain, or preferably by providing a password that unlocks grounds that the user can provision the user agent with the
said private key. private key of the domain, or preferably by providing a
password that unlocks said private key.
Authorization of the use of a particular username in the From header Authorization of the use of a particular username in the From header
field is a matter of local policy for the authentication service, one field is a matter of local policy for the authentication service, one
which depends greatly on the manner in which authentication is that 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 that do not correspond to the user-portion of SIP
SIP From headers, or a user might manage multiple accounts in the From headers, or a user might manage multiple accounts in the same
same administrative domain. In this latter case, a domain might administrative domain. In this latter case, a domain might maintain
maintain a mapping between the values in the 'username' parameter of a mapping between the values in the 'username' parameter of Proxy-
Proxy-Authorization and a set of one or more SIP URIs which might Authorization and a set of one or more SIP URIs that might
legitimately be asserted for that 'username'. For example, the legitimately be asserted for that 'username'. For example, the
username can correspond to the 'private identity' as defined in 3GPP, username can correspond to the 'private identity' as defined in Third
in which case the From header field can contain any one of the public Generation Partnership Project (3GPP), in which case the From header
identities associated with this private identity. In this instance, field can contain any one of the public identities associated with
another policy might be as follows: the URI in the From header field this private identity. In this instance, another policy might be as
MUST correspond exactly to one of the mapped URIs associated with the follows: the URI in the From header field MUST correspond exactly to
'username' given in the Proxy-Authorization header. Various one of the mapped URIs associated with the 'username' given in the
exceptions to such policies might arise for cases like anonymity; if Proxy-Authorization header. Various exceptions to such policies
the AoR asserted in the From header field uses a form like might arise for cases like anonymity; if the AoR asserted in the From
'sip:anonymous@example.com', then the 'example.com' proxy should header field uses a form like 'sip:anonymous@example.com', then the
authenticate that the user is a valid user in the domain and insert 'example.com' proxy should authenticate that the user is a valid user
the signature over the From header field as usual. 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 display- 'sip:alice@atlanta.example.com'); it does not convert the display-
name portion of the From header field (e.g., 'Alice Atlanta'). name portion of the From header field (e.g., 'Alice Atlanta').
Authentication services MAY check and validate the display-name as Authentication services MAY check and validate the display-name as
well, and compare it to a list of acceptable display-names that may 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 be used by the sender; if the display-name does not meet policy
constraints, the authentication service MUST return a 403 response constraints, the authentication service MUST return a 403 response
code with the reason phrase "Inappropriate Display-Name". However, code. The reason phrase should indicate the nature of the problem;
the display-name is not always present, and in many environments the for example, "Inappropriate Display Name". However, the display-name
requisite operational procedures for display-name validation may not is not always present, and in many environments the requisite
exist. For more information, see Section 13.2. operational procedures for display-name validation may not exist.
For more information, see Section 13.2.
Step 3: The authentication service SHOULD ensure that any pre- Step 3:
existing Date header in the request is accurate. Local policy can
dictate precisely how accurate the Date must be, a RECOMMENDED The authentication service SHOULD ensure that any preexisting Date
maximum discrepancy of ten minutes will ensure that the request is header in the request is accurate. Local policy can dictate
unlikely to upset any verifiers. If the Date header contains a time precisely how accurate the Date must be; a RECOMMENDED maximum
different by more than ten minutes from the current time noted by the 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 authentication service, the authentication service SHOULD reject the
request. This behavior is not mandatory because a user agent client request. This behavior is not mandatory because a user agent client
could only exploit the Date header in order to cause a request to (UAC) could only exploit the Date header in order to cause a request
fail verification; the Identity header is not intended to provide a to fail verification; the Identity header is not intended to provide
source of non-repudiation or a perfect record of when messages are a source of non-repudiation or a perfect record of when messages are
processed. Finally, the authentication service MUST verify that the processed. Finally, the authentication service MUST verify that the
Date header falls within the validity period of its certificate. For Date header falls within the validity period of its certificate. For
more information on the security properties associated with the Date more information on the security properties associated with the Date
header field value, see Section 9. header field value, see Section 9.
Step 4: The authentication service MUST form the identity signature Step 4:
and add an Identity header to the request containing this signature.
After the Identity header has been added to the request, the The authentication service MUST form the identity signature and add
authentication service MUST also add an Identity-Info header. The an Identity header to the request containing this signature. After
Identity-Info header contains a URI from which its certificate can be the Identity header has been added to the request, the authentication
acquired. Details on the generation of both of these headers are service MUST also add an Identity-Info header. The Identity-Info
provided in section Section 9. header contains a URI from which its certificate can be acquired.
Details on the generation of both of these headers are provided in
Section 9.
Finally, the authentication service MUST forward the message Finally, the authentication service MUST forward the message
normally. normally.
5.1. Identity within a Dialog and Retargeting 5.1. Identity within a Dialog and Retargeting
Retargeting is broadly defined as the alteration of the Request-URI Retargeting is broadly defined as the alteration of the Request-URI
by intermediaries. More specifically, retargeting supplants the by intermediaries. More specifically, retargeting supplants the
original target URI with one that corresponds to a different user, a original target URI with one that corresponds to a different user, a
user that is not authorized to register under the original target user that is not authorized to register under the original target
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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 it should process/forward the request normally.
Any means of anticipating retargeting and so on is outside the scope Any means of anticipating retargeting, and so on, is outside the
of this document, and likely to have equal applicability to response scope of this document, and likely to have equal applicability to
identity as it does to requests in the backwards direction within a response identity as it does to requests in the backwards direction
dialog. Consequently, no special guidance is given for implementers within a dialog. Consequently, no special guidance is given for
here regarding the 'connected party' problem; authentication service implementers here regarding the 'connected party' problem;
behavior is unchanged if retargeting has occurred for a dialog- authentication service behavior is unchanged if retargeting has
forming request. Ultimately, the authentication service provides an occurred for a dialog-forming request. Ultimately, the
Identity header for requests in the backwards dialog when the user is authentication service provides an Identity header for requests in
authorized to assert the identity given in the From header field, and the backwards dialog when the user is authorized to assert the
if they are not, an Identity header is not provided. identity given in the From header field, and if they are not, an
Identity header is not provided.
For further information on the problems of response identity and the For further information on the problems of response identity and the
potential solution spaces, see [15]. potential solution spaces, see [15].
6. Verifier Behavior 6. Verifier Behavior
This document introduces a new logical role for SIP entities called a This document introduces a new logical role for SIP entities called a
'verifier', which may be instantiated by a user agent or proxy
server. When a verifier receives a SIP message containing an server. When a verifier receives a SIP message containing an
Identity header, it may inspect the signature to verify the identity Identity header, it may inspect the signature to verify the identity
of the sender of the message. Typically, the results of a of the sender of the message. Typically, the results of a
verification are provided as input to an authorization process which verification are provided as input to an authorization process that
is outside the scope of this document. If an Identity header is not 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 per-sending-domain policy, or a per-sending-user example, based on a per-sending-domain policy, or a per-sending-user
policy), then a 428 'Use Identity Header' response MUST be sent. policy), then a 428 'Use Identity Header' response MUST be sent.
In order to verify the identity of the sender of a message, an entity In order to verify the identity of the sender of a message, an entity
acting as a verifier MUST perform the following steps, in the order acting as a verifier MUST perform the following steps, in the order
here specified. here specified.
Step 1: The verifier MUST acquire the certificate for the signing Step 1:
domain. Implementations supporting this specification SHOULD have
some means of retaining domain certificates (in accordance with The verifier MUST acquire the certificate for the signing domain.
normal practices for certificate lifetimes and revocation) in order Implementations supporting this specification SHOULD have some means
to prevent themselves from needlessly downloading the same of retaining domain certificates (in accordance with normal practices
certificate every time a request from the same domain is received. for certificate lifetimes and revocation) in order to prevent
Certificates cached in this manner should be indexed by the URI given themselves from needlessly downloading the same certificate every
in the Identity-Info header field value. time a request from the same domain is received. Certificates cached
in this manner should be indexed by the URI given in the Identity-
Info header field value.
Provided that the domain certificate used to sign this message is not 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 verifier, 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 verifier processes this certificate
the usual ways, including checking that it has not expired, that the in the usual ways, including checking that it has not expired, that
chain is valid back to a trusted CA, and that it does not appear on the chain is valid back to a trusted certification authority (CA),
revocation lists. Once the certificate is acquired, it MUST be and that it does not appear on revocation lists. Once the
validated following the procedures in RFC3280 [9]. If the certificate is acquired, it MUST be validated following the
certificate cannot be validated (it is self-signed and untrusted, or procedures in RFC 3280 [9]. If the certificate cannot be validated
signed by an untrusted or unknown certificate authority, expired, or (it is self-signed and untrusted, or signed by an untrusted or
revoked), the verifier MUST send a 437 'Unsupported Certificate' unknown certificate authority, expired, or revoked), the verifier
response. MUST send a 437 'Unsupported Certificate' response.
Step 2: The verifier MUST follow the process described in Step 2:
Section 13.4 to determine if the signer is authoritative for the URI
in the From header field.
Step 3: The verifier MUST verify the signature in the Identity header The verifier MUST follow the process described in Section 13.4 to
field, following the procedures for generating the hashed digest- determine if the signer is authoritative for the URI in the From
string described in Section 9. If a verifier determines that the header field.
signature on the message does not correspond to the reconstructed
digest-string, then a 438 'Invalid Identity Header' response MUST be Step 3:
The verifier MUST verify the signature in the Identity header field,
following the procedures for generating the hashed digest-string
described in Section 9. If a verifier determines that the signature
on the message does not correspond to the reconstructed digest-
string, then a 438 'Invalid Identity Header' response MUST be
returned. returned.
Step 4: The verifier MUST validate the Date, Contact and Call-ID Step 4:
headers in the manner described in Section 13.1; recipients that wish
to verify Identity signatures MUST support all of the operations The verifier MUST validate the Date, Contact, and Call-ID headers in
described there. It must furthermore ensure that the value of the the manner described in Section 13.1; recipients that wish to verify
Date header falls within the validity period of the certificate whose Identity signatures MUST support all of the operations described
there. It must furthermore ensure that the value of the Date header
falls within the validity period of the certificate whose
corresponding private key was used to sign the Identity header. corresponding private key was used to sign the Identity header.
7. Considerations for User Agent 7. 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 13.3 for more information about securing the channel between Section 13.3 for more information about securing the channel between
the UAC and the authentication service. the UAC and the authentication service.
skipping to change at page 12, line 17 skipping to change at page 12, line 38
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 13.3 for more information about securing the channel between Section 13.3 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 From When a UAC sends a request, it MUST accurately populate the From
header field with a value corresponding to an identity that it header field with a value corresponding to an identity that it
believes it is authorized to claim. In a request it MUST set the URI believes it is authorized to claim. In a request, it MUST set the
portion of its From header to match a SIP, SIPS or TEL URI AoR which URI portion of its From header to match a SIP, SIPS, or TEL URI AoR
it is authorized to use in the domain (including anonymous URIs, as that it is authorized to use in the domain (including anonymous URIs,
described in RFC 3323 [3]). In general, UACs SHOULD NOT use the TEL as described in RFC 3323 [3]). In general, UACs SHOULD NOT use the
URI form in the From header field (see Section 11). TEL URI form in the From header field (see Section 11).
Note that this document defines a number of new 4xx response codes. Note that this document defines a number of new 4xx response codes.
If user agents support these response codes, they will be able to If user agents support these response codes, they will be able to
respond intelligently to Identity-based error conditions. 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. For a given domain, if an entity that can instantiate loose routing. For a given domain, if an entity that can instantiate
the authentication service role is not in the path of dialog-forming the authentication service role is not in the path of dialog-forming
requests, identity for mid-dialog requests in the backwards direction requests, identity for mid-dialog requests in the backwards direction
cannot be provided. 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.
8. Considerations for Proxy Servers 8. Considerations for Proxy Servers
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 validate the Identity header act as an authentication service, it MAY validate the Identity header
before it makes a forwarding decision for a request. Proxy servers before it makes a forwarding decision for a request. Proxy servers
MUST NOT remove or modify an existing Identity or Identity-Info MUST NOT remove or modify an existing Identity or Identity-Info
header in a request. header in a request.
9. Header Syntax 9. Header Syntax
This document specifies two new SIP headers: Identity and Identity- This document specifies two new SIP headers: Identity and Identity-
Info. Each of these headers can appear only once in a SIP message. Info. Each of these headers can appear only once in a SIP message.
The grammar for these two headers is (following the ABNF [6] in s): The grammar for these two headers is (following the ABNF [6] in RFC
(following the ABNF [6] in RFC3261 [1]): 3261 [1]):
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 *( SEMI ident-info-params ) Identity-Info = "Identity-Info" HCOLON ident-info
*( SEMI ident-info-params )
ident-info = LAQUOT absoluteURI RAQUOT ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-info-extension ident-info-params = ident-info-alg / ident-info-extension
ident-info-alg = "alg" EQUAL token ident-info-alg = "alg" EQUAL token
ident-info-extension = generic-param 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 be placed in a bit-exact string in the order SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character: specified here, separated by a vertical line, "|" or %x7C, character:
skipping to change at page 13, line 27 skipping to change at page 13, line 47
ident-info = LAQUOT absoluteURI RAQUOT ident-info = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-info-extension ident-info-params = ident-info-alg / ident-info-extension
ident-info-alg = "alg" EQUAL token ident-info-alg = "alg" EQUAL token
ident-info-extension = generic-param 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 be placed in a bit-exact string in the order SIP message MUST be placed in a bit-exact string in the order
specified here, separated by a vertical line, "|" or %x7C, character: specified here, separated by a vertical line, "|" or %x7C, character:
o The AoR of the UA sending the message, or addr-spec of the From o The AoR of the UA sending the message, or addr-spec of the From
header field (referred to occasionally here as the 'identity header field (referred to occasionally here as the 'identity
field'). 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 linear whitespace (LWS) rather than
digit and method portions, and thus the CSeq header field may need SP to separate the digit and method portions, and thus the CSeq
to be transformed in order to be canonicalized. The header field may need to be transformed in order to be
authentication service MUST strip leading zeros from the 'digit' canonicalized. The authentication service MUST strip leading
portion of the Cseq before generating the digest-string. 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. RFC the weekday and month items case set as shown in BNF in RFC 3261.
3261 specifies that the BNF for weekday and month are a choice RFC 3261 specifies that the BNF for weekday and month is a choice
amongst a set of tokens. The RFC 2234 rules for the BNF specify amongst a set of tokens. The RFC 2234 rules for the BNF specify
that tokens are case sensitive. However, when used to construct that tokens are case sensitive. However, when used to construct
the canonical string defined here, the first letter of each week the canonical string defined here, the first letter of each week
and month MUST be capitalized, and the remaining two letters must and month MUST be capitalized, and the remaining two letters must
be lowercase. This matches the capitalization provided in the be lowercase. This matches the capitalization provided in the
definition of each token. All requests that use the Identity definition of each token. All requests that use the Identity
mechanism MUST contain a Date header. 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 "|" characters 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). This in the Message (in the ABNF for SIP, the message-body). This
includes all components of multipart message bodies. Note that 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
skipping to change at page 14, line 21 skipping to change at page 14, line 42
includes all components of multipart message bodies. Note that 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 "|" 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 13 and [5]. why their inclusion mitigates replay attacks, see Section 13 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 RFC 3261 [1]):
digest-string = addr-spec "|" addr-spec "|" callid "|" 1*DIGIT SP Method "|" digest-string = addr-spec "|" addr-spec "|" callid "|"
SIP-date "|" [ addr-spec ] "|" message-body 1*DIGIT SP Method "|" 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. The hashing and signing algorithm is the certificate for the domain. The hashing and signing algorithm is
specified by the 'alg' parameter of the Identity-Info header (see specified by the 'alg' parameter of the Identity-Info header (see
below for more information on Identity-Info header parameters). This below for more information on Identity-Info header parameters). This
document defines only one value for the 'alg' parameter: 'rsa-sha1'; document defines only one value for the 'alg' parameter: 'rsa-sha1';
further values MUST be defined in a Standards Track RFC, see further values MUST be defined in a Standards Track RFC, see Section
Section 14.7 for more information. All implementations of this 14.7 for more information. All implementations of this specification
specification MUST support 'rsa-sha1'. When the 'rsa-sha1' algorithm MUST support 'rsa-sha1'. When the 'rsa-sha1' algorithm is specified
is specified in the 'alg' parameter of Identity-Info, the hash and in the 'alg' parameter of Identity-Info, the hash and signature MUST
signature MUST be generated as follows: compute the results of be generated as follows: compute the results of signing this string
signing this string with sha1WithRSAEncryption as described in RFC with sha1WithRSAEncryption as described in RFC 3370 [7] and base64
3370 [7] and base64 encode the results as specified in RFC 3548 [8]. encode the results as specified in RFC 3548 [8]. A 1024-bit or
A 1024 bit or longer RSA key MUST be used. The result is placed in longer RSA key MUST be used. The result is placed in the Identity
the Identity header field. For detailed examples of the usage of header field. For detailed examples of the usage of this algorithm,
this algorithm, see Section 10. see Section 10.
The 'absoluteURI' portion of the Identity-Info header MUST contain The 'absoluteURI' portion of the Identity-Info header MUST contain a
either an HTTP or HTTPS URI which dereferences to a resource that URI which dereferences to a resource containing the certificate of
contains a single MIME body containing the certificate of the the authentication service. All implementations of this
authentication service. These URIs MUST follow the conventions of specification MUST support the use of HTTP and HTTPS URIs in the
RFC2585 [10] and the indicated resource MUST be of the form Identity-Info header. Such HTTP and HTTPS URIs MUST follow the
'application/pkix-cert' described in that specification. Note that conventions of RFC 2585 [10], and for those URIs the indicated
this introduces key lifecycle management concerns; were a domain to resource MUST be of the form 'application/pkix-cert' described in
change the key available at the Identity-Info URI before a verifier that specification. Note that this introduces key lifecycle
evaluates a request signed by an authentication service, this would management concerns; were a domain to change the key available at the
cause obvious verifier failures. When a rollover occurs, Identity-Info URI before a verifier evaluates a request signed by an
authentication services SHOULD thus provide new Identity-Info URIs authentication service, this would cause obvious verifier failures.
for each new certificate, and SHOULD continue to make older key When a rollover occurs, authentication services SHOULD thus provide
acquisition URIs available for a duration longer than the plausible new Identity-Info URIs for each new certificate, and SHOULD continue
lifetime of a SIP message (an hour would most likely suffice). 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 The Identity-Info header field MUST contain an 'alg' parameter. No
other parameters are defined for the Identity-Info header in this other parameters are defined for the Identity-Info header in this
document. Future Standards Track RFCs may define additional document. Future Standards Track RFCs may define additional
Identity-Info header parameters. Identity-Info header parameters.
This document adds the following entries to Table 2 of RFC 3261 [1]: This document adds the following entries to Table 2 of RFC 3261 [1]:
Header field where proxy ACK BYE CAN INV OPT REG Header field where proxy ACK BYE CAN INV OPT REG
------------ ----- ----- --- --- --- --- --- --- ------------ ----- ----- --- --- --- --- --- ---
skipping to change at page 18, line 34 skipping to change at page 18, line 45
When the authentication service receives the INVITE, it authenticates When the authentication service receives the INVITE, it 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|
sip: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:
kjOP4YVZXmF0X3/4RUfAG6ffwbVQepNGRBz58b3dJq3prEV4h5GnS4F6udDRCI4/ ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
rSK9cl+TFv45nu0Qu2d/0WPPOvvc3JWwuUmHrCwGwC+tW7fOWnC07QKgQn40uwg5 ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
7WaXixQev5N0JfoLXnO3UDoum89JRhXPAIp2vffJbD4= FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=
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: the following:
INVITE sip:bob@biloxi.exmple.org SIP/2.0 INVITE sip:bob@biloxi.example.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:"kjOP4YVZXmF0X3/4RUfAG6ffwbVQepNGRBz58b3dJq3prEV4h5GnS4F6udDRCI4/ Identity:
rSK9cl+TFv45nu0Qu2d/0WPPOvvc3JWwuUmHrCwGwC+tW7fOWnC07QKgQn40uwg5 "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
7WaXixQev5N0JfoLXnO3UDoum89JRhXPAIp2vffJbD4=" ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1 Identity-Info: <https://atlanta.example.com/atlanta.cer>;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
atlanta.example.com then forwards the request normally. When Bob atlanta.example.com then forwards the request normally. When Bob
receives the request, if he does not already know the certificate of receives the request, if he does not already know the certificate of
atlanta.example.com, he de-references the URL in the Identity-Info atlanta.example.com, he dereferences the URL in the Identity-Info
header to acquire the certificate. Bob then generates the same header to acquire the certificate. Bob then generates the same
canonical string given above, from the same headers of the SIP canonical string given above, from the same headers of the SIP
request. Using this canonical string, the signed digest in the request. Using this canonical string, the signed digest in the
Identity header, and the certificate discovered by de-referencing the Identity header, and the certificate discovered by dereferencing the
Identity-Info header, Bob can verify that the given set of headers Identity-Info header, Bob can verify that the given set of headers
and the message body have not been modified. and the message body have not been modified.
10.2. Identity for a Request with no MIME body or Contact 10.2. Identity for a Request with No MIME Body or Contact
Consider the following private key and certificate pair assigned to Consider the following private key and certificate pair assigned to
"biloxi.example.org". "biloxi.example.org".
-----BEGIN RSA PRIVATE KEY----- -----BEGIN RSA PRIVATE KEY-----
MIICXgIBAAKBgQC/obBYLRMPjskrAqWOiGPAUxI3/m2ti7ix4caqCTAuFX5cLegQ MIICXgIBAAKBgQC/obBYLRMPjskrAqWOiGPAUxI3/m2ti7ix4caqCTAuFX5cLegQ
7nmquLOHfIhxVIqT2f06UA0lOo2NVofK9G7MTkVbVNiyAlLYUDEj7XWLDICf3ZHL 7nmquLOHfIhxVIqT2f06UA0lOo2NVofK9G7MTkVbVNiyAlLYUDEj7XWLDICf3ZHL
6Fr/+CF7wrQ9r4kv7XiJKxodVCCd/DhCT9Gp+VDoe8HymqOW/KsneriyIwIDAQAB 6Fr/+CF7wrQ9r4kv7XiJKxodVCCd/DhCT9Gp+VDoe8HymqOW/KsneriyIwIDAQAB
AoGBAJ7fsFIKXKkjWgj8ksGOthS3Sn19xPSCyEdBxfEm2Pj7/Nzzeli/PcOaic0k AoGBAJ7fsFIKXKkjWgj8ksGOthS3Sn19xPSCyEdBxfEm2Pj7/Nzzeli/PcOaic0k
JALBcnqN2fHEeIGK/9xUBxTufgQYVJqvyHERs6rXX/iT4Ynm9t1905EiQ9ZpHsrI JALBcnqN2fHEeIGK/9xUBxTufgQYVJqvyHERs6rXX/iT4Ynm9t1905EiQ9ZpHsrI
skipping to change at page 20, line 41 skipping to change at page 20, line 48
CzAJBgNVBAYTAlVTMQswCQYDVQQIDAJNUzEPMA0GA1UEBwwGQmlsb3hpMQ0wCwYD CzAJBgNVBAYTAlVTMQswCQYDVQQIDAJNUzEPMA0GA1UEBwwGQmlsb3hpMQ0wCwYD
VQQKDARJRVRGMRswGQYDVQQDDBJiaWxveGkuZXhhbXBsZS5jb22CAQAwDAYDVR0T VQQKDARJRVRGMRswGQYDVQQDDBJiaWxveGkuZXhhbXBsZS5jb22CAQAwDAYDVR0T
BAUwAwEB/zANBgkqhkiG9w0BAQUFAAOBgQBiyKHIt8TXfGNfpnJXi5jCizOxmY8Y BAUwAwEB/zANBgkqhkiG9w0BAQUFAAOBgQBiyKHIt8TXfGNfpnJXi5jCizOxmY8Y
gln8tyPFaeyq95TGcvTCWzdoBLVpBD+fpRWrX/II5sE6VHbbAPjjVmKbZwzQAtpp gln8tyPFaeyq95TGcvTCWzdoBLVpBD+fpRWrX/II5sE6VHbbAPjjVmKbZwzQAtpp
P2Fauj28t94ZeDHN2vqzjfnHjCO24kG3Juf2T80ilp9YHcDwxjUFrt86UnlC+yid P2Fauj28t94ZeDHN2vqzjfnHjCO24kG3Juf2T80ilp9YHcDwxjUFrt86UnlC+yid
yaTeusW5Gu7v1g== yaTeusW5Gu7v1g==
-----END CERTIFICATE----- -----END CERTIFICATE-----
Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
at the end of the dialog initiated in the previous example. He at the end of the dialog initiated in the previous example. He
therefore creates the following BYE request which he forwards to the therefore creates the following BYE request, which he forwards to the
'biloxi.example.org' proxy server that instantiates the 'biloxi.example.org' proxy server that instantiates the
authentication service role: authentication service role:
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
Call-ID: a84b4c76e66710 Call-ID: a84b4c76e66710
CSeq: 231 BYE CSeq: 231 BYE
skipping to change at page 21, line 34 skipping to change at page 21, line 44
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 vertical bars. The canonical terminate, in this case, with two vertical bars. The canonical
string over which the identity signature will be generated is the string over which the identity signature will be generated is the
following (note that the first line wraps because of RFC editorial following (note that the first line wraps because of RFC editorial
conventions): 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:
vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc= pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=
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 the
following:
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: "vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv Identity:
wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc=" eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="
Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1 Identity-Info: <https://biloxi.example.org/biloxi.cer>;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.
11. Identity and the TEL URI Scheme 11. Identity and the TEL URI Scheme
Since many SIP applications provide a VoIP service, telephone numbers Since many SIP applications provide a Voice over IP (VoIP) service,
are commonly used as identities in SIP deployments. In the majority telephone numbers are commonly used as identities in SIP deployments.
of cases, this is not problematic for the identity mechanism In the majority of cases, this is not problematic for the identity
described in this document. Telephone numbers commonly appear in the mechanism described in this document. Telephone numbers commonly
username portion of a SIP URI (e.g., appear in the username portion of a SIP URI (e.g.,
'sip:+17005551008@chicago.example.com;user=phone'). That username 'sip:+17005551008@chicago.example.com;user=phone'). That username
conforms to the syntax of the TEL URI scheme (RFC3966 [13]). For conforms to the syntax of the TEL URI scheme (RFC3966 [13]). For
this sort of SIP address-of-record, chicago.example.com is the this sort of SIP address-of-record, chicago.example.com is the
appropriate 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
skipping to change at page 23, line 13 skipping to change at page 23, line 23
sign a request containing a TEL URI in the From header field. This sign a request containing a TEL URI in the From header field. This
is permitted in this specification strictly for forward compatibility is permitted in this specification strictly for forward compatibility
purposes. In the longer-term, it is possible that ENUM [14] may purposes. In the longer-term, it is possible that ENUM [14] 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.
12. Privacy Considerations 12. Privacy Considerations
The identity mechanism presented in this draft is compatible with the The identity mechanism presented in this document is compatible with
standard SIP practices for privacy described in RFC3323 [3]. A SIP the standard SIP practices for privacy described in RFC 3323 [3]. A
proxy server can act both as a privacy service and as an SIP 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 that the authentication service is willing to
authorize, there is no reason why private SIP URIs which contain authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity by an authentication service. The construction of the Identity
header is the same for private URIs as it is for any other sort of header is the same for private URIs as it is for any other sort of
URIs. 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 'anonymous.invalid' will not be possible for privacy domains like 'anonymous.invalid' will not be possible for privacy
services that also act as authentication services. The assurance services that also act as authentication services. The assurance
offered by the usage of anonymous URIs with a valid domain portion is offered by the usage of anonymous URIs with a valid domain portion is
"this is a known user in my domain that I have authenticated, but I "this is a known user in my domain that I have authenticated, but I
am keeping their identity private". The use of the domain am keeping its identity private". The use of the domain
'anonymous.invalid' entails that no corresponding authority for the 'anonymous.invalid' entails that no corresponding authority for the
domain can exist, and as a consequence, authentication service domain can exist, and as a consequence, authentication service
functions are meaningless. 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 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 [12] defines the "id" priv-value token which is specific to RFC 3325 [12] defines the "id" priv-value token, which is specific to
the P-Asserted-Identity header. The sort of assertion provided by the P-Asserted-Identity header. 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 that 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 Finally, note that unlike RFC3325, the mechanism described in this
specification adds no information to SIP requests that has privacy specification adds no information to SIP requests that has privacy
implications. implications.
13. Security Considerations 13. Security Considerations
13.1. Handling of digest-string Elements 13.1. Handling of digest-string Elements
This document describes a mechanism which provides a signature over This document describes a mechanism that 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
requests. 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 following section details the individual security properties The following section details the individual security properties
obtained by including each of these header fields within the obtained by including each of these header fields within the
skipping to change at page 24, line 45 skipping to change at page 25, line 6
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 serves two purposes: first, it prevents cut-and-paste signature serves two purposes: first, it prevents cut-and-paste
attacks in which an Identity header from legitimate request for one attacks in which an Identity header from legitimate request for one
user is cut-and-pasted into a request for a different user; second, user is cut-and-pasted into a request for a different user; second,
it preserves the starting URI scheme of the request, which helps it preserves the starting URI scheme of the request, which helps
prevent downgrade attacks against the use of SIPS. prevent downgrade 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 RFC 3261, Section 23.4.2.
of this specification MUST NOT deem valid a request with an outdated Implementations of this specification MUST NOT deem valid a request
Date header field (the RECOMMENDED interval is that the Date header with an outdated Date header field (the RECOMMENDED interval is that
must indicate a time within 3600 seconds of the receipt of a the Date header must indicate a time within 3600 seconds of the
message). Implementations MUST also record Call-IDs received in receipt of a message). Implementations MUST also record Call-IDs
valid requests containing an Identity header, and MUST remember those received in valid requests containing an Identity header, and MUST
Call-IDs for at least the duration of a single Date interval (i.e. remember those Call-IDs for at least the duration of a single Date
commonly 3600 seconds). Because a SIP-compliant UA never generates interval (i.e., commonly 3600 seconds). Because a SIP-compliant UA
the same Call-ID twice, verifiers can use the Call-ID to recognize never generates the same Call-ID twice, verifiers can use the Call-ID
cut-and-paste attacks; the Call-ID serves as a nonce. The result of to recognize cut-and-paste attacks; the Call-ID serves as a nonce.
this is that if an Identity header is replayed within the Date The result of this is that if an Identity header is replayed within
interval, verifiers will recognize that it is invalid because of a the Date interval, verifiers will recognize that it is invalid
Call-ID duplication; if an Identity header is replayed after the Date because of a Call-ID duplication; if an Identity header is replayed
interval, verifiers will recognize that it is invalid because the after the Date interval, verifiers will recognize that it is invalid
Date is stale. The CSeq header field contains a numbered identifier because the Date is stale. The CSeq header field contains a numbered
for the transaction, and the name of the method of the request; identifier for the transaction, and the name of the method of the
without this information, an INVITE request could be cut-and-pasted request; without this information, an INVITE request could be cut-
by an attacker and transformed into a BYE request without changing and-pasted by an attacker and transformed into a BYE request without
any fields covered by the Identity header, and moreover requests changing any fields covered by the Identity header, and moreover
within a certain transaction could be replayed in potentially requests within a certain transaction could be replayed in
confusing or malicious ways. 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 user agent 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 its 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), the attacker would not be eligible to
requests from the called user agent, since those requests are routed receive SIP requests from the called user agent, since those requests
to the URI identified in the Contact header field. However, the are routed to the URI identified in the Contact header field.
Contact header is only included in dialog-forming requests, so it However, the Contact header is only included in dialog-forming
does not provide this protection in all cases. requests, so it 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
information present in the Via header field value(s). For example, information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header, without a signature over the sent-by field of the topmost Via header,
an attacker could remove that Via header and insert their own in a an attacker could remove that Via header and insert its own in a
cut-and-paste attack, which would cause all responses to the request cut-and-paste attack, which would cause all responses to the request
to be routed to a host of the attacker's choosing. However, a 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 thereby preempting responses. It is necessary for the
proper operation of SIP for subsequent intermediaries to be capable proper operation of SIP for subsequent intermediaries to be capable
of inserting such Via header fields, and thus it cannot be prevented. of inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS. known practice for securing Via is the use of SIPS.
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
bodies carried in SIP requests. There is little purpose in Session Description Protocol (SDP) bodies carried in SIP requests.
establishing the identity of the user that originated a SIP request There is little purpose in establishing the identity of the user that
if this assurance is not coupled with a comparable assurance over the originated a SIP request if this assurance is not coupled with a
media descriptors. Note however that this is not perfect end-to-end comparable assurance over the media descriptors. Note, however, that
security. The authentication service itself, when instantiated at a this is not perfect end-to-end security. The authentication service
intermediary, could conceivably change the SDP (and SIP headers, for itself, when instantiated at a intermediary, could conceivably change
that matter) before providing a signature. Thus, while this the SDP (and SIP headers, for that matter) before providing a
mechanism reduces the chance that a replayer or man-in-the-middle signature. Thus, while this mechanism reduces the chance that a
will modify SDP, it does not eliminate it entirely. Since it is a replayer or man-in-the-middle will modify SDP, it does not eliminate
foundational assumption of this mechanism that the user trusts their it entirely. Since it is a foundational assumption of this mechanism
local domain to vouch for their security, they must also trust the that the users trust their local domain to vouch for their security,
service not to violate the integrity of their message without good they must also trust the service not to violate the integrity of
reason. Note that RFC3261 16.6 states that SIP proxy servers "MUST their message without good reason. Note that RFC 3261, Section 16.6,
NOT add to, modify, or remove the message body." states that SIP proxy servers "MUST NOT add to, modify, or remove the
message body."
In the end analysis, the Identity and Identity-Info headers cannot In the end analysis, the Identity and Identity-Info headers cannot
protect themselves. Any attacker could remove these headers from a protect themselves. Any attacker could remove these headers from a
SIP request, and modify the request arbitrarily afterwards. However, SIP request, and modify the request arbitrarily afterwards. However,
this mechanism is not intended to protect requests from men-in-the- this mechanism is not intended to protect requests from men-in-the-
middle who interfere with SIP messages; it is intended only to middle who interfere with SIP messages; it is intended only to
provide a way that SIP users can prove definitively that they are who 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 they claim to be. At best, by stripping identity information from a
request, a man-in-the-middle could make it impossible to distinguish request, a man-in-the-middle could make it impossible to distinguish
any illegitimate messages he would like to send from those messages any illegitimate messages he would like to send from those messages
sent by an authorized user. However, it requires a considerably sent by an authorized user. However, it requires a considerably
greater amount of energy to mount such an attack than it does to greater amount of energy to mount such an attack than it does to
mount trivial impersonations by just copying someone else's From mount trivial impersonations by just copying someone else's From
header field. This mechanism provides a way that an authorized user header field. This mechanism provides a way that an authorized user
can provide a definitive assurance of their identity which an can provide a definitive assurance of his identity that an
unauthorized user, an impersonator, cannot. unauthorized user, an impersonator, cannot.
One additional respect in which the Identity-Info header cannot One additional respect in which the Identity-Info header cannot
protect itself is the 'alg' parameter. The 'alg' parameter is not protect itself is the 'alg' parameter. The 'alg' parameter is not
included in the digest-string, and accordingly, a man-in-the-middle included in the digest-string, and accordingly, a man-in-the-middle
might attempt to modify the 'alg' parameter. However, it is might attempt to modify the 'alg' parameter. However, it is
important to note that preventing men-in-the-middle is not the important to note that preventing men-in-the-middle is not the
primary impetus for this mechanism. Moreover, changing the 'alg' primary impetus for this mechanism. Moreover, changing the 'alg'
would at worst result in some sort of bid-down attack, and at best would at worst result in some sort of bid-down attack, and at best
cause a failure in the verifier. Note that only one valid 'alg' cause a failure in the verifier. Note that only one valid 'alg'
parameter is defined in this document, and that thus there is parameter is defined in this document and that thus there is
currently no weaker algorithm to which the mechanism can be bid-down. currently no weaker algorithm to which the mechanism can be bid down.
'alg' has been incorporated into this mechanism for forward- 'alg' has been incorporated into this mechanism for forward-
compatibility reasons in case the current algorithm exhibits compatibility reasons in case the current algorithm exhibits
weaknesses, and requires swift replacement, in the future. weaknesses, and requires swift replacement, in the future.
13.2. Display Names and Identity 13.2. Display-Names and Identity
As a matter of interface design, SIP user agents might render the 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 display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email identity of the caller; there is a significant precedent in email
user interfaces for this practice. As such, it might seem that the user interfaces for this practice. As such, it might seem that the
lack of a signature over the display-name is a significant omission. lack of a signature over the display-name is a significant omission.
However, there are several important senses in which a signature over However, there are several important senses in which a signature over
the display-name does not prevent impersonation. In the first place, the display-name does not prevent impersonation. In the first place,
a particular display-name, like "Jon Peterson", is not unique in the a particular display-name, like "Jon Peterson", is not unique in the
world; many users in different administrative domains might world; many users in different administrative domains might
legitimately claim that name. Furthermore, enrollment practices for legitimately claim that name. Furthermore, enrollment practices for
SIP-based services might have a difficult time discerning the SIP-based services might have a difficult time discerning the
legitimate display-name for a user; it is safe to assume that legitimate display-name for a user; it is safe to assume that
impersonators will be capable of creating SIP accounts with impersonators will be capable of creating SIP accounts with arbitrary
arbitrarily display-names. The same situation prevails in email display-names. The same situation prevails in email today. Note
today. Note that an impersonator who attempted to replay a message that an impersonator who attempted to replay a message with an
with an Identity header, changing only the display-name in the From Identity header, changing only the display-name in the From header
header field, would be detected by the other replay protection field, would be detected by the other replay protection mechanisms
mechanisms described in Section 13.1. described in Section 13.1.
Of course, an authentication service can enforce policies about the Of course, an authentication service can enforce policies about the
display-name even if the display-name is not signed. The exact display-name even if the display-name is not signed. The exact
mechanics for creating and operationalizing such policies is outside mechanics for creating and operationalizing such policies is outside
the scope of this document. The effect of this policy would not be the scope of this document. The effect of this policy would not be
to prevent impersonation of a particular unique identifier like a SIP to prevent impersonation of a particular unique identifier like a SIP
URI (since display-names are not unique identifiers), but to allow a 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 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 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 choosing. In the absence of a signature, man-in-the-middle attackers
could conceivably alter the display-names in a request with impunity. could conceivably alter the display-names in a request with impunity.
Note that the scope of this specification is impersonation attacks, Note that the scope of this specification is impersonation attacks,
however, and that a man-in-the-middle might also strip the Identity however, and that a man-in-the-middle might also strip the Identity
and Identity-Info headers from a message. and Identity-Info headers from a message.
There are many environments in which policies regarding the display- There are many environments in which policies regarding the display-
name aren't feasible. Distributing bit-exact and internationalizable name aren't feasible. Distributing bit-exact and internationalizable
display-names to end users as part of the enrollment or registration display-names to end-users as part of the enrollment or registration
process would require mechanisms that are not explored in this process would require mechanisms that are not explored in this
document. In the absence of policy enforcement regarding domain document. In the absence of policy enforcement regarding domain
names, there are conceivably attacks that an adversary could mount names, there are conceivably attacks that an adversary could mount
against SIP systems that rely too heavily on the display-name in against SIP systems that rely too heavily on the display-name in
their user interface, but this argues for intelligent interface their user interface, but this argues for intelligent interface
design, not changes to the mechanisms. Relying on a non-unique design, not changes to the mechanisms. Relying on a non-unique
identifier for identity would ultimately result in a weak mechanism. identifier for identity would ultimately result in a weak mechanism.
13.3. Securing the Connection to the Authentication Service 13.3. Securing the Connection to the Authentication Service
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their user interface, but this argues for intelligent interface their user interface, but this argues for intelligent interface
design, not changes to the mechanisms. Relying on a non-unique design, not changes to the mechanisms. Relying on a non-unique
identifier for identity would ultimately result in a weak mechanism. identifier for identity would ultimately result in a weak mechanism.
13.3. Securing the Connection to the Authentication Service 13.3. 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 the user receives a challenge via a
such as Digest), then it is possible that a rogue server is mechanism such as Digest), then it is possible that a rogue server
attempting to pose as an authentication service for a domain that is attempting to pose as an authentication service for a domain
it does not control, possibly in an attempt to collect shared that it does not control, possibly in an attempt to collect shared
secrets for that domain. secrets for that domain.
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 with the request request will not have integrity protection when 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 possible that applications relying on replay attacks. However, it is possible that applications relying on
the presence of the Identity header could leverage this integrity the presence of the Identity header could leverage this integrity
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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.
13.4. Domain Names and Subordination 13.4. Domain Names and Subordination
When a verifier processes a request containing an Identity-Info When a verifier processes a request containing an Identity-Info
header, it must compare the domain portion of the URI in the From header, it must compare the domain portion of the URI in the From
header field of the request with the domain name which is the subject header field of the request with the domain name that is the subject
of the certificate acquired from the Identity-Info header. While it of the certificate acquired from the Identity-Info header. While it
might seem that this should be a straightforward process, it is might seem that this should be a straightforward process, it is
complicated by two deployment realities. In the first place, complicated by two deployment realities. In the first place,
certificates have varying ways of describing their subjects, and may certificates have varying ways of describing their subjects, and may
indeed have multiple subjects, especially in 'virtual hosting' cases indeed have multiple subjects, especially in 'virtual hosting' cases
where multiple domains are managed by a single application. where multiple domains are managed by a single application.
Secondly, some SIP services may delegate SIP functions to a Secondly, some SIP services may delegate SIP functions to a
subordinate domain and utilize the procedures in RFC3263 [4] allow subordinate domain and utilize the procedures in RFC 3263 [4] that
requests for, say, 'example.com' to be routed to 'sip.example.com'; allow requests for, say, 'example.com' to be routed to
as a result, a user with the AoR 'sip:jon@example.com' may process 'sip.example.com'. As a result, a user with the AoR
their requests through a host like 'sip.example.com', and it may be 'sip:jon@example.com' may process its requests through a host like
that latter host which acts as an authentication service. 'sip.example.com', and it may be that latter host that acts as an
authentication service.
To meet the second of these problems, a domain that deploys an To meet the second of these problems, a domain that deploys an
authentication service on a subordinate host MUST be willing to authentication service on a subordinate host MUST be willing to
supply that host with the private keying material associated with a supply that host with the private keying material associated with a
certificate whose subject is a domain name that corresponds to the certificate whose subject is a domain name that corresponds to the
domain portion of the AoRs that the domain distributes to users. domain portion of the AoRs that the domain distributes to users.
Note that this corresponds to the comparable case of routing inbound Note that this corresponds to the comparable case of routing inbound
SIP requests to a domain. When the NAPTR and SRV procedures of SIP requests to a domain. When the NAPTR and SRV procedures of RFC
RFC3263 are used to direct requests to a domain name other than the 3263 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', domain in the original Request-URI (e.g., for 'sip:jon@example.com',
the corresponding SRV records point to the service the corresponding SRV records point to the service
'sip1.example.org'), the client expects that the certificate passed 'sip1.example.org'), the client expects that the certificate passed
back in any TLS exchange with that host will correspond exactly with back in any TLS exchange with that host will correspond exactly with
the domain of the original Request-URI, not the domain name of the the domain of the original Request-URI, not the domain name of the
host. Consequently, in order to make inbound routing to such SIP host. Consequently, in order to make inbound routing to such SIP
services work, a domain administrator must similarly be willing to services work, a domain administrator must similarly be willing to
share the domain's private key with the service. This design share the domain's private key with the service. This design
decision was made to compensate for the insecurity of the DNS, and it decision was made to compensate for the insecurity of the DNS, and it
makes certain potential approaches to DNS-based 'virtual hosting' makes certain potential approaches to DNS-based 'virtual hosting'
unsecurable for SIP in environments where domain administrators are unsecurable for SIP in environments where domain administrators are
unwilling to share keys with hosting services. unwilling to share keys with hosting services.
A verifier MUST evaluate the correspondence between the user's A verifier MUST evaluate the correspondence between the user's
identity and the signing certificate by following the procedures identity and the signing certificate by following the procedures
defined in RFC 2818 [11] Section 3.1. While RFC2818 deals with the defined in RFC 2818 [11], Section 3.1. While RFC 2818 deals with the
use of HTTP in TLS, the procedures described are applicable to use of HTTP in TLS, the procedures described are applicable to
verifying identity if one subtitutes the "hostname of the server" in verifying identity if one substitutes the "hostname of the server" in
HTTP for the domain portion of the user's identity in the From header HTTP for the domain portion of the user's identity in the From header
field of a SIP request with an Identity header. field of a SIP request with an Identity header.
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 to support the root certificates of all user agents will be able to 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
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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 previous key exchange has not be trusted by verifiers, unless some previous key exchange has
justified such trust. justified such trust.
For further information on certificate security and practices see For further information on certificate security and practices, see
RFC3280 [9]. The Security Considerations of RFC3280 are applicable RFC3280 [9]. The Security Considerations of RFC3280 are applicable
to this document. to this document.
13.5. Authorization and Transitional Strategies 13.5. Authorization and Transitional Strategies
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 used its administrative domain assumes that the issuing domain used its
administrative practices to authenticate its users. However, it is administrative practices to authenticate its users. However, it is
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This document requests changes to the header and response-code sub- This document requests changes to the header and response-code sub-
registries of the SIP parameters IANA registry, and requests the registries of the SIP parameters IANA registry, and requests the
creation of two new registries for parameters for the Identity-Info creation of two new registries for parameters for the Identity-Info
header. header.
14.1. Header Field Names 14.1. Header Field Names
This document specifies two new SIP headers: Identity and Identity- This document specifies two new SIP headers: Identity and Identity-
Info. Their syntax is given in Section 9. These headers are defined Info. Their syntax is given in Section 9. These headers are defined
by the following information, which is to be added to the header sub- by the following information, which has been added to the header
registry under http://www.iana.org/assignments/sip-parameters. sub-registry under 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: n Compact Form: n
14.2. 428 'Use Identity Header' Response Code 14.2. 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
Section 6. It is sent when a verifier receives a SIP request that in Section 6. It is sent when a verifier receives a SIP request that
lacks an Identity header in order to indicate that the request should lacks an Identity header in order to indicate that the request should
be re-sent with an Identity header. This response code is defined by 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 the following information, which has been added to the 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
14.3. 436 'Bad Identity-Info' Response Code 14.3. 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
Section 6. It is used when the Identity-Info header contains a URI in Section 6. It is used when the Identity-Info header contains a
that cannot be dereferenced by the verifier (either the URI scheme is URI that cannot be dereferenced by the verifier (either the URI
unsupported by the verifier, or the resource designated by the URI is scheme is unsupported by the verifier, or the resource designated by
otherwise unavailable). This response code is defined by the the URI is otherwise unavailable). This response code is defined by
following information, which is to be added to the method and the following information, which has been added to the 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: 436 Response Code Number: 436
Default Reason Phrase: Bad Identity-Info Default Reason Phrase: Bad Identity-Info
14.4. 437 'Unsupported Certificate' Response Code 14.4. 437 'Unsupported Certificate' 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
Section 6. It is used when the verifier cannot validate the in Section 6. 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 has been 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
14.5. 438 'Invalid Identity Header' Response Code 14.5. 438 'Invalid 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
Section 6. It is used when the verifier receives a message with an in Section 6. It is used when the verifier receives a message with
Identity signature that does not correspond to the digest-string an Identity signature that does not correspond to the digest-string
calculated by the verifier. This response code is defined by the calculated by the verifier. This response code is defined by the
following information, which is to be added to the method and following information, which has been added to the 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: 438 Response Code Number: 438
Default Reason Phrase: Invalid Identity Header Default Reason Phrase: Invalid Identity Header
14.6. Identity-Info Parameters 14.6. Identity-Info Parameters
This document requests that the IANA create a new registry for The IANA has created a new registry for Identity-Info headers. This
Identity-Info headers. This registry is to be prepopulated with a registry is to be prepopulated with a single entry for a parameter
single entry for a parameter called 'alg', which describes the called 'alg', which describes the algorithm used to create the
algorithm used to create the signature which appears in the Identity signature that appears in the Identity header. Registry entries must
header. Registry entries must contain the name of the parameter and contain the name of the parameter and the specification in which the
the specification in which the parameter is defined. New parameters parameter is defined. New parameters for the Identity-Info header
for the Identity-Info header may be defined only in Standards Track may be defined only in Standards Track RFCs.
RFCs.
14.7. Identity-Info Algorithm Parameter Values 14.7. Identity-Info Algorithm Parameter Values
This document requests that the IANA create a new registry for The IANA has created a new registry for Identity-Info 'alg' parameter
Identity-Info 'alg' parameter values. This registry is to be values. This registry is to be prepopulated with a single entry for
prepopulated with a single entry for a value called 'rsa-sha1', which a value called 'rsa-sha1', which describes the algorithm used to
describes the algorithm used to create the signature which appears in create the signature that appears in the Identity header. Registry
the Identity header. Registry entries must contain the name of the entries must contain the name of the 'alg' parameter value and the
'alg' parameter value and the specification in which the value is specification in which the value is described. New values for the
described. New values for the 'alg' parameter may be defined only in 'alg' parameter may be defined only in Standards Track RFCs.
Standards Track RFCs.
Appendix A. Acknowledgments Appendix A. Acknowledgements
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,
Aki Niemi, and Jim Schaad for their comments. Jonathan Rosenberg Aki Niemi, and Jim Schaad for their comments. Jonathan Rosenberg
provided detailed fixes to innumerable sections of the document. The provided detailed fixes to innumerable sections of the document. The
bit-archive presented in Appendix B follows the pioneering example of bit-archive presented in Appendix B follows the pioneering example of
Robert Sparks' torture-test draft. Thanks to Hans Persson and Tao RFC 4475 [16]. Thanks to Hans Persson and Tao Wan for thorough nit
Wan for thorough nit reviews. reviews.
Appendix B. Bit-exact archive of example messages Appendix B. Bit-Exact Archive of Examples of Messages
The following text block is an encoded, gzip-compressed TAR archive
of files that represent the transformations performed on the examples
of messages discussed in Section 10. It includes for each example:
The following text block is an encoded, gzip compressed TAR archive
of files that represent the transformations performed on the example
messages discussed in Section 10. It includes for each example:
o (foo).message: the original message o (foo).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)
o (foo).signed: the RSA-signed SHA1 hash of the canonical string o (foo).signed: the RSA-signed SHA1 hash of the canonical string
(binary) (binary)
o (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash o (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
of the canonical string as it would appear in the request of the canonical string as it would appear in the request
o (foo).identity: the original message with the Identity and o (foo).identity: the original message with the Identity and
Identity-Info headers added Identity-Info headers added
skipping to change at page 34, line 28 skipping to change at page 35, line 25
} }
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
-- BEGIN MESSAGE ARCHIVE -- -- BEGIN MESSAGE ARCHIVE --
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-- END MESSAGE ARCHIVE -- -- END MESSAGE ARCHIVE --
Appendix C. Original Requirements Appendix C. Original Requirements
The following requirements were crafted throughout the development of The following requirements were crafted throughout the development of
the mechanism described in this document. They are preserved here the mechanism described in this document. They are preserved here
for historical reasons. for historical reasons.
o The mechanism must allow a UAC or a proxy server to provide a o The mechanism must allow a UAC or a proxy server to provide a
strong cryptographic identity assurance in a request that can be strong cryptographic identity assurance in a request that can be
verified by a proxy 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
skipping to change at page 37, line 4 skipping to change at page 38, line 24
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 In order to provide full replay protection, the mechanism must be o In order to provide full replay protection, the mechanism must be
capable of protecting the integrity of SIP message bodies (to capable of protecting the integrity of SIP message bodies (to
ensure that media offers and answers are linked to the signaling ensure that media offers and answers are linked to the signaling
identity). 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) which it is authorized to use within a accounts or aliases) that it is authorized to use within a
domain, and for the UAC to assert one identity while domain, and for the UAC to assert one identity while
authenticating itself as another, related, identity, as permitted authenticating itself as another, related, identity, as permitted
by the local policy of the domain. by the local policy of the domain.
Appendix D. Changelog References
NOTE TO THE RFC-EDITOR: Please remove this section prior to
publication as an RFC.
Changes from draft-ietf-sip-identity-06:
- Disambiguated 428 response code, added new 438 for invalid
Identity headers
- Used RFC2585 format for Identity-Info URIs
- Updated example certificates to comply with RFC3280
- Replaced certificate validation and mapping procedures with
reference to RFC2818
- Numerous editorial fixes
Changes from draft-ietf-sip-identity-05:
- Removed the requirements section
- Numerous editorial fixes
Changes from draft-ietf-sip-identity-04:
- Changed the delimiter of the digest-string from ":" to "|"
- Removed support for the SIPS URI scheme from the Identity-Info
header
- Made the Identity-Info header extensible; added an Identity-Info
header for algorithm with an initial defined value of 'rsa-sha1'
- Broke up the Security Considerations into smaller chunks for
organizational reasons; expanded discussion of most issues
- Added some guidelines for authentication service certificate
rollover and lifecycle management (also now based HTTP certificate
retrieval on RFC2585)
Changes from draft-ietf-sip-identity-03:
- Softened requirement for TLS and direct connections; now SHOULD-
strength, SIPS and Digest auth-int listed as alternatives.
- Added non-normative section about authentication service
behavior for backwards-direction requests within a dialog
- Added support for CID URI in Identity Info
- Added new response codes (436 and 437) corresponding to error
cases for an unsupported URI scheme and an unsupported
certificate, respectively
Changes from draft-ietf-sip-identity-02:
- Extracted text relating to providing identity in SIP responses;
this text will appear in a separate draft
- Added compliance testing/example section
- Added CSeq to the signature of the Identity header to prevent a
specific cut-and-paste attack; also added addr-spec of the To
header to the signature of the Identity header for similar reasons
- Added text about why neither Via headers nor display-names are
protected by this mechanism
- Added bit-exact reference files for compliance testing
- Added privacy considerations
Changes from draft-ietf-sip-identity-01:
- Completely changed underlying mechanism - instead of using an
AIB, the mechanism now recommends the use of the Identity header
and Identity-Info header
- Numerous other changes resulting from the above
- Various other editorial corrections
Changes from draft-peterson-sip-identity-01:
- Split off child draft-ietf-sip-authid-body-00 for defining of
the AIB
- Clarified scope in introduction
- Removed a lot of text that was redundant with RFC3261
(especially about authentication practices)
- Added mention of content indirection mechanism for adding token
to requests and responses
- Improved Security Considerations (added piece about credential
strength)
Changes from draft-peterson-sip-identity-00:
- Added a section on authenticated identities in responses
- Removed hostname convention for authentication services
- Added text about using 'message/sip' or 'message/sipfrag' in
authenticated identity bodies, also RECOMMENDED a few more headers
in sipfrags to increase reference integrity
- Various other editorial corrections
15. References
15.1. Normative References 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", BCP 14, 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. and P. Overell, "Augmented BNF for Syntax
RFC 2234, November 1997. Specifications: ABNF", RFC 4234, October 2005.
[7] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms", [7] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms",
RFC 3370, August 2002. RFC 3370, August 2002.
[8] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", [8] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003. RFC 3548, July 2003.
[9] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 [9] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002. Revocation List (CRL) Profile", RFC 3280, April 2002.
[10] Housley, R. and P. Hoffman, "Internet X.509 Public Key [10] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP", RFC 2585, Infrastructure Operational Protocols: FTP and HTTP", RFC 2585,
May 1999. May 1999.
[11] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000. [11] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
15.2. Informative References Informative References
[12] Jennings, C., Peterson, J., and M. Watson, "Private Extensions [12] 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.
[13] Schulzrinne, H., "The TEL URI for Telephone Numbers", RFC 3966, [13] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004. December 2004.
[14] Faltstrom, P. and M. Mealling, "The E.164 to URI DDDS [14] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Application", RFC 3761, April 2004. Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[15] Peterson, J., "Retargeting and Security in SIP: A Framework and [15] Peterson, J., "Retargeting and Security in SIP: A Framework and
Requirements", draft-peterson-sipping-retarget-00 (work in Requirements", Work in Progress, February 2005.
progress), February 2005.
[16] Sparks, R., Ed., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
and H. Schulzrinne, "Session Initiation Protocol (SIP) Torture
Test Messages, RFC 4475, May 2006.
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
Intellectual Property Statement Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
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made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
skipping to change at page 42, line 29 skipping to change at page 41, line 45
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
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The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
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ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Disclaimer of Validity Acknowledgement
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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