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rfc:rfc4474

Network Working Group J. Peterson Request for Comments: 4474 NeuStar Category: Standards Track C. Jennings

                                                         Cisco Systems
                                                           August 2006
     Enhancements for Authenticated Identity Management in the
                 Session Initiation Protocol (SIP)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 The existing security mechanisms in the Session Initiation Protocol
 (SIP) are inadequate for cryptographically assuring the identity of
 the end users that originate SIP requests, especially in an
 interdomain context.  This document defines a mechanism for securely
 identifying originators of SIP messages.  It does so by defining two
 new SIP header fields, Identity, for conveying a signature used for
 validating the identity, and Identity-Info, for conveying a reference
 to the certificate of the signer.

Peterson & Jennings Standards Track [Page 1] RFC 4474 SIP Identity August 2006

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................3
 3. Background ......................................................3
 4. Overview of Operations ..........................................6
 5. Authentication Service Behavior .................................7
    5.1. Identity within a Dialog and Retargeting ..................10
 6. Verifier Behavior ..............................................11
 7. Considerations for User Agent ..................................12
 8. Considerations for Proxy Servers ...............................13
 9. Header Syntax ..................................................13
 10. Compliance Tests and Examples .................................16
    10.1. Identity-Info with a Singlepart MIME body ................17
    10.2. Identity for a Request with No MIME Body or Contact ......20
 11. Identity and the TEL URI Scheme ...............................22
 12. Privacy Considerations ........................................23
 13. Security Considerations .......................................24
    13.1. Handling of digest-string Elements .......................24
    13.2. Display-Names and Identity ...............................27
    13.3. Securing the Connection to the Authentication Service ....28
    13.4. Domain Names and Subordination ...........................29
    13.5. Authorization and Transitional Strategies ................30
 14. IANA Considerations ...........................................31
    14.1. Header Field Names .......................................31
    14.2. 428 'Use Identity Header' Response Code ..................32
    14.3. 436 'Bad Identity-Info' Response Code ....................32
    14.4. 437 'Unsupported Certificate' Response Code ..............32
    14.5. 438 'Invalid Identity Header' Response Code ..............33
    14.6. Identity-Info Parameters .................................33
    14.7. Identity-Info Algorithm Parameter Values .................33
 Appendix A. Acknowledgements ......................................34
 Appendix B. Bit-Exact Archive of Examples of Messages .............34
    B.1. Encoded Reference Files ...................................35
 Appendix C. Original Requirements .................................38
 References ........................................................39
    Normative References ...........................................39
    Informative References .........................................39

Peterson & Jennings Standards Track [Page 2] RFC 4474 SIP Identity August 2006

1. Introduction

 This document provides enhancements to the existing mechanisms for
 authenticated identity management in the Session Initiation Protocol
 (SIP, RFC 3261 [1]).  An identity, for the purposes of this document,
 is defined as a SIP URI, commonly a canonical address-of-record (AoR)
 employed to reach a user (such as 'sip:alice@atlanta.example.com').
 RFC 3261 stipulates several places within a SIP request where a user
 can express an identity for themselves, notably the user-populated
 From header field.  However, the recipient of a SIP request has no
 way to verify that the From header field has been populated
 appropriately, in the absence of some sort of cryptographic
 authentication mechanism.
 RFC 3261 specifies a number of security mechanisms that can be
 employed by SIP user agents (UAs), including Digest, Transport Layer
 Security (TLS), and S/MIME (implementations may support other
 security schemes as well).  However, few SIP user agents today
 support the end-user certificates necessary to authenticate
 themselves (via S/MIME, for example), and furthermore Digest
 authentication is limited by the fact that the originator and
 destination must share a prearranged secret.  It is desirable for SIP
 user agents to be able to send requests to destinations with which
 they have no previous association -- just as in the telephone network
 today, one can receive a call from someone with whom one has no
 previous association, and still have a reasonable assurance that the
 person's displayed Caller-ID is accurate.  A cryptographic approach,
 like the one described in this document, can probably provide a much
 stronger and less-spoofable assurance of identity than the telephone
 network provides today.

2. Terminology

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
 described in RFC 2119 [2] and indicate requirement levels for
 compliant SIP implementations.

3. Background

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

Peterson & Jennings Standards Track [Page 3] RFC 4474 SIP Identity August 2006

 of potential subscribers to a whitelist before determining whether it
 should accept or reject the subscription.  In both of these cases,
 attackers might attempt to circumvent these authorization policies
 through impersonation.  Since the primary identifier of the sender of
 a SIP request, the From header field, can be populated arbitrarily by
 the controller of a user agent, impersonation is very simple today.
 The mechanism described in this document aspires to provide a strong
 identity system for SIP in which authorization policies cannot be
 circumvented by impersonation.
 All RFC 3261-compliant user agents support Digest authentication,
 which utilizes a shared secret, as a means for authenticating
 themselves to a SIP registrar.  Registration allows a user agent to
 express that it is an appropriate entity to which requests should be
 sent for a particular SIP AoR URI (e.g.,
 'sip:alice@atlanta.example.com').
 By the definition of identity used in this document, registration is
 a proof of the identity of the user to a registrar.  However, the
 credentials with which a user agent proves its identity to a
 registrar cannot be validated by just any user agent or proxy server
 -- these credentials are only shared between the user agent and their
 domain administrator.  So this shared secret does not immediately
 help a user to authenticate to a wide range of recipients.
 Recipients require a means of determining whether or not the 'return
 address' identity of a non-REGISTER request (i.e., the From header
 field value) has legitimately been asserted.
 The AoR URI used for registration is also the URI with which a UA
 commonly populates the From header field of requests in order to
 provide a 'return address' identity to recipients.  From an
 authorization perspective, if you can prove you are eligible to
 register in a domain under a particular AoR, you can prove you can
 legitimately receive requests for that AoR, and accordingly, when you
 place that AoR in the From header field of a SIP request other than a
 registration (like an INVITE), you are providing a 'return address'
 where you can legitimately be reached.  In other words, if you are
 authorized to receive requests for that 'return address', logically,
 it follows that you are also authorized to assert that 'return
 address' in your From header field.  This is of course only one
 manner in which a domain might determine how a particular user is
 authorized to populate the From header field; as an aside, for other
 sorts of URIs in the From (like anonymous URIs), other authorization
 policies would apply.
 Ideally, then, SIP user agents should have some way of proving to
 recipients of SIP requests that their local domain has authenticated
 them and authorized the population of the From header field.  This

Peterson & Jennings Standards Track [Page 4] RFC 4474 SIP Identity August 2006

 document proposes a mediated authentication architecture for SIP in
 which requests are sent to a server in the user's local domain, which
 authenticates such requests (using the same practices by which the
 domain would authenticate REGISTER requests).  Once a message has
 been authenticated, the local domain then needs some way to
 communicate to other SIP entities that the sending user has been
 authenticated and its use of the From header field has been
 authorized.  This document addresses how that imprimatur of
 authentication can be shared.
 RFC 3261 already describes an architecture very similar to this in
 Section 26.3.2.2, in which a user agent authenticates itself to a
 local proxy server, which in turn authenticates itself to a remote
 proxy server via mutual TLS, creating a two-link chain of transitive
 authentication between the originator and the remote domain.  While
 this works well in some architectures, there are a few respects in
 which this is impractical.  For one, transitive trust is inherently
 weaker than an assertion that can be validated end-to-end.  It is
 possible for SIP requests to cross multiple intermediaries in
 separate administrative domains, in which case transitive trust
 becomes even less compelling.
 One solution to this problem is to use 'trusted' SIP intermediaries
 that assert an identity for users in the form of a privileged SIP
 header.  A mechanism for doing so (with the P-Asserted-Identity
 header) is given in [12].  However, this solution allows only hop-
 by-hop trust between intermediaries, not end-to-end cryptographic
 authentication, and it assumes a managed network of nodes with strict
 mutual trust relationships, an assumption that is incompatible with
 widespread Internet deployment.
 Accordingly, this document specifies a means of sharing a
 cryptographic assurance of end-user SIP identity in an interdomain or
 intradomain context that is based on the concept of an
 'authentication service' and a new SIP header, the Identity header.
 Note that the scope of this document is limited to providing this
 identity assurance for SIP requests; solving this problem for SIP
 responses is more complicated and is a subject for future work.
 This specification allows either a user agent or a proxy server to
 provide identity services and to verify identities.  To maximize
 end-to-end security, it is obviously preferable for end-users to
 acquire their own certificates and corresponding private keys; if
 they do, they can act as an authentication service.  However, end-
 user certificates may be neither practical nor affordable, given the
 difficulties of establishing a Public Key Infrastructure (PKI) that
 extends to end-users, and moreover, given the potentially large
 number of SIP user agents (phones, PCs, laptops, PDAs, gaming

Peterson & Jennings Standards Track [Page 5] RFC 4474 SIP Identity August 2006

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

4. Overview of Operations

 This section provides an informative (non-normative) high-level
 overview of the mechanisms described in this document.
 Imagine the case where Alice, who has the home proxy of example.com
 and the address-of-record sip:alice@example.com, wants to communicate
 with sip:bob@example.org.
 Alice generates an INVITE and places her identity in the From header
 field of the request.  She then sends an INVITE over TLS to an
 authentication service proxy for her domain.
 The authentication service authenticates Alice (possibly by sending a
 Digest authentication challenge) and validates that she is authorized
 to assert the identity that is populated in the From header field.
 This value may be Alice's AoR, or it may be some other value that the
 policy of the proxy server permits her to use.  It then computes a
 hash over some particular headers, including the From header field
 and the bodies in the message.  This hash is signed with the
 certificate for the domain (example.com, in Alice's case) and
 inserted in a new header field in the SIP message, the 'Identity'
 header.
 The proxy, as the holder of the private key of its domain, is
 asserting that the originator of this request has been authenticated
 and that she is authorized to claim the identity (the SIP address-
 of-record) that appears in the From header field.  The proxy also
 inserts a companion header field, Identity-Info, that tells Bob how
 to acquire its certificate, if he doesn't already have it.
 When Bob's domain receives the request, it verifies the signature
 provided in the Identity header, and thus can validate that the
 domain indicated by the host portion of the AoR in the From header
 field authenticated the user, and permitted the user to assert that
 From header field value.  This same validation operation may be
 performed by Bob's user agent server (UAS).

Peterson & Jennings Standards Track [Page 6] RFC 4474 SIP Identity August 2006

5. Authentication Service Behavior

 This document defines a new role for SIP entities called an
 authentication service.  The authentication service role can be
 instantiated by a proxy server or a user agent.  Any entity that
 instantiates the authentication service role MUST possess the private
 key of a domain certificate.  Intermediaries that instantiate this
 role MUST be capable of authenticating one or more SIP users that can
 register in that domain.  Commonly, this role will be instantiated by
 a proxy server, since these entities are more likely to have a static
 hostname, hold a corresponding certificate, and have access to SIP
 registrar capabilities that allow them to authenticate users in their
 domain.  It is also possible that the authentication service role
 might be instantiated by an entity that acts as a redirect server,
 but that is left as a topic for future work.
 SIP entities that act as an authentication service MUST add a Date
 header field to SIP requests if one is not already present (see
 Section 9 for information on how the Date header field assists
 verifiers).  Similarly, authentication services MUST add a Content-
 Length header field to SIP requests if one is not already present;
 this can help verifiers to double-check that they are hashing exactly
 as many bytes of message-body as the authentication service when they
 verify the message.
 Entities instantiating the authentication service role perform the
 following steps, in order, to generate an Identity header for a SIP
 request:
 Step 1:
 The authentication service MUST extract the identity of the sender
 from the request.  The authentication service takes this value from
 the From header field; this AoR will be referred to here as the
 'identity field'.  If the identity field contains a SIP or SIP Secure
 (SIPS) URI, the authentication service MUST extract the hostname
 portion of the identity field and compare it to the domain(s) for
 which it is responsible (following the procedures in RFC 3261,
 Section 16.4, used by a proxy server to determine the domain(s) for
 which it is responsible).  If the identity field uses the TEL URI
 scheme, the policy of the authentication service determines whether
 or not it is responsible for this identity; see Section 11 for more
 information.  If the authentication service is not responsible for
 the identity in question, it SHOULD process and forward the request
 normally, but it MUST NOT add an Identity header; see below for more
 information on authentication service handling of an existing
 Identity header.

Peterson & Jennings Standards Track [Page 7] RFC 4474 SIP Identity August 2006

 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
       intermediary (proxy server), it may challenge the request with
       a 407 response code using the Digest authentication scheme (or
       viewing a Proxy-Authentication header sent in the request,
       which was sent in anticipation of a challenge using cached
       credentials, as described in RFC 3261, Section 22.3).  Note
       that if that proxy server is maintaining a TLS connection with
       the client over which the client had previously authenticated
       itself using Digest authentication, the identity value obtained
       from that previous authentication step can be reused without an
       additional Digest challenge.
       If the authentication service is instantiated by a SIP user
       agent, a user agent can be said to authenticate its user on the
       grounds that the user can provision the user agent with the
       private key of the domain, or preferably by providing a
       password that unlocks said private key.
 Authorization of the use of a particular username in the From header
 field is a matter of local policy for the authentication service, one
 that depends greatly on the manner in which authentication is
 performed.  For example, one policy might be as follows: the username
 given in the 'username' parameter of the Proxy-Authorization header
 MUST correspond exactly to the username in the From header field of
 the SIP message.  However, there are many cases in which this is too
 limiting or inappropriate; a realm might use 'username' parameters in
 Proxy-Authorization that do not correspond to the user-portion of SIP
 From headers, or a user might manage multiple accounts in the same
 administrative domain.  In this latter case, a domain might maintain
 a mapping between the values in the 'username' parameter of Proxy-
 Authorization and a set of one or more SIP URIs that might
 legitimately be asserted for that 'username'.  For example, the
 username can correspond to the 'private identity' as defined in Third
 Generation Partnership Project (3GPP), in which case the From header
 field can contain any one of the public identities associated with
 this private identity.  In this instance, another policy might be as
 follows: the URI in the From header field MUST correspond exactly to
 one of the mapped URIs associated with the 'username' given in the
 Proxy-Authorization header.  Various exceptions to such policies
 might arise for cases like anonymity; if the AoR asserted in the From

Peterson & Jennings Standards Track [Page 8] RFC 4474 SIP Identity August 2006

 header field uses a form like 'sip:anonymous@example.com', then the
 'example.com' proxy should authenticate that the user is a valid user
 in the domain and insert the signature over the From header field as
 usual.
 Note that this check is performed on the addr-spec in the From header
 field (e.g., the URI of the sender, like
 'sip:alice@atlanta.example.com'); it does not convert the display-
 name portion of the From header field (e.g., 'Alice Atlanta').
 Authentication services MAY check and validate the display-name as
 well, and compare it to a list of acceptable display-names that may
 be used by the sender; if the display-name does not meet policy
 constraints, the authentication service MUST return a 403 response
 code.  The reason phrase should indicate the nature of the problem;
 for example, "Inappropriate Display Name".  However, the display-name
 is not always present, and in many environments the requisite
 operational procedures for display-name validation may not exist.
 For more information, see Section 13.2.
 Step 3:
 The authentication service SHOULD ensure that any preexisting Date
 header in the request is accurate.  Local policy can dictate
 precisely how accurate the Date must be; a RECOMMENDED maximum
 discrepancy of ten minutes will ensure that the request is unlikely
 to upset any verifiers.  If the Date header contains a time different
 by more than ten minutes from the current time noted by the
 authentication service, the authentication service SHOULD reject the
 request.  This behavior is not mandatory because a user agent client
 (UAC) could only exploit the Date header in order to cause a request
 to fail verification; the Identity header is not intended to provide
 a source of non-repudiation or a perfect record of when messages are
 processed.  Finally, the authentication service MUST verify that the
 Date header falls within the validity period of its certificate.  For
 more information on the security properties associated with the Date
 header field value, see Section 9.
 Step 4:
 The authentication service MUST form the identity signature and add
 an Identity header to the request containing this signature.  After
 the Identity header has been added to the request, the authentication
 service MUST also add an Identity-Info header.  The Identity-Info
 header contains a URI from which its certificate can be acquired.
 Details on the generation of both of these headers are provided in
 Section 9.

Peterson & Jennings Standards Track [Page 9] RFC 4474 SIP Identity August 2006

 Finally, the authentication service MUST forward the message
 normally.

5.1. Identity within a Dialog and Retargeting

 Retargeting is broadly defined as the alteration of the Request-URI
 by intermediaries.  More specifically, retargeting supplants the
 original target URI with one that corresponds to a different user, a
 user that is not authorized to register under the original target
 URI.  By this definition, retargeting does not include translation of
 the Request-URI to a contact address of an endpoint that has
 registered under the original target URI, for example.
 When a dialog-forming request is retargeted, this can cause a few
 wrinkles for the Identity mechanism when it is applied to requests
 sent in the backwards direction within a dialog.  This section
 provides some non-normative considerations related to this case.
 When a request is retargeted, it may reach a SIP endpoint whose user
 is not identified by the URI designated in the To header field value.
 The value in the To header field of a dialog-forming request is used
 as the From header field of requests sent in the backwards direction
 during the dialog, and is accordingly the header that would be signed
 by an authentication service for requests sent in the backwards
 direction.  In retargeting cases, if the URI in the From header does
 not identify the sender of the request in the backwards direction,
 then clearly it would be inappropriate to provide an Identity
 signature over that From header.  As specified above, if the
 authentication service is not responsible for the domain in the From
 header field of the request, it MUST NOT add an Identity header to
 the request, and it should process/forward the request normally.
 Any means of anticipating retargeting, and so on, is outside the
 scope of this document, and likely to have equal applicability to
 response identity as it does to requests in the backwards direction
 within a dialog.  Consequently, no special guidance is given for
 implementers here regarding the 'connected party' problem;
 authentication service behavior is unchanged if retargeting has
 occurred for a dialog-forming request.  Ultimately, the
 authentication service provides an Identity header for requests in
 the backwards dialog when the user is authorized to assert the
 identity given in the From header field, and if they are not, an
 Identity header is not provided.
 For further information on the problems of response identity and the
 potential solution spaces, see [15].

Peterson & Jennings Standards Track [Page 10] RFC 4474 SIP Identity August 2006

6. Verifier Behavior

 This document introduces a new logical role for SIP entities called a
 server.  When a verifier receives a SIP message containing an
 Identity header, it may inspect the signature to verify the identity
 of the sender of the message.  Typically, the results of a
 verification are provided as input to an authorization process that
 is outside the scope of this document.  If an Identity header is not
 present in a request, and one is required by local policy (for
 example, based on a per-sending-domain policy, or a per-sending-user
 policy), then a 428 'Use Identity Header' response MUST be sent.
 In order to verify the identity of the sender of a message, an entity
 acting as a verifier MUST perform the following steps, in the order
 here specified.
 Step 1:
 The verifier MUST acquire the certificate for the signing domain.
 Implementations supporting this specification SHOULD have some means
 of retaining domain certificates (in accordance with normal practices
 for certificate lifetimes and revocation) in order to prevent
 themselves from needlessly downloading the same certificate every
 time a request from the same domain is received.  Certificates cached
 in this manner should be indexed by the URI given in the Identity-
 Info header field value.
 Provided that the domain certificate used to sign this message is not
 previously known to the verifier, SIP entities SHOULD discover this
 certificate by dereferencing the Identity-Info header, unless they
 have some more efficient implementation-specific way of acquiring
 certificates for that domain.  If the URI scheme in the Identity-Info
 header cannot be dereferenced, then a 436 'Bad Identity-Info'
 response MUST be returned.  The verifier processes this certificate
 in the usual ways, including checking that it has not expired, that
 the chain is valid back to a trusted certification authority (CA),
 and that it does not appear on revocation lists.  Once the
 certificate is acquired, it MUST be validated following the
 procedures in RFC 3280 [9].  If the certificate cannot be validated
 (it is self-signed and untrusted, or signed by an untrusted or
 unknown certificate authority, expired, or revoked), the verifier
 MUST send a 437 'Unsupported Certificate' response.
 Step 2:
 The verifier MUST follow the process described in Section 13.4 to
 determine if the signer is authoritative for the URI in the From
 header field.

Peterson & Jennings Standards Track [Page 11] RFC 4474 SIP Identity August 2006

 Step 3:
 The verifier MUST verify the signature in the Identity header field,
 following the procedures for generating the hashed digest-string
 described in Section 9.  If a verifier determines that the signature
 on the message does not correspond to the reconstructed digest-
 string, then a 438 'Invalid Identity Header' response MUST be
 returned.
 Step 4:
 The verifier MUST validate the Date, Contact, and Call-ID headers in
 the manner described in Section 13.1; recipients that wish to verify
 Identity signatures MUST support all of the operations described
 there.  It must furthermore ensure that the value of the Date header
 falls within the validity period of the certificate whose
 corresponding private key was used to sign the Identity header.

7. Considerations for User Agent

 This mechanism can be applied opportunistically to existing SIP
 deployments; accordingly, it requires no change to SIP user agent
 behavior in order for it to be effective.  However, because this
 mechanism does not provide integrity protection between the UAC and
 the authentication service, a UAC SHOULD implement some means of
 providing this integrity.  TLS would be one such mechanism, which is
 attractive because it MUST be supported by SIP proxy servers, but is
 potentially problematic because it is a hop-by-hop mechanism.  See
 Section 13.3 for more information about securing the channel between
 the UAC and the authentication service.
 When a UAC sends a request, it MUST accurately populate the From
 header field with a value corresponding to an identity that it
 believes it is authorized to claim.  In a request, it MUST set the
 URI portion of its From header to match a SIP, SIPS, or TEL URI AoR
 that it is authorized to use in the domain (including anonymous URIs,
 as described in RFC 3323 [3]).  In general, UACs SHOULD NOT use the
 TEL URI form in the From header field (see Section 11).
 Note that this document defines a number of new 4xx response codes.
 If user agents support these response codes, they will be able to
 respond intelligently to Identity-based error conditions.
 The UAC MUST also be capable of sending requests, including mid-call
 requests, through an 'outbound' proxy (the authentication service).
 The best way to accomplish this is using pre-loaded Route headers and
 loose routing.  For a given domain, if an entity that can instantiate
 the authentication service role is not in the path of dialog-forming

Peterson & Jennings Standards Track [Page 12] RFC 4474 SIP Identity August 2006

 requests, identity for mid-dialog requests in the backwards direction
 cannot be provided.
 As a recipient of a request, a user agent that can verify signed
 identities should also support an appropriate user interface to
 render the validity of identity to a user.  User agent
 implementations SHOULD differentiate signed From header field values
 from unsigned From header field values when rendering to an end-user
 the identity of the sender of a request.

8. Considerations for Proxy Servers

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

9. Header Syntax

 This document specifies two new SIP headers: Identity and Identity-
 Info.  Each of these headers can appear only once in a SIP message.
 The grammar for these two headers is (following the ABNF [6] in RFC
 3261 [1]):
 Identity = "Identity" HCOLON signed-identity-digest
 signed-identity-digest = LDQUOT 32LHEX RDQUOT
 Identity-Info = "Identity-Info" HCOLON ident-info
                  *( SEMI ident-info-params )
 ident-info = LAQUOT absoluteURI RAQUOT
 ident-info-params = ident-info-alg / ident-info-extension
 ident-info-alg = "alg" EQUAL token
 ident-info-extension = generic-param
 The signed-identity-digest is a signed hash of a canonical string
 generated from certain components of a SIP request.  To create the
 contents of the signed-identity-digest, the following elements of a
 SIP message MUST be placed in a bit-exact string in the order
 specified here, separated by a vertical line, "|" or %x7C, character:
 o  The AoR of the UA sending the message, or addr-spec of the From
    header field (referred to occasionally here as the 'identity
    field').

Peterson & Jennings Standards Track [Page 13] RFC 4474 SIP Identity August 2006

 o  The addr-spec component of the To header field, which is the AoR
    to which the request is being sent.
 o  The callid from Call-Id header field.
 o  The digit (1*DIGIT) and method (method) portions from CSeq header
    field, separated by a single space (ABNF SP, or %x20).  Note that
    the CSeq header field allows linear whitespace (LWS) rather than
    SP to separate the digit and method portions, and thus the CSeq
    header field may need to be transformed in order to be
    canonicalized.  The authentication service MUST strip leading
    zeros from the 'digit' portion of the Cseq before generating the
    digest-string.
 o  The Date header field, with exactly one space each for each SP and
    the weekday and month items case set as shown in BNF in RFC 3261.
    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
    that tokens are case sensitive.  However, when used to construct
    the canonical string defined here, the first letter of each week
    and month MUST be capitalized, and the remaining two letters must
    be lowercase.  This matches the capitalization provided in the
    definition of each token.  All requests that use the Identity
    mechanism MUST contain a Date header.
 o  The addr-spec component of the Contact header field value.  If the
    request does not contain a Contact header, this field MUST be
    empty (i.e., there will be no whitespace between the fourth and
    fifth "|" characters in the canonical string).
 o  The body content of the message with the bits exactly as they are
    in the Message (in the ABNF for SIP, the message-body).  This
    includes all components of multipart message bodies.  Note that
    the message-body does NOT include the CRLF separating the SIP
    headers from the message-body, but does include everything that
    follows that CRLF.  If the message has no body, then message-body
    will be empty, and the final "|" will not be followed by any
    additional characters.
 For more information on the security properties of these headers, and
 why their inclusion mitigates replay attacks, see Section 13 and [5].
 The precise formulation of this digest-string is, therefore
 (following the ABNF [6] in RFC 3261 [1]):
 digest-string = addr-spec "|" addr-spec "|" callid "|"
                 1*DIGIT SP Method "|" SIP-date "|" [ addr-spec ] "|"
                 message-body
 Note again that the first addr-spec MUST be taken from the From
 header field value, the second addr-spec MUST be taken from the To
 header field value, and the third addr-spec MUST be taken from the
 Contact header field value, provided the Contact header is present in
 the request.

Peterson & Jennings Standards Track [Page 14] RFC 4474 SIP Identity August 2006

 After the digest-string is formed, it MUST be hashed and signed with
 the certificate for the domain.  The hashing and signing algorithm is
 specified by the 'alg' parameter of the Identity-Info header (see
 below for more information on Identity-Info header parameters).  This
 document defines only one value for the 'alg' parameter: 'rsa-sha1';
 further values MUST be defined in a Standards Track RFC, see Section
 14.7 for more information.  All implementations of this specification
 MUST support 'rsa-sha1'.  When the 'rsa-sha1' algorithm is specified
 in the 'alg' parameter of Identity-Info, the hash and signature MUST
 be generated as follows: compute the results of signing this string
 with sha1WithRSAEncryption as described in RFC 3370 [7] and base64
 encode the results as specified in RFC 3548 [8].  A 1024-bit or
 longer RSA key MUST be used.  The result is placed in the Identity
 header field.  For detailed examples of the usage of this algorithm,
 see Section 10.
 The 'absoluteURI' portion of the Identity-Info header MUST contain a
 URI which dereferences to a resource containing the certificate of
 the authentication service.  All implementations of this
 specification MUST support the use of HTTP and HTTPS URIs in the
 Identity-Info header.  Such HTTP and HTTPS URIs MUST follow the
 conventions of RFC 2585 [10], and for those URIs the indicated
 resource MUST be of the form 'application/pkix-cert' described in
 that specification.  Note that this introduces key lifecycle
 management concerns; were a domain to change the key available at the
 Identity-Info URI before a verifier evaluates a request signed by an
 authentication service, this would cause obvious verifier failures.
 When a rollover occurs, authentication services SHOULD thus provide
 new Identity-Info URIs for each new certificate, and SHOULD continue
 to make older key acquisition URIs available for a duration longer
 than the plausible lifetime of a SIP message (an hour would most
 likely suffice).
 The Identity-Info header field MUST contain an 'alg' parameter.  No
 other parameters are defined for the Identity-Info header in this
 document.  Future Standards Track RFCs may define additional
 Identity-Info header parameters.

Peterson & Jennings Standards Track [Page 15] RFC 4474 SIP Identity August 2006

 This document adds the following entries to Table 2 of RFC 3261 [1]:
    Header field         where   proxy   ACK  BYE  CAN  INV  OPT  REG
    ------------         -----   -----   ---  ---  ---  ---  ---  ---
    Identity               R       a      o    o    -    o    o    o
                                         SUB  NOT  REF  INF  UPD  PRA
                                         ---  ---  ---  ---  ---  ---
                                          o    o    o    o    o    o
    Header field         where   proxy   ACK  BYE  CAN  INV  OPT  REG
    ------------         -----   -----   ---  ---  ---  ---  ---  ---
    Identity-Info          R       a      o    o    -    o    o    o
                                         SUB  NOT  REF  INF  UPD  PRA
                                         ---  ---  ---  ---  ---  ---
                                          o    o    o    o    o    o
 Note, in the table above, that this mechanism does not protect the
 CANCEL method.  The CANCEL method cannot be challenged, because it is
 hop-by-hop, and accordingly authentication service behavior for
 CANCEL would be significantly limited.  Note as well that the
 REGISTER method uses Contact header fields in very unusual ways that
 complicate its applicability to this mechanism, and the use of
 Identity with REGISTER is consequently a subject for future study,
 although it is left as optional here for forward-compatibility
 reasons.  The Identity and Identity-Info header MUST NOT appear in
 CANCEL.

10. Compliance Tests and Examples

 The examples in this section illustrate the use of the Identity
 header in the context of a SIP transaction.  Implementers are advised
 to verify their compliance with the specification against the
 following criteria:
 o  Implementations of the authentication service role MUST generate
    identical base64 identity strings to the ones shown in the
    Identity headers in these examples when presented with the source
    message and utilizing the appropriate supplied private key for the
    domain in question.
 o  Implementations of the verifier role MUST correctly validate the
    given messages containing the Identity header when utilizing the
    supplied certificates (with the caveat about self-signed
    certificates below).

Peterson & Jennings Standards Track [Page 16] RFC 4474 SIP Identity August 2006

 Note that the following examples use self-signed certificates, rather
 than certificates issued by a recognized certificate authority.  The
 use of self-signed certificates for this mechanism is NOT
 RECOMMENDED, and it appears here only for illustrative purposes.
 Therefore, in compliance testing, implementations of verifiers SHOULD
 generate appropriate warnings about the use of self-signed
 certificates.  Also, the example certificates in this section have
 placed their domain name subject in the subjectAltName field; in
 practice, certificate authorities may place domain names in other
 locations in the certificate (see Section 13.4 for more information).
 Note that all examples in this section use the 'rsa-sha1' algorithm.
 Bit-exact reference files for these messages and their various
 transformations are supplied in Appendix B.

10.1. Identity-Info with a Singlepart MIME body

 Consider the following private key and certificate pair assigned to
 'atlanta.example.com' (rendered in OpenSSL format).
  1. —-BEGIN RSA PRIVATE KEY—–

MIICXQIBAAKBgQDPPMBtHVoPkXV+Z6jq1LsgfTELVWpy2BVUffJMPH06LL0cJSQO

 aIeVzIojzWtpauB7IylZKlAjB5f429tRuoUiedCwMLKblWAqZt6eHWpCNZJ7lONc
 IEwnmh2nAccKk83Lp/VH3tgAS/43DQoX2sndnYh+g8522Pzwg7EGWspzzwIDAQAB
 AoGBAK0W3tnEFD7AjVQAnJNXDtx59Aa1Vu2JEXe6oi+OrkFysJjbZJwsLmKtrgtt
 PXOU8t2mZpi0wK4hX4tZhntiwGKkUPC3h9Bjp+GerifP341RMyMO+6fPgjqOzUDw
 +rPjjMpwD7AkcEcqDgbTrZnWv/QnCSaaF3xkUGfFkLx5OKcRAkEA7UxnsE8XaT30
 tP/UUc51gNk2KGKgxQQTHopBcew9yfeCRFhvdL7jpaGatEi5iZwGGQQDVOVHUN1H
 0YLpHQjRowJBAN+R2bvA/Nimq464ZgnelEDPqaEAZWaD3kOfhS9+vL7oqES+u5E0
 J7kXb7ZkiSVUg9XU/8PxMKx/DAz0dUmOL+UCQH8C9ETUMI2uEbqHbBdVUGNk364C
 DFcndSxVh+34KqJdjiYSx6VPPv26X9m7S0OydTkSgs3/4ooPxo8HaMqXm80CQB+r
 xbB3UlpOohcBwFK9mTrlMB6Cs9ql66KgwnlL9ukEhHHYozGatdXeoBCyhUsogdSU
 6/aSAFcvWEGtj7/vyJECQQCCS1lKgEXoNQPqONalvYhyyMZRXFLdD4gbwRPK1uXK
 Ypk3CkfFzOyfjeLcGPxXzq2qzuHzGTDxZ9PAepwX4RSk
 -----END RSA PRIVATE KEY-----
 -----BEGIN CERTIFICATE-----
 MIIC3TCCAkagAwIBAgIBADANBgkqhkiG9w0BAQUFADBZMQswCQYDVQQGEwJVUzEL
 MAkGA1UECAwCR0ExEDAOBgNVBAcMB0F0bGFudGExDTALBgNVBAoMBElFVEYxHDAa
 BgNVBAMME2F0bGFudGEuZXhhbXBsZS5jb20wHhcNMDUxMDI0MDYzNjA2WhcNMDYx
 MDI0MDYzNjA2WjBZMQswCQYDVQQGEwJVUzELMAkGA1UECAwCR0ExEDAOBgNVBAcM
 B0F0bGFudGExDTALBgNVBAoMBElFVEYxHDAaBgNVBAMME2F0bGFudGEuZXhhbXBs
 ZS5jb20wgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBAM88wG0dWg+RdX5nqOrU
 uyB9MQtVanLYFVR98kw8fTosvRwlJA5oh5XMiiPNa2lq4HsjKVkqUCMHl/jb21G6
 hSJ50LAwspuVYCpm3p4dakI1knuU41wgTCeaHacBxwqTzcun9Ufe2ABL/jcNChfa
 yd2diH6DznbY/PCDsQZaynPPAgMBAAGjgbQwgbEwHQYDVR0OBBYEFNmU/MrbVYcE
 KDr/20WISrG1j1rNMIGBBgNVHSMEejB4gBTZlPzK21WHBCg6/9tFiEqxtY9azaFd
 pFswWTELMAkGA1UEBhMCVVMxCzAJBgNVBAgMAkdBMRAwDgYDVQQHDAdBdGxhbnRh

Peterson & Jennings Standards Track [Page 17] RFC 4474 SIP Identity August 2006

 MQ0wCwYDVQQKDARJRVRGMRwwGgYDVQQDDBNhdGxhbnRhLmV4YW1wbGUuY29tggEA
 MAwGA1UdEwQFMAMBAf8wDQYJKoZIhvcNAQEFBQADgYEADdQYtswBDmTSTq0mt211
 7alm/XGFrb2zdbU0vorxRdOZ04qMyrIpXG1LEmnEOgcocyrXRBvq5p6WbZAcEQk0
 DsE3Ve0Nc8x9nmvljW7GsMGFCnCuo4ODTf/1lGdVr9DeCzcj10YUQ3MRemDMXhY2
 CtDisLWl7SXOORcZAi1oU9w=
 -----END CERTIFICATE-----
 A user of atlanta.example.com, Alice, wants to send an INVITE to
 bob@biloxi.example.org.  She therefore creates the following INVITE
 request, which she forwards to the atlanta.example.org proxy server
 that instantiates the authentication service role:
       INVITE sip:bob@biloxi.example.org SIP/2.0
       Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
       To: Bob <sip:bob@biloxi.example.org>
       From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
       Call-ID: a84b4c76e66710
       CSeq: 314159 INVITE
       Max-Forwards: 70
       Date: Thu, 21 Feb 2002 13:02:03 GMT
       Contact: <sip:alice@pc33.atlanta.example.com>
       Content-Type: application/sdp
       Content-Length: 147
       v=0
       o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
       s=Session SDP
       c=IN IP4 pc33.atlanta.example.com
       t=0 0
       m=audio 49172 RTP/AVP 0
       a=rtpmap:0 PCMU/8000
 When the authentication service receives the INVITE, it authenticates
 Alice by sending a 407 response.  As a result, Alice adds an
 Authorization header to her request, and resends to the
 atlanta.example.com authentication service.  Now that the service is
 sure of Alice's identity, it calculates an Identity header for the
 request.  The canonical string over which the identity signature will
 be generated is the following (note that the first line wraps because
 of RFC editorial conventions):
 sip:alice@atlanta.example.com|sip:bob@biloxi.example.org|
 a84b4c76e66710|314159 INVITE|Thu, 21 Feb 2002 13:02:03 GMT|
 sip:alice@pc33.atlanta.example.com|v=0
 o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
 s=Session SDP
 c=IN IP4 pc33.atlanta.example.com
 t=0 0

Peterson & Jennings Standards Track [Page 18] RFC 4474 SIP Identity August 2006

 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 The resulting signature (sha1WithRsaEncryption) using the private RSA
 key given above, with base64 encoding, is the following:
 ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
 ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
 FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=
 Accordingly, the atlanta.example.com authentication service will
 create an Identity header containing that base64 signature string
 (175 bytes).  It will also add an HTTPS URL where its certificate is
 made available.  With those two headers added, the message looks like
 the following:
 INVITE sip:bob@biloxi.example.org SIP/2.0
 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
 To: Bob <sip:bob@biloxi.example.org>
 From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
 Call-ID: a84b4c76e66710
 CSeq: 314159 INVITE
 Max-Forwards: 70
 Date: Thu, 21 Feb 2002 13:02:03 GMT
 Contact: <sip:alice@pc33.atlanta.example.com>
 Identity:
   "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
    ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
    FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="
 Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
 Content-Type: application/sdp
 Content-Length: 147
 v=0
 o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
 s=Session SDP
 c=IN IP4 pc33.atlanta.example.com
 t=0 0
 m=audio 49172 RTP/AVP 0
 a=rtpmap:0 PCMU/8000
 atlanta.example.com then forwards the request normally.  When Bob
 receives the request, if he does not already know the certificate of
 atlanta.example.com, he dereferences the URL in the Identity-Info
 header to acquire the certificate.  Bob then generates the same
 canonical string given above, from the same headers of the SIP
 request.  Using this canonical string, the signed digest in the
 Identity header, and the certificate discovered by dereferencing the

Peterson & Jennings Standards Track [Page 19] RFC 4474 SIP Identity August 2006

 Identity-Info header, Bob can verify that the given set of headers
 and the message body have not been modified.

10.2. Identity for a Request with No MIME Body or Contact

 Consider the following private key and certificate pair assigned to
 "biloxi.example.org".
  1. —-BEGIN RSA PRIVATE KEY—–

MIICXgIBAAKBgQC/obBYLRMPjskrAqWOiGPAUxI3/m2ti7ix4caqCTAuFX5cLegQ

 7nmquLOHfIhxVIqT2f06UA0lOo2NVofK9G7MTkVbVNiyAlLYUDEj7XWLDICf3ZHL
 6Fr/+CF7wrQ9r4kv7XiJKxodVCCd/DhCT9Gp+VDoe8HymqOW/KsneriyIwIDAQAB
 AoGBAJ7fsFIKXKkjWgj8ksGOthS3Sn19xPSCyEdBxfEm2Pj7/Nzzeli/PcOaic0k
 JALBcnqN2fHEeIGK/9xUBxTufgQYVJqvyHERs6rXX/iT4Ynm9t1905EiQ9ZpHsrI
 /AMMUYA1QrGgAIHvZLVLzq+9KLDEZ+HQbuCLJXF+6bl0Eb5BAkEA636oMANp0Qa3
 mYWEQ2utmGsYxkXSfyBb18TCOwCty0ndBR24zyOJF2NbZS98Lz+Ga25hfIGw/JHK
 nD9bOE88UwJBANBRSpd4bmS+m48R/13tRESAtHqydNinX0kS/RhwHr7mkHTU3k/M
 FxQtx34I3GKzaZxMn0A66KS9v/SHdnF+ePECQQCGe7QshyZ8uitLPtZDclCWhEKH
 qAQHmUEZvUF2VHLrbukLLOgHUrHNa24cILv4d3yaCVUetymNcuyTwhKj24wFAkAO
 z/jx1EplN3hwL+NsllZoWI58uvu7/Aq2c3czqaVGBbb317sHCYgKk0bAG3kwO3mi
 93/LXWT1cdiYVpmBcHDBAkEAmpgkFj+xZu5gWASY5ujv+FCMP0WwaH5hTnXu+tKe
 PJ3d2IJZKxGnl6itKRN7GeRh9PSK0kZSqGFeVrvsJ4Nopg==
 -----END RSA PRIVATE KEY-----
 -----BEGIN CERTIFICATE-----
 MIIC1jCCAj+gAwIBAgIBADANBgkqhkiG9w0BAQUFADBXMQswCQYDVQQGEwJVUzEL
 MAkGA1UECAwCTVMxDzANBgNVBAcMBkJpbG94aTENMAsGA1UECgwESUVURjEbMBkG
 A1UEAwwSYmlsb3hpLmV4YW1wbGUuY29tMB4XDTA1MTAyNDA2NDAyNloXDTA2MTAy
 NDA2NDAyNlowVzELMAkGA1UEBhMCVVMxCzAJBgNVBAgMAk1TMQ8wDQYDVQQHDAZC
 aWxveGkxDTALBgNVBAoMBElFVEYxGzAZBgNVBAMMEmJpbG94aS5leGFtcGxlLmNv
 bTCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAv6GwWC0TD47JKwKljohjwFMS
 N/5trYu4seHGqgkwLhV+XC3oEO55qrizh3yIcVSKk9n9OlANJTqNjVaHyvRuzE5F
 W1TYsgJS2FAxI+11iwyAn92Ry+ha//ghe8K0Pa+JL+14iSsaHVQgnfw4Qk/RqflQ
 6HvB8pqjlvyrJ3q4siMCAwEAAaOBsTCBrjAdBgNVHQ4EFgQU0Z+RL47W/APDtc5B
 fSoQXuEFE/wwfwYDVR0jBHgwdoAU0Z+RL47W/APDtc5BfSoQXuEFE/yhW6RZMFcx
 CzAJBgNVBAYTAlVTMQswCQYDVQQIDAJNUzEPMA0GA1UEBwwGQmlsb3hpMQ0wCwYD
 VQQKDARJRVRGMRswGQYDVQQDDBJiaWxveGkuZXhhbXBsZS5jb22CAQAwDAYDVR0T
 BAUwAwEB/zANBgkqhkiG9w0BAQUFAAOBgQBiyKHIt8TXfGNfpnJXi5jCizOxmY8Y
 gln8tyPFaeyq95TGcvTCWzdoBLVpBD+fpRWrX/II5sE6VHbbAPjjVmKbZwzQAtpp
 P2Fauj28t94ZeDHN2vqzjfnHjCO24kG3Juf2T80ilp9YHcDwxjUFrt86UnlC+yid
 yaTeusW5Gu7v1g==
 -----END CERTIFICATE-----
 Bob (bob@biloxi.example.org) now wants to send a BYE request to Alice
 at the end of the dialog initiated in the previous example.  He
 therefore creates the following BYE request, which he forwards to the
 'biloxi.example.org' proxy server that instantiates the
 authentication service role:

Peterson & Jennings Standards Track [Page 20] RFC 4474 SIP Identity August 2006

 BYE sip:alice@pc33.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
 To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
 Call-ID: a84b4c76e66710
 CSeq: 231 BYE
 Content-Length: 0
 When the authentication service receives the BYE, it authenticates
 Bob by sending a 407 response.  As a result, Bob adds an
 Authorization header to his request, and resends to the
 biloxi.example.org authentication service.  Now that the service is
 sure of Bob's identity, it prepares to calculate an Identity header
 for the request.  Note that this request does not have a Date header
 field.  Accordingly, the biloxi.example.org will add a Date header to
 the request before calculating the identity signature.  If the
 Content-Length header were not present, the authentication service
 would add it as well.  The baseline message is thus:
 BYE sip:alice@pc33.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
 To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
 Date: Thu, 21 Feb 2002 14:19:51 GMT
 Call-ID: a84b4c76e66710
 CSeq: 231 BYE
 Content-Length: 0
 Also note that this request contains no Contact header field.
 Accordingly, biloxi.example.org will place no value in the canonical
 string for the addr-spec of the Contact address.  Also note that
 there is no message body, and accordingly, the signature string will
 terminate, in this case, with two vertical bars.  The canonical
 string over which the identity signature will be generated is the
 following (note that the first line wraps because of RFC editorial
 conventions):
 sip:bob@biloxi.example.org|sip:alice@atlanta.example.com|
 a84b4c76e66710|231 BYE|Thu, 21 Feb 2002 14:19:51 GMT||
 The resulting signature (sha1WithRsaEncryption) using the private RSA
 key given above for biloxi.example.org, with base64 encoding, is the
 following:

Peterson & Jennings Standards Track [Page 21] RFC 4474 SIP Identity August 2006

 sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
 eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
 pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=
 Accordingly, the biloxi.example.org authentication service will
 create an Identity header containing that base64 signature string.
 It will also add an HTTPS URL where its certificate is made
 available.  With those two headers added, the message looks like the
 following:
 BYE sip:alice@pc33.atlanta.example.com SIP/2.0
 Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
 Max-Forwards: 70
 From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
 To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
 Date: Thu, 21 Feb 2002 14:19:51 GMT
 Call-ID: a84b4c76e66710
 CSeq: 231 BYE
 Identity:
   "sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
    eU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgtw0Lu5csIp
    pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs="
 Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
 Content-Length: 0
 biloxi.example.org then forwards the request normally.

11. Identity and the TEL URI Scheme

 Since many SIP applications provide a Voice over IP (VoIP) service,
 telephone numbers are commonly used as identities in SIP deployments.
 In the majority of cases, this is not problematic for the identity
 mechanism described in this document.  Telephone numbers commonly
 appear in the username portion of a SIP URI (e.g.,
 'sip:+17005551008@chicago.example.com;user=phone').  That username
 conforms to the syntax of the TEL URI scheme (RFC 3966 [13]).  For
 this sort of SIP address-of-record, chicago.example.com is the
 appropriate signatory.
 It is also possible for a TEL URI to appear in the SIP To or From
 header field outside the context of a SIP or SIPS URI (e.g.,
 'tel:+17005551008').  In this case, it is much less clear which
 signatory is appropriate for the identity.  Fortunately for the
 identity mechanism, this form of the TEL URI is more common for the
 To header field and Request-URI in SIP than in the From header field,
 since the UAC has no option but to provide a TEL URI alone when the
 remote domain to which a request is sent is unknown.  The local
 domain, however, is usually known by the UAC, and accordingly it can

Peterson & Jennings Standards Track [Page 22] RFC 4474 SIP Identity August 2006

 form a proper From header field containing a SIP URI with a username
 in TEL URI form.  Implementations that intend to send their requests
 through an authentication service SHOULD put telephone numbers in the
 From header field into SIP or SIPS URIs whenever possible.
 If the local domain is unknown to a UAC formulating a request, it
 most likely will not be able to locate an authentication service for
 its request, and therefore the question of providing identity in
 these cases is somewhat moot.  However, an authentication service MAY
 sign a request containing a TEL URI in the From header field.  This
 is permitted in this specification strictly for forward compatibility
 purposes.  In the longer-term, it is possible that ENUM [14] may
 provide a way to determine which administrative domain is responsible
 for a telephone number, and this may aid in the signing and
 verification of SIP identities that contain telephone numbers.  This
 is a subject for future work.

12. Privacy Considerations

 The identity mechanism presented in this document is compatible with
 the standard SIP practices for privacy described in RFC 3323 [3].  A
 SIP proxy server can act both as a privacy service and as an
 authentication service.  Since a user agent can provide any From
 header field value that the authentication service is willing to
 authorize, there is no reason why private SIP URIs that contain
 legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
 by an authentication service.  The construction of the Identity
 header is the same for private URIs as it is for any other sort of
 URIs.
 Note, however, that an authentication service must possess a
 certificate corresponding to the host portion of the addr-spec of the
 From header field of any request that it signs; accordingly, using
 domains like 'anonymous.invalid' will not be possible for privacy
 services that also act as authentication services.  The assurance
 offered by the usage of anonymous URIs with a valid domain portion is
 "this is a known user in my domain that I have authenticated, but I
 am keeping its identity private".  The use of the domain
 'anonymous.invalid' entails that no corresponding authority for the
 domain can exist, and as a consequence, authentication service
 functions are meaningless.
 The "header" level of privacy described in RFC 3323 requests that a
 privacy service alter the Contact header field value of a SIP
 message.  Since the Contact header field is protected by the
 signature in an Identity header, privacy services cannot be applied
 after authentication services without a resulting integrity
 violation.

Peterson & Jennings Standards Track [Page 23] RFC 4474 SIP Identity August 2006

 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 is very different from the Identity
 header presented in this document.  It contains additional
 information about the sender of a message that may go beyond what
 appears in the From header field; P-Asserted-Identity holds a
 definitive identity for the sender that is somehow known to a closed
 network of intermediaries that presumably the network will use this
 identity for billing or security purposes.  The danger of this
 network-specific information leaking outside of the closed network
 motivated the "id" priv-value token.  The "id" priv-value token has
 no implications for the Identity header, and privacy services MUST
 NOT remove the Identity header when a priv-value of "id" appears in a
 Privacy header.
 Finally, note that unlike RFC 3325, the mechanism described in this
 specification adds no information to SIP requests that has privacy
 implications.

13. Security Considerations

13.1. Handling of digest-string Elements

 This document describes a mechanism that provides a signature over
 the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
 requests.  While a signature over the From header field would be
 sufficient to secure a URI alone, the additional headers provide
 replay protection and reference integrity necessary to make sure that
 the Identity header will not be used in cut-and-paste attacks.  In
 general, the considerations related to the security of these headers
 are the same as those given in RFC 3261 for including headers in
 tunneled 'message/sip' MIME bodies (see Section 23 in particular).
 The following section details the individual security properties
 obtained by including each of these header fields within the
 signature; collectively, this set of header fields provides the
 necessary properties to prevent impersonation.
 The From header field indicates the identity of the sender of the
 message, and the SIP address-of-record URI in the From header field
 is the identity of a SIP user, for the purposes of this document.
 The To header field provides the identity of the SIP user that this
 request targets.  Providing the To header field in the Identity
 signature serves two purposes: first, it prevents cut-and-paste
 attacks in which an Identity header from legitimate request for one
 user is cut-and-pasted into a request for a different user; second,
 it preserves the starting URI scheme of the request, which helps
 prevent downgrade attacks against the use of SIPS.

Peterson & Jennings Standards Track [Page 24] RFC 4474 SIP Identity August 2006

 The Date and Contact headers provide reference integrity and replay
 protection, as described in RFC 3261, Section 23.4.2.
 Implementations of this specification MUST NOT deem valid a request
 with an outdated Date header field (the RECOMMENDED interval is that
 the Date header must indicate a time within 3600 seconds of the
 receipt of a message).  Implementations MUST also record Call-IDs
 received in valid requests containing an Identity header, and MUST
 remember those Call-IDs for at least the duration of a single Date
 interval (i.e., commonly 3600 seconds).  Because a SIP-compliant UA
 never generates the same Call-ID twice, verifiers can use the Call-ID
 to recognize cut-and-paste attacks; the Call-ID serves as a nonce.
 The result of this is that if an Identity header is replayed within
 the Date interval, verifiers will recognize that it is invalid
 because of a Call-ID duplication; if an Identity header is replayed
 after the Date interval, verifiers will recognize that it is invalid
 because the Date is stale.  The CSeq header field contains a numbered
 identifier for the transaction, and the name of the method of the
 request; without this information, an INVITE request could be cut-
 and-pasted by an attacker and transformed into a BYE request without
 changing any fields covered by the Identity header, and moreover
 requests within a certain transaction could be replayed in
 potentially confusing or malicious ways.
 The Contact header field is included to tie the Identity header to a
 particular user agent instance that generated the request.  Were an
 active attacker to intercept a request containing an Identity header,
 and cut-and-paste the Identity header field into its own request
 (reusing the From, To, Contact, Date, and Call-ID fields that appear
 in the original message), the attacker would not be eligible to
 receive SIP requests from the called user agent, since those requests
 are routed to the URI identified in the Contact header field.
 However, the Contact header is only included in dialog-forming
 requests, so it does not provide this protection in all cases.
 It might seem attractive to provide a signature over some of the
 information present in the Via header field value(s).  For example,
 without a signature over the sent-by field of the topmost Via header,
 an attacker could remove that Via header and insert its own in a
 cut-and-paste attack, which would cause all responses to the request
 to be routed to a host of the attacker's choosing.  However, a
 signature over the topmost Via header does not prevent attacks of
 this nature, since the attacker could leave the topmost Via intact
 and merely insert a new Via header field directly after it, which
 would cause responses to be routed to the attacker's host "on their
 way" to the valid host, which has exactly the same end result.
 Although it is possible that an intermediary-based authentication
 service could guarantee that no Via hops are inserted between the
 sending user agent and the authentication service, it could not

Peterson & Jennings Standards Track [Page 25] RFC 4474 SIP Identity August 2006

 prevent an attacker from adding a Via hop after the authentication
 service, and thereby preempting responses.  It is necessary for the
 proper operation of SIP for subsequent intermediaries to be capable
 of inserting such Via header fields, and thus it cannot be prevented.
 As such, though it is desirable, securing Via is not possible through
 the sort of identity mechanism described in this document; the best
 known practice for securing Via is the use of SIPS.
 This mechanism also provides a signature over the bodies of SIP
 requests.  The most important reason for doing so is to protect
 Session Description Protocol (SDP) bodies carried in SIP requests.
 There is little purpose in establishing the identity of the user that
 originated a SIP request if this assurance is not coupled with a
 comparable assurance over the media descriptors.  Note, however, that
 this is not perfect end-to-end security.  The authentication service
 itself, when instantiated at a intermediary, could conceivably change
 the SDP (and SIP headers, for that matter) before providing a
 signature.  Thus, while this mechanism reduces the chance that a
 replayer or man-in-the-middle will modify SDP, it does not eliminate
 it entirely.  Since it is a foundational assumption of this mechanism
 that the users trust their local domain to vouch for their security,
 they must also trust the service not to violate the integrity of
 their message without good reason.  Note that RFC 3261, Section 16.6,
 states that SIP proxy servers "MUST NOT add to, modify, or remove the
 message body."
 In the end analysis, the Identity and Identity-Info headers cannot
 protect themselves.  Any attacker could remove these headers from a
 SIP request, and modify the request arbitrarily afterwards.  However,
 this mechanism is not intended to protect requests from men-in-the-
 middle who interfere with SIP messages; it is intended only to
 provide a way that SIP users can prove definitively that they are who
 they claim to be.  At best, by stripping identity information from a
 request, a man-in-the-middle could make it impossible to distinguish
 any illegitimate messages he would like to send from those messages
 sent by an authorized user.  However, it requires a considerably
 greater amount of energy to mount such an attack than it does to
 mount trivial impersonations by just copying someone else's From
 header field.  This mechanism provides a way that an authorized user
 can provide a definitive assurance of his identity that an
 unauthorized user, an impersonator, cannot.
 One additional respect in which the Identity-Info header cannot
 protect itself is the 'alg' parameter.  The 'alg' parameter is not
 included in the digest-string, and accordingly, a man-in-the-middle
 might attempt to modify the 'alg' parameter.  However, it is
 important to note that preventing men-in-the-middle is not the
 primary impetus for this mechanism.  Moreover, changing the 'alg'

Peterson & Jennings Standards Track [Page 26] RFC 4474 SIP Identity August 2006

 would at worst result in some sort of bid-down attack, and at best
 cause a failure in the verifier.  Note that only one valid 'alg'
 parameter is defined in this document and that thus there is
 currently no weaker algorithm to which the mechanism can be bid down.
 'alg' has been incorporated into this mechanism for forward-
 compatibility reasons in case the current algorithm exhibits
 weaknesses, and requires swift replacement, in the future.

13.2. Display-Names and Identity

 As a matter of interface design, SIP user agents might render the
 display-name portion of the From header field of a caller as the
 identity of the caller; there is a significant precedent in email
 user interfaces for this practice.  As such, it might seem that the
 lack of a signature over the display-name is a significant omission.
 However, there are several important senses in which a signature over
 the display-name does not prevent impersonation.  In the first place,
 a particular display-name, like "Jon Peterson", is not unique in the
 world; many users in different administrative domains might
 legitimately claim that name.  Furthermore, enrollment practices for
 SIP-based services might have a difficult time discerning the
 legitimate display-name for a user; it is safe to assume that
 impersonators will be capable of creating SIP accounts with arbitrary
 display-names.  The same situation prevails in email today.  Note
 that an impersonator who attempted to replay a message with an
 Identity header, changing only the display-name in the From header
 field, would be detected by the other replay protection mechanisms
 described in Section 13.1.
 Of course, an authentication service can enforce policies about the
 display-name even if the display-name is not signed.  The exact
 mechanics for creating and operationalizing such policies is outside
 the scope of this document.  The effect of this policy would not be
 to prevent impersonation of a particular unique identifier like a SIP
 URI (since display-names are not unique identifiers), but to allow a
 domain to manage the claims made by its users.  If such policies are
 enforced, users would not be free to claim any display-name of their
 choosing.  In the absence of a signature, man-in-the-middle attackers
 could conceivably alter the display-names in a request with impunity.
 Note that the scope of this specification is impersonation attacks,
 however, and that a man-in-the-middle might also strip the Identity
 and Identity-Info headers from a message.
 There are many environments in which policies regarding the display-
 name aren't feasible.  Distributing bit-exact and internationalizable
 display-names to end-users as part of the enrollment or registration
 process would require mechanisms that are not explored in this

Peterson & Jennings Standards Track [Page 27] RFC 4474 SIP Identity August 2006

 document.  In the absence of policy enforcement regarding domain
 names, there are conceivably attacks that an adversary could mount
 against SIP systems that rely too heavily on the display-name in
 their user interface, but this argues for intelligent interface
 design, not changes to the mechanisms.  Relying on a non-unique
 identifier for identity would ultimately result in a weak mechanism.

13.3. Securing the Connection to the Authentication Service

 The assurance provided by this mechanism is strongest when a user
 agent forms a direct connection, preferably one secured by TLS, to an
 intermediary-based authentication service.  The reasons for this are
 twofold:
    If a user does not receive a certificate from the authentication
    service over this TLS connection that corresponds to the expected
    domain (especially when the user receives a challenge via a
    mechanism such as Digest), then it is possible that a rogue server
    is attempting to pose as an authentication service for a domain
    that it does not control, possibly in an attempt to collect shared
    secrets for that domain.
    Without TLS, the various header field values and the body of the
    request will not have integrity protection when the request
    arrives at an authentication service.  Accordingly, a prior
    legitimate or illegitimate intermediary could modify the message
    arbitrarily.
 Of these two concerns, the first is most material to the intended
 scope of this mechanism.  This mechanism is intended to prevent
 impersonation attacks, not man-in-the-middle attacks; integrity over
 the header and bodies is provided by this mechanism only to prevent
 replay attacks.  However, it is possible that applications relying on
 the presence of the Identity header could leverage this integrity
 protection, especially body integrity, for services other than replay
 protection.
 Accordingly, direct TLS connections SHOULD be used between the UAC
 and the authentication service whenever possible.  The opportunistic
 nature of this mechanism, however, makes it very difficult to
 constrain UAC behavior, and moreover there will be some deployment
 architectures where a direct connection is simply infeasible and the
 UAC cannot act as an authentication service itself.  Accordingly,
 when a direct connection and TLS are not possible, a UAC should use
 the SIPS mechanism, Digest 'auth-int' for body integrity, or both
 when it can.  The ultimate decision to add an Identity header to a

Peterson & Jennings Standards Track [Page 28] RFC 4474 SIP Identity August 2006

 request lies with the authentication service, of course; domain
 policy must identify those cases where the UAC's security association
 with the authentication service is too weak.

13.4. Domain Names and Subordination

 When a verifier processes a request containing an Identity-Info
 header, it must compare the domain portion of the URI in the From
 header field of the request with the domain name that is the subject
 of the certificate acquired from the Identity-Info header.  While it
 might seem that this should be a straightforward process, it is
 complicated by two deployment realities.  In the first place,
 certificates have varying ways of describing their subjects, and may
 indeed have multiple subjects, especially in 'virtual hosting' cases
 where multiple domains are managed by a single application.
 Secondly, some SIP services may delegate SIP functions to a
 subordinate domain and utilize the procedures in RFC 3263 [4] that
 allow requests for, say, 'example.com' to be routed to
 'sip.example.com'.  As a result, a user with the AoR
 'sip:jon@example.com' may process its requests through a host like
 '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
 authentication service on a subordinate host MUST be willing to
 supply that host with the private keying material associated with a
 certificate whose subject is a domain name that corresponds to the
 domain portion of the AoRs that the domain distributes to users.
 Note that this corresponds to the comparable case of routing inbound
 SIP requests to a domain.  When the NAPTR and SRV procedures of RFC
 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',
 the corresponding SRV records point to the service
 'sip1.example.org'), the client expects that the certificate passed
 back in any TLS exchange with that host will correspond exactly with
 the domain of the original Request-URI, not the domain name of the
 host.  Consequently, in order to make inbound routing to such SIP
 services work, a domain administrator must similarly be willing to
 share the domain's private key with the service.  This design
 decision was made to compensate for the insecurity of the DNS, and it
 makes certain potential approaches to DNS-based 'virtual hosting'
 unsecurable for SIP in environments where domain administrators are
 unwilling to share keys with hosting services.
 A verifier MUST evaluate the correspondence between the user's
 identity and the signing certificate by following the procedures
 defined in RFC 2818 [11], Section 3.1.  While RFC 2818 deals with the
 use of HTTP in TLS, the procedures described are applicable to

Peterson & Jennings Standards Track [Page 29] RFC 4474 SIP Identity August 2006

 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
 field of a SIP request with an Identity header.
 Because the domain certificates that can be used by authentication
 services need to assert only the hostname of the authentication
 service, existing certificate authorities can provide adequate
 certificates for this mechanism.  However, not all proxy servers and
 user agents will be able to support the root certificates of all
 certificate authorities, and moreover there are some significant
 differences in the policies by which certificate authorities issue
 their certificates.  This document makes no recommendations for the
 usage of particular certificate authorities, nor does it describe any
 particular policies that certificate authorities should follow, but
 it is anticipated that operational experience will create de facto
 standards for authentication services.  Some federations of service
 providers, for example, might only trust certificates that have been
 provided by a certificate authority operated by the federation.  It
 is strongly RECOMMENDED that self-signed domain certificates should
 not be trusted by verifiers, unless some previous key exchange has
 justified such trust.
 For further information on certificate security and practices, see
 RFC 3280 [9].  The Security Considerations of RFC 3280 are applicable
 to this document.

13.5. Authorization and Transitional Strategies

 Ultimately, the worth of an assurance provided by an Identity header
 is limited by the security practices of the domain that issues the
 assurance.  Relying on an Identity header generated by a remote
 administrative domain assumes that the issuing domain used its
 administrative practices to authenticate its users.  However, it is
 possible that some domains will implement policies that effectively
 make users unaccountable (e.g., ones that accept unauthenticated
 registrations from arbitrary users).  The value of an Identity header
 from such domains is questionable.  While there is no magic way for a
 verifier to distinguish "good" from "bad" domains by inspecting a SIP
 request, it is expected that further work in authorization practices
 could be built on top of this identity solution; without such an
 identity solution, many promising approaches to authorization policy
 are impossible.  That much said, it is RECOMMENDED that
 authentication services based on proxy servers employ strong
 authentication practices such as token-based identifiers.
 One cannot expect the Identity and Identity-Info headers to be
 supported by every SIP entity overnight.  This leaves the verifier in
 a compromising position; when it receives a request from a given SIP

Peterson & Jennings Standards Track [Page 30] RFC 4474 SIP Identity August 2006

 user, how can it know whether or not the sender's domain supports
 Identity?  In the absence of ubiquitous support for identity, some
 transitional strategies are necessary.
    A verifier could remember when it receives a request from a domain
    that uses Identity, and in the future, view messages received from
    that domain without Identity headers with skepticism.
    A verifier could query the domain through some sort of callback
    system to determine whether or not it is running an authentication
    service.  There are a number of potential ways in which this could
    be implemented; use of the SIP OPTIONS method is one possibility.
    This is left as a subject for future work.
 In the long term, some sort of identity mechanism, either the one
 documented in this specification or a successor, must become
 mandatory-to-use for the SIP protocol; that is the only way to
 guarantee that this protection can always be expected by verifiers.
 Finally, it is worth noting that the presence or absence of the
 Identity headers cannot be the sole factor in making an authorization
 decision.  Permissions might be granted to a message on the basis of
 the specific verified Identity or really on any other aspect of a SIP
 request.  Authorization policies are outside the scope of this
 specification, but this specification advises any future
 authorization work not to assume that messages with valid Identity
 headers are always good.

14. IANA Considerations

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

14.1. Header Field Names

 This document specifies two new SIP headers: Identity and Identity-
 Info.  Their syntax is given in Section 9.  These headers are defined
 by the following information, which has been added to the header
 sub-registry under http://www.iana.org/assignments/sip-parameters.
       Header Name: Identity
       Compact Form: y
       Header Name: Identity-Info
       Compact Form: n

Peterson & Jennings Standards Track [Page 31] RFC 4474 SIP Identity August 2006

14.2. 428 'Use Identity Header' Response Code

 This document registers a new SIP response code, which is described
 in Section 6.  It is sent when a verifier receives a SIP request that
 lacks an Identity header in order to indicate that the request should
 be re-sent with an Identity header.  This response code is defined by
 the following information, which has been added to the method and
 response-code sub-registry under
 http://www.iana.org/assignments/sip-parameters.
       Response Code Number: 428
       Default Reason Phrase: Use Identity Header

14.3. 436 'Bad Identity-Info' Response Code

 This document registers a new SIP response code, which is described
 in Section 6.  It is used when the Identity-Info header contains a
 URI that cannot be dereferenced by the verifier (either the URI
 scheme is unsupported by the verifier, or the resource designated by
 the URI is otherwise unavailable).  This response code is defined by
 the following information, which has been added to the method and
 response-code sub-registry under
 http://www.iana.org/assignments/sip-parameters.
       Response Code Number: 436
       Default Reason Phrase: Bad Identity-Info

14.4. 437 'Unsupported Certificate' Response Code

 This document registers a new SIP response code, which is described
 in Section 6.  It is used when the verifier cannot validate the
 certificate referenced by the URI of the Identity-Info header,
 because, for example, the certificate is self-signed, or signed by a
 root certificate authority for whom the verifier does not possess a
 root certificate.  This response code is defined by the following
 information, which has been added to the method and response-code
 sub-registry under http://www.iana.org/assignments/sip-parameters.
       Response Code Number: 437
       Default Reason Phrase: Unsupported Certificate

Peterson & Jennings Standards Track [Page 32] RFC 4474 SIP Identity August 2006

14.5. 438 'Invalid Identity Header' Response Code

 This document registers a new SIP response code, which is described
 in Section 6.  It is used when the verifier receives a message with
 an Identity signature that does not correspond to the digest-string
 calculated by the verifier.  This response code is defined by the
 following information, which has been added to the method and
 response-code sub-registry under
 http://www.iana.org/assignments/sip-parameters.
       Response Code Number: 438
       Default Reason Phrase: Invalid Identity Header

14.6. Identity-Info Parameters

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

14.7. Identity-Info Algorithm Parameter Values

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

Peterson & Jennings Standards Track [Page 33] RFC 4474 SIP Identity August 2006

Appendix A. Acknowledgements

 The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
 Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
 Johnston, Patrik Faltstrom, Paul Kyzviat, Adam Roach, John Elwell,
 Aki Niemi, and Jim Schaad for their comments.  Jonathan Rosenberg
 provided detailed fixes to innumerable sections of the document.  The
 bit-archive presented in Appendix B follows the pioneering example of
 RFC 4475 [16].  Thanks to Hans Persson and Tao Wan for thorough nit
 reviews.

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:
 o  (foo).message: the original message
 o  (foo).canonical: the canonical string constructed from that
    message
 o  (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
 o  (foo).signed: the RSA-signed SHA1 hash of the canonical string
    (binary)
 o  (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
    of the canonical string as it would appear in the request
 o  (foo).identity: the original message with the Identity and
    Identity-Info headers added
 Also included in the archive are two public key/certificate pairs,
 for atlanta.example.com and biloxi.example.org, respectively,
 including:
 o  (foo).cer: the certificate of the domain
 o  (foo).privkey: the private key of the domain
 o  (foo).pubkey: the public key of the domain, extracted from the
    cert file for convenience
 To recover the compressed archive file intact, the text of this
 document may be passed as input to the following Perl script (the
 output should be redirected to a file or piped to "tar -xzvf -").

Peterson & Jennings Standards Track [Page 34] RFC 4474 SIP Identity August 2006

 #!/usr/bin/perl
 use strict;
 my $bdata = "";
 use MIME::Base64;
 while(<>) {
  if (/-- BEGIN MESSAGE ARCHIVE --/ .. /-- END MESSAGE ARCHIVE --/) {
      if ( m/^\s*[^\s]+\s*$/) {
          $bdata = $bdata . $_;
      }
   }
 }
 print decode_base64($bdata);
 Alternatively, the base-64 encoded block can be edited by hand to
 remove document structure lines and fed as input to any base-64
 decoding utility.

B.1. Encoded Reference Files

  1. - BEGIN MESSAGE ARCHIVE –

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 d8z/a4OerNZze4z+iczvXqwBtvrI+7TMhDq3WqlMK9nlKt3a0z2RHGGlCQ8jMtub
 akAY2zocFupKgghFgbyFoS8BZx7Yl3mZXDZt5ZwYcj5kezmjEwY/YCO4rk+lFQc+
 26mK7GYb+rhviUDaVKy2X5DZUvOAOd8VeYQUtOfJ6QxVKtCW0DakDRBDOb3cIk3h
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Peterson & Jennings Standards Track [Page 35] RFC 4474 SIP Identity August 2006

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Peterson & Jennings Standards Track [Page 36] RFC 4474 SIP Identity August 2006

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 -- END MESSAGE ARCHIVE --

Peterson & Jennings Standards Track [Page 37] RFC 4474 SIP Identity August 2006

Appendix C. Original Requirements

 The following requirements were crafted throughout the development of
 the mechanism described in this document.  They are preserved here
 for historical reasons.
 o  The mechanism must allow a UAC or a proxy server to provide a
    strong cryptographic identity assurance in a request that can be
    verified by a proxy server or UAS.
 o  User agents that receive identity assurances must be able to
    validate these assurances without performing any network lookup.
 o  User agents that hold certificates on behalf of their user must be
    capable of adding this identity assurance to requests.
 o  Proxy servers that hold certificates on behalf of their domain
    must be capable of adding this identity assurance to requests; a
    UAC is not required to support this mechanism in order for an
    identity assurance to be added to a request in this fashion.
 o  The mechanism must prevent replay of the identity assurance by an
    attacker.
 o  In order to provide full replay protection, the mechanism must be
    capable of protecting the integrity of SIP message bodies (to
    ensure that media offers and answers are linked to the signaling
    identity).
 o  It must be possible for a user to have multiple AoRs (i.e.,
    accounts or aliases) that it is authorized to use within a
    domain, and for the UAC to assert one identity while
    authenticating itself as another, related, identity, as permitted
    by the local policy of the domain.

Peterson & Jennings Standards Track [Page 38] RFC 4474 SIP Identity August 2006

References

Normative References

 [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
       Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
       Session Initiation Protocol", RFC 3261, June 2002.
 [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [3]   Peterson, J., "A Privacy Mechanism for the Session Initiation
       Protocol (SIP)", RFC 3323, November 2002.
 [4]   Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
       (SIP): Locating SIP Servers", RFC 3263, June 2002.
 [5]   Peterson, J., "Session Initiation Protocol (SIP) Authenticated
       Identity Body (AIB) Format", RFC 3893, September 2004.
 [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 4234, October 2005.
 [7]   Housley, R., "Cryptographic Message Syntax (CMS) Algorithms",
       RFC 3370, August 2002.
 [8]   Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
       RFC 3548, July 2003.
 [9]   Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
       Public Key Infrastructure Certificate and Certificate
       Revocation List (CRL) Profile", RFC 3280, April 2002.
 [10]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
       Infrastructure Operational Protocols: FTP and HTTP", RFC 2585,
       May 1999.
 [11]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

Informative References

 [12]  Jennings, C., Peterson, J., and M. Watson, "Private Extensions
       to the Session Initiation Protocol (SIP) for Asserted Identity
       within Trusted Networks", RFC 3325, November 2002.
 [13]  Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
       December 2004.

Peterson & Jennings Standards Track [Page 39] RFC 4474 SIP Identity August 2006

 [14]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
       Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
       Application (ENUM)", RFC 3761, April 2004.
 [15]  Peterson, J., "Retargeting and Security in SIP: A Framework and
       Requirements", Work in 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

 Jon Peterson
 NeuStar, Inc.
 1800 Sutter St
 Suite 570
 Concord, CA  94520
 US
 Phone: +1 925/363-8720
 EMail: jon.peterson@neustar.biz
 URI:   http://www.neustar.biz/
 Cullen Jennings
 Cisco Systems
 170 West Tasman Drive
 MS: SJC-21/2
 San Jose, CA  95134
 USA
 Phone: +1 408 902-3341
 EMail: fluffy@cisco.com

Peterson & Jennings Standards Track [Page 40] RFC 4474 SIP Identity August 2006

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
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Acknowledgement

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 Administrative Support Activity (IASA).

Peterson & Jennings Standards Track [Page 41]

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