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Versions: (draft-peterson-sip-identity) 00 01 02 03 04 05 06 RFC 4474

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


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

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

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

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



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


Table of Contents

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






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1.  Introduction

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

   RFC3261 stipulates several places within a SIP request where a user
   can express an identity for themselves, notably the user-populated
   From header field.  However, the recipient of a SIP request has no
   way to verify that the From header field has been populated
   appropriately, in the absence of some sort of cryptographic
   authentication mechanism.

   RFC3261 specifies a number of security mechanisms that can be
   employed by SIP UAs, including Digest, TLS and S/MIME
   (implementations may support other security schemes as well).
   However, few SIP user agents today support the end-user certificates
   necessary to authenticate themselves (via S/MIME, for example), and
   furthermore Digest authentication is limited by the fact that the
   originator and destination must share a pre-arranged secret.  It is
   desirable for SIP user agents to be able to send requests to
   destinations with which they have no previous association - just as
   in the telephone network today, one can receive a call from someone
   with whom one has no previous association, and still have a
   reasonable assurance that their displayed Caller-ID is accurate.  A
   cryptographic approach, like the one described in this document, can
   probably provide a much stronger and less-spoofable assurance of
   identity than the telephone network provides today.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in RFC2119 [2] and indicate requirement levels for
   compliant SIP implementations.


3.  Background

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



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

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

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

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

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



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   them and authorized the population of the From header field.  This
   document proposes a mediated authentication architecture for SIP in
   which requests are sent to a server in the user's local domain, which
   authenticates such requests (using the same practices by which the
   domain would authenticate REGISTER requests).  Once a message has
   been authenticated, the local domain then needs some way to
   communicate to other SIP entities that the sending user has been
   authenticated and their use of the From header field has been
   authorized.  This draft addresses how that imprimatur of
   authentication can be shared.

   RFC3261 already describes an architecture very similar to this in
   Section 26.3.2.2, in which a user agent authenticates itself to a
   local proxy server which in turn authenticates itself to a remote
   proxy server via mutual TLS, creating a two-link chain of transitive
   authentication between the originator and the remote domain.  While
   this works well in some architectures, there are a few respects in
   which this is impractical.  For one, transitive trust is inherently
   weaker than an assertion that can be validated end-to-end.  It is
   possible for SIP requests to cross multiple intermediaries in
   separate administrative domains, in which case transitive trust
   becomes even less compelling.

   One solution to this problem is to use 'trusted' SIP intermediaries
   that assert an identity for users in the form of a privileged SIP
   header.  A mechanism for doing so (with the P-Asserted-Identity
   header) is given in [12].  However, this solution allows only hop-by-
   hop trust between intermediaries, not end-to-end cryptographic
   authentication, and it assumes a managed network of nodes with strict
   mutual trust relationships, an assumption that is incompatible with
   widespread Internet deployment.

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

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



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


4.  Overview of Operations

   This section provides an informative (non-normative) high-level
   overview of the mechanisms described in this document.

   Imagine the case where Alice, who has the home proxy of example.com
   and the address-of-record sip:alice@example.com, wants to communicate
   with sip:bob@example.org.

   Alice generates an INVITE and places her identity in the From header
   field of the request.  She then sends an INVITE over TLS to an
   authentication service proxy for her domain.

   The authentication service authenticates Alice (possibly by sending a
   Digest authentication challenge) and validates that she is authorized
   to assert the identity which is populated in the From header field.
   This value may be Alice's AoR, or it may be some other value that the
   policy of the proxy server permits her to use.  It then computes a
   hash over some particular headers, including the From header field
   and the bodies in the message.  This hash is signed with the
   certificate for the domain (example.com, in Alice's case) and
   inserted in a new header field in the SIP message, the 'Identity'
   header.

   The proxy, as the holder of the private key of its domain, is
   asserting that the originator of this request has been authenticated
   and that she is authorized to claim the identity (the SIP address-of-
   record) which appears in the From header field.  The proxy also
   inserts a companion header field, Identity-Info, that tells Bob how
   to acquire its certificate, if he doesn't already have it.

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




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5.  Authentication Service Behavior

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

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

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

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

   Step 2: The authentication service MUST determine whether or not the
   sender of the request is authorized to claim the identity given in
   the identity field.  In order to do so, the authentication service



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   MUST authenticate the sender of the message.  Some possible ways in
   which this authentication might be performed include:
      If the authentication service is instantiated by a SIP
      intermediary (proxy server), it may challenge the request with a
      407 response code using the Digest authentication scheme (or
      viewing a Proxy-Authentication header sent in the request which
      was sent in anticipation of a challenge using cached credentials,
      as described in RFC 3261 Section 22.3).  Note that if that proxy
      server is maintaining a TLS connection with the client over which
      the client had previously authenticated itself using Digest
      authentication, the identity value obtained from that previous
      authentication step can be reused without an additional Digest
      challenge.
      If the authentication service is instantiated by a SIP user agent,
      a user agent can be said to authenticate its user on the grounds
      that the user can provision the user agent with the private key of
      the domain, or preferably by providing a password that unlocks
      said private key.

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

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



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   name portion of the From header field (e.g., 'Alice Atlanta').
   Authentication services MAY check and validate the display-name as
   well, and compare it to a list of acceptable display-names that may
   be used by the sender; if the display-name does not meet policy
   constraints, the authentication service MUST return a 403 response
   code with the reason phrase "Inappropriate Display-Name".  However,
   the display-name is not always present, and in many environments the
   requisite operational procedures for display-name validation may not
   exist.  For more information, see Section 13.2.

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

   Step 4: The authentication service MUST form the identity signature
   and add an Identity header to the request containing this signature.
   After the Identity header has been added to the request, the
   authentication service MUST also add an Identity-Info header.  The
   Identity-Info header contains a URI from which its certificate can be
   acquired.  Details on the generation of both of these headers are
   provided in section Section 9.

   Finally, the authentication service MUST forward the message
   normally.

5.1.  Identity within a Dialog and Retargeting

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

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



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

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

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

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


6.  Verifier Behavior

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

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



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   here specified.

   Step 1: The verifier MUST acquire the certificate for the signing
   domain.  Implementations supporting this specification SHOULD have
   some means of retaining domain certificates (in accordance with
   normal practices for certificate lifetimes and revocation) in order
   to prevent themselves from needlessly downloading the same
   certificate every time a request from the same domain is received.
   Certificates cached in this manner should be indexed by the URI given
   in the Identity-Info header field value.

   Provided that the domain certificate used to sign this message is not
   previously known to the recipient, SIP entities SHOULD discover this
   certificate by dereferencing the Identity-Info header, unless they
   have some more efficient implementation-specific way of acquiring
   certificates for that domain.  If the URI scheme in the Identity-Info
   header cannot be dereferenced, then a 436 'Bad Identity-Info'
   response MUST be returned.  The client processes this certificate in
   the usual ways, including checking that it has not expired, that the
   chain is valid back to a trusted CA, and that it does not appear on
   revocation lists.  Once the certificate is acquired, it MUST be
   validated following the procedures in RFC3280 [9].  If the
   certificate cannot be validated (it is self-signed and untrusted, or
   signed by an untrusted or unknown certificate authority, expired, or
   revoked), the verifier MUST send a 437 'Unsupported Certificate'
   response.

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

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

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


7.  Considerations for User Agent




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

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

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

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

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


8.  Considerations for Proxy Servers

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



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   header in a request.


9.  Header Syntax

   This document specifies two new SIP headers: Identity and Identity-
   Info.  Each of these headers can appear only once in a SIP message.
   The grammar for these two headers is (following the ABNF [6] in s):
   (following the ABNF [6] in RFC3261 [1]):

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

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

   The signed-identity-digest is a signed hash of a canonical string
   generated from certain components of a SIP request.  To create the
   contents of the signed-identity-digest, the following elements of a
   SIP message MUST be placed in a bit-exact string in the order
   specified here, separated by a vertical line, "|" or %x7C, character:
   o  The AoR of the UA sending the message, or addr-spec of the From
      header field (referred to occasionally here as the 'identity
      field').
   o  The addr-spec component of the To header field, which is the AoR
      to which the request is being sent.
   o  The callid from Call-Id header field.
   o  The digit (1*DIGIT) and method (method) portions from CSeq header
      field, separated by a single space (ABNF SP, or %x20).  Note that
      the CSeq header field allows LWS rather than SP to separate the
      digit and method portions, and thus the CSeq header field may need
      to be transformed in order to be canonicalized.  The
      authentication service MUST strip leading zeros from the 'digit'
      portion of the Cseq before generating the digest-string.
   o  The Date header field, with exactly one space each for each SP and
      the weekday and month items case set as shown in BNF in 3261.  RFC
      3261 specifies that the BNF for weekday and month are a choice
      amongst a set of tokens.  The RFC 2234 rules for the BNF specify
      that tokens are case sensitive.  However, when used to construct
      the canonical string defined here, the first letter of each week
      and month MUST be capitalized, and the remaining two letters must
      be lowercase.  This matches the capitalization provided in the
      definition of each token.  All requests that use the Identity
      mechanism MUST contain a Date header.




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

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

   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.

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

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



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

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

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

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

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


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

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

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


10.  Compliance Tests and Examples




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

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

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

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

10.1.  Identity-Info with a Singlepart MIME body

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

















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   -----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
   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:













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         INVITE sip:bob@biloxi.example.org SIP/2.0
         Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
         To: Bob <sip:bob@biloxi.example.org>
         From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
         Call-ID: a84b4c76e66710
         CSeq: 314159 INVITE
         Max-Forwards: 70
         Date: Thu, 21 Feb 2002 13:02:03 GMT
         Contact: <sip:alice@pc33.atlanta.example.com>
         Content-Type: application/sdp
         Content-Length: 147

         v=0
         o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
         s=Session SDP
         c=IN IP4 pc33.atlanta.example.com
         t=0 0
         m=audio 49172 RTP/AVP 0
         a=rtpmap:0 PCMU/8000

   When the authentication service receives the INVITE, it authenticates
   Alice by sending a 407 response.  As a result, Alice adds an
   Authorization header to her request, and resends to the
   atlanta.example.com authentication service.  Now that the service is
   sure of Alice's identity, it calculates an Identity header for the
   request.  The canonical string over which the identity signature will
   be generated is the following (note that the first line wraps because
   of RFC editorial conventions):

   sip:alice@atlanta.example.com|sip:bob@biloxi.example.org|a84b4c76e66710|314159 INVITE|Thu, 21 Feb 2002 13:02:03 GMT|alice@pc33.atlanta.example.com|v=0
   o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
   s=Session SDP
   c=IN IP4 pc33.atlanta.example.com
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   The resulting signature (sha1WithRsaEncryption) using the private RSA
   key given above, with base64 encoding, is the following:

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

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



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   like:

         INVITE sip:bob@biloxi.exmple.org SIP/2.0
         Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
         To: Bob <sip:bob@biloxi.example.org>
         From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
         Call-ID: a84b4c76e66710
         CSeq: 314159 INVITE
         Max-Forwards: 70
         Date: Thu, 21 Feb 2002 13:02:03 GMT
         Contact: <sip:alice@pc33.atlanta.example.com>
         Identity:"kjOP4YVZXmF0X3/4RUfAG6ffwbVQepNGRBz58b3dJq3prEV4h5GnS4F6udDRCI4/
                   rSK9cl+TFv45nu0Qu2d/0WPPOvvc3JWwuUmHrCwGwC+tW7fOWnC07QKgQn40uwg5
                   7WaXixQev5N0JfoLXnO3UDoum89JRhXPAIp2vffJbD4="
         Identity-Info: <https://atlanta.example.com/atlanta.cer>;alg=rsa-sha1
         Content-Type: application/sdp
         Content-Length: 147

         v=0
         o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
         s=Session SDP
         c=IN IP4 pc33.atlanta.example.com
         t=0 0
         m=audio 49172 RTP/AVP 0
         a=rtpmap:0 PCMU/8000

   atlanta.example.com then forwards the request normally.  When Bob
   receives the request, if he does not already know the certificate of
   atlanta.example.com, he de-references the URL in the Identity-Info
   header to acquire the certificate.  Bob then generates the same
   canonical string given above, from the same headers of the SIP
   request.  Using this canonical string, the signed digest in the
   Identity header, and the certificate discovered by de-referencing the
   Identity-Info header, Bob can verify that the given set of headers
   and the message body have not been modified.

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

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











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   -----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:

   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



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

   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:

   vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv
   wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM
   p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc=

   Accordingly, the biloxi.example.org authentication service will
   create an Identity header containing that base64 signature string.
   It will also add an HTTPS URL where its certificate is made
   available.  With those two headers added, the message looks like:







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   BYE sip:alice@pc33.atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnashds10
   Max-Forwards: 70
   From: Bob <sip:bob@biloxi.example.org>;tag=a6c85cf
   To: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Date: Thu, 21 Feb 2002 14:19:51 GMT
   Call-ID: a84b4c76e66710
   CSeq: 231 BYE
   Identity: "vvXEPaukq60Jd1M7Ag0CeCiI0cGfgV0uAyJA7UdpkT82E1TkWFJhc8DTDV5xnafv
              wKtekBNpfc0sbW2gfK7i/FRMNLuYOIk9aH9Oc+GhvR5J+m1uw1e2WBSYXH3FQJKM
              p94gYvRM3hD0P081WBGgxXlaN5LFplIKE25n4FzLhBc="
   Identity-Info: <https://biloxi.example.org/biloxi.cer>;alg=rsa-sha1
   Content-Length: 0

   biloxi.example.org then forwards the request normally.


11.  Identity and the TEL URI Scheme

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

   It is also possible for a TEL URI to appear in the SIP To or From
   header field outside the context of a SIP or SIPS URI (e.g.,
   'tel:+17005551008').  In this case, it is much less clear which
   signatory is appropriate for the identity.  Fortunately for the
   identity mechanism, this form of the TEL URI is more common for the
   To header field and Request-URI in SIP than in the From header field,
   since the UAC has no option but to provide a TEL URI alone when the
   remote domain to which a request is sent is unknown.  The local
   domain, however, is usually known by the UAC, and accordingly it can
   form a proper From header field containing a SIP URI with a username
   in TEL URI form.  Implementations that intend to send their requests
   through an authentication service SHOULD put telephone numbers in the
   From header field into SIP or SIPS URIs whenever possible.

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



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   is permitted in this specification strictly for forward compatibility
   purposes.  In the longer-term, it is possible that ENUM [14] may
   provide a way to determine which administrative domain is responsible
   for a telephone number, and this may aid in the signing and
   verification of SIP identities that contain telephone numbers.  This
   is a subject for future work.


12.  Privacy Considerations

   The identity mechanism presented in this draft is compatible with the
   standard SIP practices for privacy described in RFC3323 [3].  A SIP
   proxy server can act both as a privacy service and as an
   authentication service.  Since a user agent can provide any From
   header field value which the authentication service is willing to
   authorize, there is no reason why private SIP URIs which contain
   legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
   by an authentication service.  The construction of the Identity
   header is the same for private URIs as it is for any other sort of
   URIs.

   Note, however, that an authentication service must possess a
   certificate corresponding to the host portion of the addr-spec of the
   From header field of any request that it signs; accordingly, using
   domains like 'anonymous.invalid' will not be possible for privacy
   services that also act as authentication services.  The assurance
   offered by the usage of anonymous URIs with a valid domain portion is
   "this is a known user in my domain that I have authenticated, but I
   am keeping their identity private".  The use of the domain
   'anonymous.invalid' entails that no corresponding authority for the
   domain can exist, and as a consequence, authentication service
   functions are meaningless.

   The "header" level of privacy described in RFC3323 requests that a
   privacy service to alter the Contact header field value of a SIP
   message.  Since the Contact header field is protected by the
   signature in an Identity header, privacy services cannot be applied
   after authentication services without a resulting integrity
   violation.

   RFC3325 [12] defines the "id" priv-value token which is specific to
   the P-Asserted-Identity header.  The sort of assertion provided by
   the P-Asserted-Identity header is very different from the Identity
   header presented in this document.  It contains additional
   information about the sender of a message that may go beyond what
   appears in the From header field; P-Asserted-Identity holds a
   definitive identity for the sender which is somehow known to a closed
   network of intermediaries that presumably the network will use this



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   identity for billing or security purposes.  The danger of this
   network-specific information leaking outside of the closed network
   motivated the "id" priv-value token.  The "id" priv-value token has
   no implications for the Identity header, and privacy services MUST
   NOT remove the Identity header when a priv-value of "id" appears in a
   Privacy header.

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


13.  Security Considerations

13.1.  Handling of digest-string Elements

   This document describes a mechanism which provides a signature over
   the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
   requests.  While a signature over the From header field would be
   sufficient to secure a URI alone, the additional headers provide
   replay protection and reference integrity necessary to make sure that
   the Identity header will not be used in cut-and-paste attacks.  In
   general, the considerations related to the security of these headers
   are the same as those given in RFC3261 for including headers in
   tunneled 'message/sip' MIME bodies (see Section 23 in particular).
   The following section details the individual security properties
   obtained by including each of these header fields within the
   signature; collectively, this set of header fields provides the
   necessary properties to prevent impersonation.

   The From header field indicates the identity of the sender of the
   message, and the SIP address-of-record URI in the From header field
   is the identity of a SIP user, for the purposes of this document.
   The To header field provides the identity of the SIP user that this
   request targets.  Providing the To header field in the Identity
   signature serves two purposes: first, it prevents cut-and-paste
   attacks in which an Identity header from legitimate request for one
   user is cut-and-pasted into a request for a different user; second,
   it preserves the starting URI scheme of the request, which helps
   prevent downgrade attacks against the use of SIPS.

   The Date and Contact headers provide reference integrity and replay
   protection, as described in RFC3261 Section 23.4.2.  Implementations
   of this specification MUST NOT deem valid a request with an outdated
   Date header field (the RECOMMENDED interval is that the Date header
   must indicate a time within 3600 seconds of the receipt of a
   message).  Implementations MUST also record Call-IDs received in
   valid requests containing an Identity header, and MUST remember those



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   Call-IDs for at least the duration of a single Date interval (i.e.
   commonly 3600 seconds).  Because a SIP-compliant UA never generates
   the same Call-ID twice, verifiers can use the Call-ID to recognize
   cut-and-paste attacks; the Call-ID serves as a nonce.  The result of
   this is that if an Identity header is replayed within the Date
   interval, verifiers will recognize that it is invalid because of a
   Call-ID duplication; if an Identity header is replayed after the Date
   interval, verifiers will recognize that it is invalid because the
   Date is stale.  The CSeq header field contains a numbered identifier
   for the transaction, and the name of the method of the request;
   without this information, an INVITE request could be cut-and-pasted
   by an attacker and transformed into a BYE request without changing
   any fields covered by the Identity header, and moreover requests
   within a certain transaction could be replayed in potentially
   confusing or malicious ways.

   The Contact header field is included to tie the Identity header to a
   particular user agent instance that generated the request.  Were an
   active attacker to intercept a request containing an Identity header,
   and cut-and-paste the Identity header field into their own request
   (reusing the From, To, Contact, Date and Call-ID fields that appear
   in the original message), they would not be eligible to receive SIP
   requests from the called user agent, since those requests are routed
   to the URI identified in the Contact header field.  However, the
   Contact header is only included in dialog-forming requests, so it
   does not provide this protection in all cases.

   It might seem attractive to provide a signature over some of the
   information present in the Via header field value(s).  For example,
   without a signature over the sent-by field of the topmost Via header,
   an attacker could remove that Via header and insert their own in a
   cut-and-paste attack, which would cause all responses to the request
   to be routed to a host of the attacker's choosing.  However, a
   signature over the topmost Via header does not prevent attacks of
   this nature, since the attacker could leave the topmost Via intact
   and merely insert a new Via header field directly after it, which
   would cause responses to be routed to the attacker's host "on their
   way" to the valid host, which has exactly the same end result.
   Although it is possible that an intermediary-based authentication
   service could guarantee that no Via hops are inserted between the
   sending user agent and the authentication service, it could not
   prevent an attacker from adding a Via hop after the authentication
   service, and thereby pre-empting responses.  It is necessary for the
   proper operation of SIP for subsequent intermediaries to be capable
   of inserting such Via header fields, and thus it cannot be prevented.
   As such, though it is desirable, securing Via is not possible through
   the sort of identity mechanism described in this document; the best
   known practice for securing Via is the use of SIPS.



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   This mechanism also provides a signature over the bodies of SIP
   requests.  The most important reason for doing so is to protect SDP
   bodies carried in SIP requests.  There is little purpose in
   establishing the identity of the user that originated a SIP request
   if this assurance is not coupled with a comparable assurance over the
   media descriptors.  Note however that this is not perfect end-to-end
   security.  The authentication service itself, when instantiated at a
   intermediary, could conceivably change the SDP (and SIP headers, for
   that matter) before providing a signature.  Thus, while this
   mechanism reduces the chance that a replayer or man-in-the-middle
   will modify SDP, it does not eliminate it entirely.  Since it is a
   foundational assumption of this mechanism that the user trusts their
   local domain to vouch for their security, they must also trust the
   service not to violate the integrity of their message without good
   reason.  Note that RFC3261 16.6 states that SIP proxy servers "MUST
   NOT add to, modify, or remove the message body."

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

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





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13.2.  Display Names and Identity

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

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

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

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





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13.3.  Securing the Connection to the Authentication Service

   The assurance provided by this mechanism is strongest when a user
   agent forms a direct connection, preferably one secured by TLS, to an
   intermediary-based authentication service.  The reasons for this are
   twofold:
      If a user does not receive a certificate from the authentication
      service over this TLS connection that corresponds to the expected
      domain (especially when they receive a challenge via a mechanism
      such as Digest), then it is possible that a rogue server is
      attempting to pose as an authentication service for a domain that
      it does not control, possibly in an attempt to collect shared
      secrets for that domain.
      Without TLS, the various header field values and the body of the
      request will not have integrity protection with the request
      arrives at an authentication service.  Accordingly, a prior
      legitimate or illegitimate intermediary could modify the message
      arbitrarily.

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

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

13.4.  Domain Names and Subordination

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



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

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

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

   Because the domain certificates that can be used by authentication
   services need to assert only the hostname of the authentication
   service, existing certificate authorities can provide adequate
   certificates for this mechanism.  However, not all proxy servers and
   user agents will be able to support the root certificates of all
   certificate authorities, and moreover there are some significant
   differences in the policies by which certificate authorities issue
   their certificates.  This document makes no recommendations for the



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   usage of particular certificate authorities, nor does it describe any
   particular policies that certificate authorities should follow, but
   it is anticipated that operational experience will create de facto
   standards for authentication services.  Some federations of service
   providers, for example, might only trust certificates that have been
   provided by a certificate authority operated by the federation.  It
   is strongly RECOMMENDED that self-signed domain certificates should
   not be trusted by verifiers, unless some previous key exchange has
   justified such trust.

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

13.5.  Authorization and Transitional Strategies

   Ultimately, the worth of an assurance provided by an Identity header
   is limited by the security practices of the domain that issues the
   assurance.  Relying on an Identity header generated by a remote
   administrative domain assumes that the issuing domain used its
   administrative practices to authenticate its users.  However, it is
   possible that some domains will implement policies that effectively
   make users unaccountable (e.g., ones that accept unauthenticated
   registrations from arbitrary users).  The value of an Identity header
   from such domains is questionable.  While there is no magic way for a
   verifier to distinguish "good" from "bad" domains by inspecting a SIP
   request, it is expected that further work in authorization practices
   could be built on top of this identity solution; without such an
   identity solution, many promising approaches to authorization policy
   are impossible.  That much said, it is RECOMMENDED that
   authentication services based on proxy servers employ strong
   authentication practices such as token-based identifiers.

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




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

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


14.  IANA Considerations

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

14.1.  Header Field Names

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

14.2.  428 'Use Identity Header' Response Code

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

14.3.  436 'Bad Identity-Info' Response Code

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



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

14.4.  437 'Unsupported Certificate' Response Code

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

14.5.  438 'Invalid Identity Header' Response Code

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

14.6.  Identity-Info Parameters

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






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14.7.  Identity-Info Algorithm Parameter Values

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


Appendix A.  Acknowledgments

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


Appendix B.  Bit-exact archive of example messages

   The following text block is an encoded, gzip compressed TAR archive
   of files that represent the transformations performed on the example
   messages discussed in Section 10.  It includes for each example:
   o  (foo).message: the original message
   o  (foo).canonical: the canonical string constructed from that
      message
   o  (foo).sha1: the SHA1 hash of the canonical string (hexadecimal)
   o  (foo).signed: the RSA-signed SHA1 hash of the canonical string
      (binary)
   o  (foo).signed.enc: the base64 encoding of the RSA-signed SHA1 hash
      of the canonical string as it would appear in the request
   o  (foo).identity: the original message with the Identity and
      Identity-Info headers added

   Also included in the archive are two public key/certificate pairs,
   for atlanta.example.com and biloxi.example.org, respectively,
   including:
   o  (foo).cer: the certificate of the domain
   o  (foo).privkey: the private key of the domain
   o  (foo).pubkey: the public key of the domain, extracted from the
      cert file for convenience




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   To recover the compressed archive file intact, the text of this
   document may be passed as input to the following Perl script (the
   output should be redirected to a file or piped to "tar -xzvf -").

   #!/usr/bin/perl
   use strict;
   my $bdata = "";
   use MIME::Base64;
   while(<>) {
    if (/-- BEGIN MESSAGE ARCHIVE --/ .. /-- END MESSAGE ARCHIVE --/) {
        if ( m/^\s*[^\s]+\s*$/) {
            $bdata = $bdata . $_;
        }
     }
   }
   print decode_base64($bdata);

   Alternatively, the base-64 encoded block can be edited by hand to
   remove document structure lines and fed as input to any base-64
   decoding utility.

B.1.  Encoded Reference Files

   -- BEGIN MESSAGE ARCHIVE --
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   bHaXS6fwwAQsdMOsKrLE20oQeRrkrgmqqeHxbU8ywEfdIzYVvJiqs/HsUIjnZpoH



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   -- END MESSAGE ARCHIVE --


Appendix C.  Original Requirements

   The following requirements were crafted throughout the development of
   the mechanism described in this document.  They are preserved here
   for historical reasons.
   o  The mechanism must allow a UAC or a proxy server to provide a
      strong cryptographic identity assurance in a request that can be
      verified by a proxy server or UAS.
   o  User agents that receive identity assurances must be able to
      validate these assurances without performing any network lookup.
   o  User agents that hold certificates on behalf of their user must be
      capable of adding this identity assurance to requests.
   o  Proxy servers that hold certificates on behalf of their domain
      must be capable of adding this identity assurance to requests; a
      UAC is not required to support this mechanism in order for an
      identity assurance to be added to a request in this fashion.
   o  The mechanism must prevent replay of the identity assurance by an
      attacker.





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   o  In order to provide full replay protection, the mechanism must be
      capable of protecting the integrity of SIP message bodies (to
      ensure that media offers and answers are linked to the signaling
      identity).
   o  It must be possible for a user to have multiple AoRs (i.e.
      accounts or aliases) which it is authorized to use within a
      domain, and for the UAC to assert one identity while
      authenticating itself as another, related, identity, as permitted
      by the local policy of the domain.


Appendix D.  Changelog

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

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

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

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


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




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

   Changes from draft-ietf-sip-identity-02:
      - Extracted text relating to providing identity in SIP responses;
      this text will appear in a separate draft
      - Added compliance testing/example section
      - Added CSeq to the signature of the Identity header to prevent a
      specific cut-and-paste attack; also added addr-spec of the To
      header to the signature of the Identity header for similar reasons
      - Added text about why neither Via headers nor display-names are
      protected by this mechanism
      - Added bit-exact reference files for compliance testing
      - Added privacy considerations

   Changes from draft-ietf-sip-identity-01:
      - Completely changed underlying mechanism - instead of using an
      AIB, the mechanism now recommends the use of the Identity header
      and Identity-Info header
      - Numerous other changes resulting from the above
      - Various other editorial corrections

   Changes from draft-peterson-sip-identity-01:
      - Split off child draft-ietf-sip-authid-body-00 for defining of
      the AIB
      - Clarified scope in introduction
      - Removed a lot of text that was redundant with RFC3261
      (especially about authentication practices)
      - Added mention of content indirection mechanism for adding token
      to requests and responses
      - Improved Security Considerations (added piece about credential
      strength)

   Changes from draft-peterson-sip-identity-00:
      - Added a section on authenticated identities in responses
      - Removed hostname convention for authentication services
      - Added text about using 'message/sip' or 'message/sipfrag' in
      authenticated identity bodies, also RECOMMENDED a few more headers
      in sipfrags to increase reference integrity
      - Various other editorial corrections


15.  References






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15.1.  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Bradner, S., "Key words for use in RFCs to indicate requirement
         levels", RFC 2119, March 1997.

   [3]   Peterson, J., "A Privacy Mechanism for the Session Initiation
         Protocol (SIP)", RFC  3323, November 2002.

   [4]   Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
         (SIP): Locating SIP Servers", RFC  3263, June 2002.

   [5]   Peterson, J., "Session Initiation Protocol (SIP) Authenticated
         Identity Body (AIB) Format", RFC 3893, September 2004.

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

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

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

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

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

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

15.2.  Informative References

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

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

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



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
















































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Authors' Addresses

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St
   Suite 570
   Concord, CA  94520
   US

   Phone: +1 925/363-8720
   Email: jon.peterson@neustar.biz
   URI:   http://www.neustar.biz/


   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone: +1 408 902-3341
   Email: fluffy@cisco.com




























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