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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: August 17, 2005                                     C. Jennings
                                                           Cisco Systems
                                                       February 16, 2005


   Enhancements for Authenticated Identity Management in the Session
                       Initiation Protocol (SIP)
                       draft-ietf-sip-identity-04

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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   This Internet-Draft will expire on August 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

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 recommends practices and conventions for
   identifying end users in SIP messages, and proposes a way to
   distribute cryptographically-secure authenticated identities.




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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Background . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Requirements . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.   Overview of Operations . . . . . . . . . . . . . . . . . . .   6
   6.   Authentication Service Behavior  . . . . . . . . . . . . . .   7
     6.1  Identity within a Dialog and Retargeting . . . . . . . . .   9
   7.   Verifying Identity . . . . . . . . . . . . . . . . . . . . .  11
   8.   User Agent Behavior  . . . . . . . . . . . . . . . . . . . .  12
   9.   Proxy Server Behavior  . . . . . . . . . . . . . . . . . . .  12
   10.  Header Syntax  . . . . . . . . . . . . . . . . . . . . . . .  13
   11.  Compliance Tests and Examples  . . . . . . . . . . . . . . .  15
     11.1   Identity-Info with a Singlepart MIME body  . . . . . . .  16
     11.2   Identity for a Request with no MIME body or Contact  . .  18
   12.  Identity and the TEL URI Scheme  . . . . . . . . . . . . . .  21
   13.  Privacy Considerations . . . . . . . . . . . . . . . . . . .  22
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  23
   15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  27
     15.1   Header Field Names . . . . . . . . . . . . . . . . . . .  27
     15.2   428 'Use Identity Header' Response Code  . . . . . . . .  27
     15.3   436 'Bad Identity-Info' Response Code  . . . . . . . . .  28
     15.4   437 'Unsupported Certificate' Response Code  . . . . . .  28
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  29
   A.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  30
   B.   Bit-exact archive of example messages  . . . . . . . . . . .  30
     B.1  Encoded Reference Files  . . . . . . . . . . . . . . . . .  31
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  28
   16.1   Normative References . . . . . . . . . . . . . . . . . . .  28
   16.2   Informative References . . . . . . . . . . . . . . . . . .  29
   C.   Changelog  . . . . . . . . . . . . . . . . . . . . . . . . .  33
        Intellectual Property and Copyright Statements . . . . . . .  35


















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1.  Introduction

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

   RFC3261 stipulates several places within a SIP request that 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
   accurately, 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 TLS or S/MIME, 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.

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
   of the former would be a presence service that compares the identity
   of potential subscribers to a whitelist before determining whether it
   should begin sending presence notifications.  In both of these cases,
   attackers might attempt to circumvent these authorization policies
   through impersonation.  Since the primary identifier of the sender of



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   a SIP request, the From header field, can be populated arbitrarily be
   the controller of a user agent, impersonation is very simple today.
   The mechanism described in this document aspires to provide a strong
   identity system for SIP in which authorization policies cannot be
   circumvented by impersonation.

   All RFC3261-compliant SIP user agents support a means of
   authenticating themselves to a SIP registrar, commonly with a shared
   secret; Digest authentication, which MUST be supported by SIP user
   agents, is typically used for this purpose.  Registration allows a
   user agent to express that it is an appropriate entity to which
   requests should be sent for a particular address-of-record SIP 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 their identity to a
   registrar cannot be validated by just any user agent or proxy server
   - these credentials are only shared between the user agent and their
   domain administrator.  So this shared secret does not immediately
   help a user to authenticate to a wide range of recipients.
   Recipients require a means of determining whether or not the 'return
   address' identity of a non-REGISTER request (i.e., the From header
   field value) has legitimately been asserted.

   The address-of-record URI used for registration is also the URI with
   which a UA commonly populates the From header of requests in order to
   provide a 'return address' identity to recipients.  The identity
   mechanism specified in this document derives from the following
   principle: if you can prove you are eligible to register in a domain
   under a particular address-of-record, you are proving that you are
   capable of legitimately receiving requests for that
   address-of-record, and accordingly, when you place that
   address-of-record 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.

   Ideally, then, SIP user agents should have some way of proving to
   recipients of SIP requests that their local domain has authenticated
   them and authorized the population of the From header field.  This
   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



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   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 [8].  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
   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
   act as an authentication service.  To maximize end-to-end security,
   it is obviously preferable for end users to hold their own
   certificates; 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 (phones, PCs, laptops, PDAs, gaming devices) that may be
   employed by a single user.  In such environments, synchronizing
   certificates 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



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   instantiate the authentication service role.

4.  Requirements

   This draft addresses the following requirements:
   o  The mechanism must allow a UAC to provide a strong cryptographic
      identity assurance in a request that can be verified by a proxy
      server or UAS.
   o  User agents that receive identity assurances must be able to
      validate these assurances without performing any network lookup.
   o  User agents that hold certificates on behalf of their user must be
      capable of adding this identity assurance to requests.
   o  Proxy servers that hold certificates on behalf of their domain
      must be capable of adding this identity assurance to requests; a
      UAC is not required to support this mechanism in order for an
      identity assurance to be added to a request in this fashion.
   o  The mechanism must prevent replay of the identity assurance by an
      attacker.
   o  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) under which it is known at 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.
   o  It must be possible, in cases where a request has been retargeted
      to a different AoR than the one designated in the To header field,
      for the UAC to ascertain the AoR to which the request has been
      sent.

5.  Overview of Operations

   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 populate the value of the From header field (which may be Alice's
   AoR, or it may be some other value that the policy of the proxy



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   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 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 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 authenticate 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.

6.  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 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 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.
   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.

   The authentication service authenticates the identity of the message
   sender and validates that the identity given in the message can
   legitimately be asserted by the sender.  Then it computes a signature
   over the canonical form of several headers and all the bodies, and
   inserts this signature into the message.




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   First, an 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.  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 12 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.

   Second, the authentication service needs to determine whether or not
   the sender of the request is authorized to claim the identity given
   in the identity field.  In order to do so, the authentication service
   MUST authenticate the sender of the message.  Some possible ways in
   which this authentication might be performed include:
   o  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).
   o  If the authentication service is instantiated by a SIP user agent,
      a user agent can be said to authenticate its user on the grounds
      that the user can provision the user agent with the private key of
      the domain, or by 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 authorization 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'.  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



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   exceptions to such policies might arise for cases like anonymity; if
   the AoR asserted in the From header field is anonymous (per RFC3323
   [3]), then the proxy should authenticate that the user is a valid
   user in the domain and insert the signature over the From header
   field as usual.

   Note that this check is performed on the addr-spec in the From header
   field (e.g., the URI of the sender, like
   'sip:alice@atlanta.example.com'); it does not convert the
   display-name portion of the From header field (e.g., 'Alice
   Atlanta').  Some SIP user agents that receive requests render the
   display-name of the caller as the identity of the caller.  However,
   there are many environments in which legislating the display-name
   isn't feasible, judging from experience with email, where users
   frequent make slight textual changes to their display-names.
   Ultimately, there is more value in focusing on the SIP address of the
   sender (which has some meaning in the network and provides a chain of
   accountability) than trying to constrain how the display-name is set.
   As such, authentication services MAY check the display-name as well,
   and compare it to a list of acceptable display-names that may be used
   by the sender; if the display-name does not meet policy constraints,
   the authentication service MUST return a 403 'Inappropriate
   Display-Name' response code.  However, in many environments this will
   not make sense.  For more information on rendering identity in a user
   interface, see Section 8.

   Third, the authentication service MUST form the identity signature
   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 are provided in section Section 10.

   Finally, the authentication service MUST forward the message
   normally.

6.1  Identity within a Dialog and Retargeting

   Retargeting, the alteration by intermediaries of the Request-URI of a
   SIP request, can cause a few wrinkles for the Identity mechanism when
   it is applied to requests sent in the backwards direction within a
   dialog.  This section provides some non-normative considerations
   related to this case.

   When a request is retargeted, it may reach a SIP endpoint whose user
   is not identified by the URI designated in the To header field value.
   The value in the To header field of a dialog-forming request is used
   as the From header field of requests sent in the backwards direction



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   during the dialog, and is accordingly the header that would be signed
   by an authentication service for requests sent in the backwards
   direction.  In retargeting cases, if the URI in the From header does
   not identify the sender of the request in the backwards direction,
   then clearly it would be inappropriate to provide an Identity
   signature over that From header.  As specified above, if the
   authentication service is not responsible for the domain in the From
   header field of the request, it must not add an Identity header to
   the request, and should process/forward the request normally.

   If there were a means in backwards-direction requests to signify a
   'connected party', an identity of the unanticipated user whose SIP
   endpoint was reached by the dialog-forming request, it isn't clear
   that it would actually be beneficial to provide a corresponding
   Identity header signature over that information.  The Identity header
   is designed to prevent impersonation-based attacks, and it is very
   unclear how and why an attacker might attempt to impersonate an
   unanticipated third party in a backwards-direction request within an
   existing dialog.  That is, it's unclear how the caller's potential
   authorization policies would be any more successful at thwarting
   impersonation if new requests in the backwards direction came from an
   assured unanticipated third-party instead of an unassured
   unanticipated third-party.  Thwarting impersonation is, ultimately,
   the purpose of this Identity mechanism, and it must be left to other
   mechanisms to solve other security problems for SIP.

   The mechanism in this draft cannot aid in determining whether or not
   the unanticipated party is an appropriate target of this request and,
   accordingly, solving this problem is outside the scope of this draft.
   If, however, it were possible for the sender of the dialog-forming
   request to anticipate that retargeting had occurred, and to gain some
   kind of assurance of the new target of the request before any
   requests in the backwards direction were received, this would open up
   some new approaches to authorization policy.

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





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7.  Verifying Identity

   When a user agent or proxy server receives a SIP message containing
   an Identity header, it may inspect the signature to verify the
   identity of the sender of the message.  If an Identity header is not
   present in a request, and one is required by local policy (for
   example, based on a global policy, 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, the user
   agent or proxy server MUST first 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 retained 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.  If the certificate cannot be validated (it is self-signed
   and untrusted, or signed by an untrusted or unknown certificate
   authority), the verifier MUST send a 437 'Unsupported Certificate'
   response.

   Subsequently, the recipient MUST verify the signature in the Identity
   header, and compare the identity of the signer (the subjectAltName of
   the certificate) with the domain portion of the URI in the From
   header field of the request as described in Section 14.
   Additionally, the Date, Contact and Call-ID headers MUST be analyzed
   in the manner described in Section 14; recipients that wish to verify
   Identity signatures MUST support all of the operations described
   there.

   If a verifier determines that the signature on the message does not
   correspond to the text of the message, then a 428 'Invalid Identity
   Header' response MUST be returned.




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   Once the identity of the sender of a request has been ascertained,
   various policies MAY be used to make authorization decisions about
   accepting communications and the like.  Such policies are outside the
   scope of this document.

8.  User Agent Behavior

   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 14 for more information about securing the channel between
   the UAC and the authentication service.

   When a UAC sends a request, it MUST accurately populate the header
   field that asserts its identity (for a SIP request, this is the From
   header field).  In a request it MUST set the URI portion of its From
   header to match a SIP, SIPS or TEL URI AoR under which the UAC can
   register (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 12).

   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.  UAC implementations MUST provide a way of
   provisioning pre-loaded Route headers in order for this mechanism to
   work for mid-call requests in the backwards direction of a dialog.

   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.

9.  Proxy Server Behavior

   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 verify the existence of an
   Identity before it makes a forwarding decision for a request.  Proxy



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   servers MUST NOT remove or modify an existing Identity or
   Identity-Info header in a request.

   For the purposes of identifying mid-dialog requests, proxy servers
   that instantiate the authentication service role MUST Record-Route
   themselves in dialog-forming requests.

10.  Header Syntax

   This document specifies two new SIP headers: Identity and
   Identity-Info.  Each of these headers can appear only once in a SIP
   message.

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

   Identity-Info = "Identity-Info" HCOLON ident-info
   ident-info = LAQUOT absoluteURI RAQUOT

   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 placed in a bit-exact string in the order specified
   here, separated by a colon:
   o  The AoR of the UA sending the message, or the 'identity field'.
      For a request, this is the addr-spec from the From header field.
   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.  The
      first letter is upper case and the rest of the letters are lower
      case.  All requests that use the Identity mechanism MUST contain a
      Date header.
   o  The addr-spec component of the Contact header field value.  If the
      request does not contain a Contact header, this field MUST be
      empty (i.e., there will be no whitespace between the fourth and
      fifth colons 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).  Note that
      the message-body does NOT include the CRLF separating the SIP



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      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 colon 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 14 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, as follows: compute the results of
   signing this string with sha1WithRSAEncryption as described in RFC
   3370 and base64 encode the results as specified in RFC 3548.  Put the
   result in the Identity header.

   Note on this choice: Assuming a 1024 bit RSA key, the raw signature
   will result in about 170 octets of base64 encoded data (without
   base64, as an aside, it would be about 130 bytes).  For comparison's
   sake, a typical HTTP Digest Authorization header (such as those used
   in RFC3261) with no cnonce is around 180 octets.  From a speed point
   of view, a 2.8GHz Intel processor does somewhere in the range of 250
   RSA 1024 bits signs per second or 1200 RSA 512 bits signs; verifies
   are roughly 10 times faster.  Hardware accelerator cards are
   available that speed this up.

   The Identity-Info header MUST contain either an HTTPS URI or a SIPS
   URI.  If it contains an HTTPS URI, the URI must dereference to a
   resource that contains a single MIME body containing the certificate
   of the authentication service.  If it is a SIPS URI, then the
   authentication service intends for a user agent that wishes to fetch
   the certificate to form a TLS connection to that URI, acquire the
   certificate during normal TLS negotiation, and close the connection.

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







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

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

   Note, in the table above, that this mechanism does not protect the
   REGISTER method or 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.  The
   REGISTER method uses Contact header fields in very unusual ways that
   complicate its applicability to this mechanism.  Accordingly, the
   Identity and Identity-Info header MUST NOT appear in REGISTER or
   CANCEL.

11.  Compliance Tests and Examples

   The examples in this section illustrate the use of the Identity
   header in the context of a SIP transaction.  Implementations MUST
   verify their compliance with these examples, i.e.:
   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 appear here only for illustrative purposes.
   Therefore, in compliance testing, implementations of verifiers SHOULD
   generated appropriate warnings about the use of self-signed
   certificates.



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

11.1  Identity-Info with a Singlepart MIME body

   Consider the following private key and certificate pair assigned to
   'atlanta.example.com'.

   -----BEGIN RSA PRIVATE KEY-----
   MIICXQIBAAKBgQC8HmM8b9E4WNhb7tZAoBVSkKyV9rAEX3nyQbg4hXte1oW1BxC+
   43MQHrG3nk6Kc9afPR6VloKwWoUoAcCnbTJ/zEiZ6dq+C5EsQGIOowYkSgqdO2po
   joCnRgzgjgvAl41R2J6CE1kMwOQxNCxPnTco8l8UGdKbNLXIuNdUM1MG8QIDAQAB
   AoGAAtPOGAVyNo+XSOJxE+2UBHaqMWLQyHAK7Coys57F+OnufocJqGTQwOhFMYZO
   leQh0KjhgcwOUMo7gBtuotWQUbbLHTGKXiBR6Pqbm6CvhwJSuNYv0vONuTb1SMll
   Kadg43na4B9kQeytn1y6lfkTkK2oYqkDVZ2AAmLSLrfhl1UCQQDp7VFItgmnybwK
   PKwJs8gnF+u+K9j+sac/3vgGgrOvpxVqwoMXl6eWN//pZ/cqshanDLmtr9ahjWCD
   DxYVyklrAkEAzd6JLJAhG8cZymVCS5Jf0F7FAVxpx0BgRPHwJliyUg6O4jPY+ASg
   cLP6nz9a38wWZQj6rRygffGZHXbBFm+8EwJBAJmZEf5ESSK6+5VdMTlNqubAdjJw
   aBMUY1U0+naL66AyfYWUIq+jDI8+RfLkKQ8H0IfvexvokW2SfwSPK1kzcfECQD/O
   MQW2xgwt8ThhmeKCQ1/5f2WklsRCl5PGyH+aDeqQyIgjOaPlCzTjE1I3+JpUTryR
   w9/Td4qRTrtrCv1BNDECQQCgHIzF8LFtI003w9MAEAoCyDbtHFPEj71b+qG22Yc4
   SPFBAbo3JGO+mrB0MX/GwJr+3DfgzMHaUx/tinPr+u1D
   -----END RSA PRIVATE KEY-----
   -----BEGIN CERTIFICATE-----
   MIIC/TCCAmagAwIBAgIBADANBgkqhkiG9w0BAQQFADBZMQswCQYDVQQGEwJVUzEQ
   MA4GA1UECBMHR2VvcmdpYTESMBAGA1UEBxQJQXRsYXQIbnRhMQ0wCwYDVQQKEwRJ
   RVRGMRUwEwYDVQQLFAxTT0lQCAgISVAgV0cwHhcNMDQwOTEzMTAxMzAzWhcNMDUw
   OTEzMTAxMzAzWjBZMQswCQYDVQQGEwJVUzEQMA4GA1UECBMHR2VvcmdpYTESMBAG
   A1UEBxQJQXRsYXQIbnRhMQ0wCwYDVQQKEwRJRVRGMRUwEwYDVQQLFAxTT0lQCAgI
   SVAgV0cwgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBALweYzxv0ThY2Fvu1kCg
   FVKQrJX2sARfefJBuDiFe17WhbUHEL7jcxAesbeeTopz1p89HpWWgrBahSgBwKdt
   Mn/MSJnp2r4LkSxAYg6jBiRKCp07amiOgKdGDOCOC8CXjVHYnoITWQzA5DE0LE+d
   NyjyXxQZ0ps0tci411QzUwbxAgMBAAGjgdQwgdEwHQYDVR0OBBYEFGfCU7cNxqSK
   NurvFqz8gj5px8uoMIGBBgNVHSMEejB4gBRnwlO3Dcakijbq7xas/II+acfLqKFd
   pFswWTELMAkGA1UEBhMCVVMxEDAOBgNVBAgTB0dlb3JnaWExEjAQBgNVBAcUCUF0
   bGF0CG50YTENMAsGA1UEChMESUVURjEVMBMGA1UECxQMU09JUAgICElQIFdHggEA
   MAwGA1UdEwQFMAMBAf8wHgYDVR0RBBcwFYITYXRsYW50YS5leGFtcGxlLmNvbTAN
   BgkqhkiG9w0BAQQFAAOBgQAc0a/5hU6yqRTxwqoBuRk/iSqDnJD/B0QQnSFLqdjy
   QV/Pm+aluA05aLRDWq6w/ufwX2HPLOvXYubpnNzjpaWCx3OLr4b5NwnsfNSxtKBJ
   vI9PWwhSW6VMo/cT2llhNudCmN+LXPd/SLy3gnGvXtwcrWAT8MVYmkCUQTRvbWaR
   fQ==
   -----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.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>
         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, in 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:

   CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
   3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
   sM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI=

   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: "CyI4+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2s
                   N3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGC
                   lsM9CG4hq+YJZTMaSROoMUBhikVIjnQ8ykeD6UXNOyfI="
         Identity-Info: https://atlanta.example.com/cert
         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 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.

11.2  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-----
   MIICXQIBAAKBgQDDIREMIIS9vBBET2FFHss2Lbwri/nK+AMoUZ74UT3amG/bYgDn
   H86eUUEjGfV3cfXErFXSnI86sUALoKjjwGYBoiUuaMhyerZyF+D9St2plnBeq6fq
   rbaPpL6bvIAF636/O2+GFP3LSLj6KS4HQwnsaUBr2YzykBD05PfwrH28VQIDAQAB
   AoGAZLRJFwglWcKYZpjNK54T5HdAGP1Zwo2zG3jcYW2UTZ/EguWWb7HzsbNfuZzp
   GWcgHwuOE28nYHQgCKA26avfOGuebFHz2WLAFC3TCOVjMzJEWawtxIc7oX9vziTF
   1Uk2K4ccK2zdJlPI46fHjJrI2xXKZWkxVNkZ8LeMspckUqECQQDqhD0SoLXoRGks
   h7byNZAMR5PfZTpHli7uFg9O+GoLtxQNE/rW6JPVcVkpCvs8oPPUu+1D7dHnyFiO
   heyme35tAkEA1QEiny94KRtTuP/WEyyYUkRfltYjrAX1BC73Xu395cNwjvnNw7qI
   f2dFUm5akGijk9UtL1qNxg+akBgJXkbkiQJAXbUHXkkfRrcHO4bjIDcs3us++BXP
   yskE6Zeg+FIktZerCGrCYVs/rxsCoHbF2v0JUSjibrE5nZ8dW53B6OgRpQJBAKfr
   9zFrqN0vT/eeqVQAai0g/gLZ2tF4+MpNhHLwSKNkSk5NHSxa19UowvvTR85kz+Bx
   xOd6Ch7EmmNSr8AFP5ECQQDOXmjIecxNI51of9u6g4T2ITRcHTYyCqWLO6VqAWlD
   G6ej+6/h+8DQyfJKMNbfMCGjZ7xZC3isNMmFibGQTLZD
   -----END RSA PRIVATE KEY-----
   -----BEGIN CERTIFICATE-----
   MIIC7DCCAlWgAwIBAgIBADANBgkqhkiG9w0BAQQFADBUMQswCQYDVQQGEwJVUzEU
   MBIGA1UECBMLTWlzc2lzc2lwcGkxDzANBgNVBAcTBkJpbG94aTENMAsGA1UEChME
   SUVURjEPMA0GA1UECxMGU0lQIFdHMB4XDTA0MDkxMzEwMzg1NVoXDTA1MDkxMzEw
   Mzg1NVowVDELMAkGA1UEBhMCVVMxFDASBgNVBAgTC01pc3Npc3NpcHBpMQ8wDQYD
   VQQHEwZCaWxveGkxDTALBgNVBAoTBElFVEYxDzANBgNVBAsTBlNJUCBXRzCBnzAN
   BgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAwyERDCCEvbwQRE9hRR7LNi28K4v5yvgD
   KFGe+FE92phv22IA5x/OnlFBIxn1d3H1xKxV0pyPOrFAC6Co48BmAaIlLmjIcnq2
   chfg/UrdqZZwXqun6q22j6S+m7yABet+vztvhhT9y0i4+ikuB0MJ7GlAa9mM8pAQ
   9OT38Kx9vFUCAwEAAaOBzTCByjAdBgNVHQ4EFgQUlZRLaS3Zm/b0xWcq7TSnQMHM
   7w8wfAYDVR0jBHUwc4AUlZRLaS3Zm/b0xWcq7TSnQMHM7w+hWKRWMFQxCzAJBgNV
   BAYTAlVTMRQwEgYDVQQIEwtNaXNzaXNzaXBwaTEPMA0GA1UEBxMGQmlsb3hpMQ0w
   CwYDVQQKEwRJRVRGMQ8wDQYDVQQLEwZTSVAgV0eCAQAwDAYDVR0TBAUwAwEB/zAd
   BgNVHREEFjAUghJiaWxveGkuZXhhbXBsZS5vcmcwDQYJKoZIhvcNAQEEBQADgYEA
   SufJHtereahZlkE5ssRRZRd/erLpEe2uUfHnTOydPBKOkvhVG4Vr4aoroPlE7gJK
   a/2BF9bohwAUSC5j5q3nvuhUcoK9XZYm2nLkN3IAhCU6oswVBJAxLanGUCjR5sxS
   HfGhGsqLmTEQ22HsrtLo68IYiwftXcLZbep50gRVX6c=
   -----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 calcuating 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 colons.  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:

   A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlrZ
   Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
   fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=

   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: "A5oh1tSWpbmXTyXJDhaCiHjT2xR2PAwBroi5Y8tdJ+CL3ziY72N3Y+lP8eoiXlr
             ZOuwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9
             BfxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM="
   Identity-Info: https://biloxi.example.org/cert
   Content-Length: 0

   biloxi.example.org then forwards the request normally.

12.  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').  That username conforms to
   the syntax of the TEL URI scheme (RFC2806bis [9]).  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 MUST put telephone numbers in the
   From header field into SIP or SIPS URIs, if 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 in
   accordance with its local policies.  Verifiers SHOULD NOT accept



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   signatures over From header TEL URIs in the absence of some
   pre-provisioned relationship with the signing domain that authorizes
   this usage of TEL URIs.

   The guidance in the paragraph above is largely provided for forward
   compatibility.  In the longer-term, it is possible that ENUM [10] may
   provide a way to determine which administrative domain is responsible
   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.

13.  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 (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 'invalid.net' may not be possible for privacy services
   that also act as authentication services.  The assurance offered by
   this combination service is "this is a known user in my domain that I
   have authenticated, but I am keeping their identity private".

   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 [8] 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
   identity for billing or security purposes.  The danger of this
   network-specific information leaking outside of the closed network



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   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.

14.  Security Considerations

   This document describes a mechanism which provides a signature over
   the Contact, Date, Call-ID, CSeq, To, and From header fields of SIP
   messages.  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 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 servers two purposes: first, it prevents replay 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
   Call-IDs for at least the duration of a single Date interval (i.e.
   commonly 3600 seconds).  Accordingly, if an Identity header is
   replayed within the Date interval, receivers will recognize that it
   is invalid because of a Call-ID duplication; if an Identity header is
   replayed after the Date interval, receivers 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.




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   The Contact header field is included to tie the Identity header to a
   particular device 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 accordingly 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.

   Note that this mechanism does not provide any protection for the
   display-name portion of the From header field, and thus users are
   free to use any display-name of their choosing, and attackers could
   conceivably alter the display-names in a request with impunity.  If
   an administrative domain wants to control the display-names selected
   by users, they could do so with policies outside the scope of this
   document (for example, their authentication service could reject
   requests from valid users that contain an improper display-name in
   the From header field).  While there are conceivably attacks that an
   adversary could mount against SIP systems that rely too heavily on
   the display-name in their user interface, this argues for intelligent
   interface design, not changes to the protocol.

   This mechanism also provides a signature over the bodies of SIP



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   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 agent that originated a SIP
   request if a man-in-the-middle can change the SDP and direct media to
   an different IP address.  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 man-in-the-middle will
   interfere with sessions, 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."

   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 a 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 into the request
      arrives at an authentication service.  Accordingly, a prior
      legitimate or illegitimate intermediary could modify the message
      arbitrarily.

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

   Accordingly, direct TLS connections SHOULD be used between the UAC
   and the authentication service whenever possible.  The opportunistic
   nature of this mechanism, however, makes it very difficult to
   constrain UAC behavior, and moreover there will be some deployment
   architectures where a direct connection is simply infeasible and the
   UAC cannot act as an authentication service itself.  Accordingly,
   when a direct connection and TLS is not possible, a UAC should use



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   the SIPS mechanism, Digest 'auth-int' for body integrity, or both
   when it can.  The ultimate decision to add an Identity header to a
   request lies with the authentication service, of course, domain
   policy must identify those cases where the UAC's security association
   with the authentication service is too weak.

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

   Since a domain certificate is used by an authentication service
   (rather than individual certificates for each identity), certain
   problems can arise with name subordination.  For example, if an
   authentication service holds a common certificate for the hostname
   'sip.atlanta.example.com', can it legitimately sign a token
   containing an identity of 'sip:alice@atlanta.example.com'? It is
   difficult for the recipient of a request to ascertain whether or not
   'sip.atlanta.example.com' is authoritative for the
   'atlanta.example.com' domain unless the recipient has some
   foreknowledge of the administration of 'atlanta.example.com'.
   Therefore, it is RECOMMENDED that UASs receiving signed requests
   notify end users if there is ANY discrepancy between the
   subjectAltName of the signer's certificate and the host portion of
   the identity within the From header field.  If the domain name in the
   subject of the certificate is subordinate to the domain name in the
   identity URI, then verifiers may consider this a minor discrepancy.
   Additionally, there are ways that a verifier might leverage the
   information about canonical SIP servers within a domain stored in the
   DNS (see RFC3263 [4]) to determine whether or not a particular
   authentication service is authoritative for a domain; however, this
   is a subject for future work.

   Because the domain certificates that can be used by authentication
   services need to assert only the hostname of the authentication
   service, existing certificate authorities can provide adequate



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   certificates for this mechanism.  However, not all proxy servers and
   user agents will be able support the root certificates of all
   certificate authorities, and moreover there are some significant
   differences in the policies by which certificate authorities issue
   their certificates.  This document makes no recommendations for the
   usage of particular certificate authorities, nor does it describe any
   particular policies that certificate authorities should follow, but
   it is anticipated that operational experience will create de facto
   standards for authentication services.  Some federations of service
   providers, for example, might only trust certificates that have been
   provided by a certificate authority operated by the federation.  It
   is STRONGLY RECOMMENDED that self-signed domain certificates should
   not be trusted by verifiers, unless some pre-existing key exchange
   has justified such trust.

   Finally, 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.  Accordingly,
   these headers are only truly efficacious if the would-be verifier
   knows that they must be included in a request.  In the long term,
   some sort of identity mechanism along these lines must become
   mandatory-to-use for the SIP protocol; that is the only way to
   guarantee that this protection can always be expected.  In the
   interim, however, identity reception policies at a domain level or an
   address-book level should be used by verifiers to determine whether
   or not identity is expected from a particular source of SIP requests.
   Those authorization policies are outside the scope of this document.

15.  IANA Considerations

   This document requests changes to the header and response-code
   sub-registries of the SIP parameters IANA registry.

15.1  Header Field Names

   This document specifies two new SIP headers: Identity and
   Identity-Info.  Their syntax is given in Section 10.  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: (none)

15.2  428 'Use Identity Header' Response Code

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



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   Section 7.  It is used when a verifier received a SIP request that
   lacks an Identity header as a response indicating 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

15.3  436 'Bad Identity-Info' Response Code

   This document registers a new SIP response code which is described in
   Section 7.  It is used when the Identity-Info header contains a URI
   that cannot be dereferenced by the verifier (either the URI scheme is
   unsupported by the verifier, or the resource designated by the URI is
   otherwise unavailable).  This response code is defined by the
   following information, which is to be added to the method and
   response-code sub-registry under
   http://www.iana.org/assignments/sip-parameters.
      Response Code Number: 436
      Default Reason Phrase: Bad Identity-Info

15.4  437 'Unsupported Certificate' Response Code

   This document registers a new SIP response code which is described in
   Section 7.  It is used when the verifier cannot validate the
   certificate referenced by the URI of the Identity-Info header,
   because, for example, the certificate is self-signed, or signed by a
   root certificate authority for whom the verifier does not possess a
   root certificate.  This response code is defined by the following
   information, which is to be added to the method and response-code
   sub-registry under http://www.iana.org/assignments/sip-parameters.
      Response Code Number: 437
      Default Reason Phrase: Unsupported Certificate

16.  References

16.1  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.



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   [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.

16.2  Informative References

   [7]   Kohl, J. and C. Neumann, "The Kerberos Network Authentication
         Service (V5)", RFC 1510, September 1993.

   [8]   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.

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

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


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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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,
   and Aki Niemi for their comments.  The bit-archive presented in
   Appendix B follows the pioneering example of Robert Sparks'
   torture-test draft.

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 11.  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).cert: the certificate of the domain
   o  (foo).privkey: the private key of the domain
   o  (foo).pubkey: the public key of the domain, extracted from the
      cert file for convenience

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

   #!/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 . $_;
        }
     }
   }



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   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 --
   H4sICPGlE0ICA2lkZW50cmVmLnRhcgDsW8uv43hWLsTOomaFYMEmtIQAhar47eTO
   FJqf3+/E78QakGzHb8dO7CR2spwFGok9IMSWLftZwwaxYQRCQixgC0JC/Ac491ZV
   P6pvVyF1l3q675EsJyd+/F7n+8757OTbuD62cTJ79s0ZDOMwRRDjHkMpjBz3KI6T
   yLh/a88QGIVxCkVJFHsGIxiJEs8mxLOPYKfuGLSTybNiHx/jtmvqR47blWnbnPYv
   qhfbZ98hy9/Mf3CsgvoYvIzi9vh1zz8CwySOPzb/KIJg4/wjKIJSCEpQ4/EkgqLP
   JvDT/H/j9uJmNCdI+oThTFviJQbY3L0X0iSJmdkMA3ZBCnqJBum4sUCn0/KQlbmw
   6GEaGAYPWNrXjK5njA3rGobA9bLrXDkD0gAuAMThGFoTTdQ9R7vtfmNzlkaDez89
   GLKxNrvN2pDC2sw0A+6Z/v4iCtebMmS6pqCZTs89OFUeDLYNVwYzNsVyQerCUS9m
   ka6xRr+0uatmg0G7gqt373N66HPO4sub+VWthD6kmV/VSuhNM1N/3rPGRlYaX8rO
   kQ4MjqYNwKYbGGiSIINGoIHax5vrcIbtbIPy5xNSMinEu4rRymu0A2YSJzJ9YnM+
   RigvCx2RU6kiGkDchXFsN/srsp8vxL3npS0dZFZK98r2CGn1TLPkeo+2uFpaA9ik
   ZEHnpsLsYSrY5ctU2Qrsklkyc2ZduOKmbiTbM66AYDlY5aZbSL8Ul/Vg+PC+g49R
   jiOIcXX6cADpOERAKNKt0adbrhdvA2vCS5recLyQMA4V6cPBUiD91J75w3WeFsR+
   mJ+asb80nequaGlcXNB4Spt1Xy0xNgrKvAgP1BB0M0maBlGiHhR+C+35rvdsTtVA
   +bBwMo1xXW3gWLC8XWhcmjYNb6sQk+vA4wauAMaDP3IYh4ehUOBhRiDgcV51DXQP
   851pnOW4jllwrkZrD77B0Bx4ITvj1DFcZUj8VkxTDoxLub8dMPbS4DUw9juZ92J6
   31+TpqOe30j25rZGvPEmFlHFAn+MhKFSd/o5tIEOvRM0t5YbIIKDGZE55OVg2kN/
   aOiTWc5y68DWMjujYcOoLV49bIsLZLiz1W4aVCcAE4Fqst6B7GenpF+j4kpdnteb
   U7iv9WuxDzxmwJZqi4eE3tddolvDUaFl6CwtVl6fWR7pas0sstGqyvTTltnpU3W9
   2s4s9YKltXBeH/uo9YA919zNrmQcwzbPoReYUGK8egXdQwOns+/CxbMn+w7w/77N
   z2V8+Yj8jxAk9UX+J6kn/v/Y/G9aYLIyJXcM6InCbT7NAUbaG3FeGeGKmYs7bR4u
   ONzTs5A6+qChXatULu6iBdwaqy9GmOLZ+hgjjYfQAzOFcEwzxFbA6pJUokWQrEzS
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   -- END MESSAGE ARCHIVE --


Appendix C.  Changelog

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

   Changes from draft-ietf-sip-identity-03:
      - Softened requirement for TLS and direct connections; now
      SHOULD-strength, SIPS and Digest auth-int listed as alternatives.
      - Added non-normative section about authentication service
      behavior for backwards-direction requests within a dialog
      - Added support for CID URI in Identity Info
      - Added new response codes (436 and 437) corresponding to error
      cases for an unsupported URI scheme and an unsupported
      certificate, respectively

   Changes from draft-ietf-sip-identity-02:
      - 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




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      - 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































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   Internet Society.




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