[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

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

SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: March 30, 2005                                      C. Jennings
                                                           Cisco Systems
                                                      September 29, 2004


   Enhancements for Authenticated Identity Management in the Session
                       Initiation Protocol (SIP)
                       draft-ietf-sip-identity-03

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
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on March 30, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  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.




Peterson & Jennings      Expires March 30, 2005                 [Page 1]

Internet-Draft                SIP Identity                September 2004


Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Background . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Requirements . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.   Overview of Operations . . . . . . . . . . . . . . . . . . .   6
   6.   Authentication Service Behavior  . . . . . . . . . . . . . .   7
   7.   Verifying Identity . . . . . . . . . . . . . . . . . . . . .   9
   8.   User Agent Behavior  . . . . . . . . . . . . . . . . . . . .  10
   9.   Proxy Server Behavior  . . . . . . . . . . . . . . . . . . .  10
   10.  Header Syntax  . . . . . . . . . . . . . . . . . . . . . . .  11
   11.  Compliance Tests and Examples  . . . . . . . . . . . . . . .  13
     11.1   Identity-Info with a Singlepart MIME body  . . . . . . .  14
     11.2   Identity for a Request with no MIME body or Contact  . .  16
   12.  Identity and the TEL URI Scheme  . . . . . . . . . . . . . .  19
   13.  Privacy Considerations . . . . . . . . . . . . . . . . . . .  20
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  21
   15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  25
     15.1   Header Field Names . . . . . . . . . . . . . . . . . . .  25
     15.2   Response Code  . . . . . . . . . . . . . . . . . . . . .  25
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  26
   A.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  26
   B.   Bit-exact archive of example messages  . . . . . . . . . . .  27
     B.1  Encoded Reference Files  . . . . . . . . . . . . . . . . .  27
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  25
   16.1   Normative References . . . . . . . . . . . . . . . . . . .  25
   16.2   Informative References . . . . . . . . . . . . . . . . . .  26
   C.   Changelog  . . . . . . . . . . . . . . . . . . . . . . . . .  30
        Intellectual Property and Copyright Statements . . . . . . .  32





















Peterson & Jennings      Expires March 30, 2005                 [Page 2]

Internet-Draft                SIP Identity                September 2004


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 enumerates a number of 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 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

   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 the proper entity to which requests
   should be sent for a particular address-of-record SIP URI (e.g.,
   'sip:alice@atlanta.example.com').

   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



Peterson & Jennings      Expires March 30, 2005                 [Page 3]

Internet-Draft                SIP Identity                September 2004


   provide their 'return address' identity to recipients.  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', you are also authorized to assert that
   'return address' in your From header field.

   In the context of registration, users already have a means of proving
   their identity to a registrar.  However, the credentials with which a
   user agent proves their identity to a registrar cannot be validated
   by just any user agent or proxy server - these credentials are only
   shared between the user agent and their domain administrator.  For
   the purposes of determining whether or not the 'return address' of a
   request can legitimately be asserted in the From header field of a
   request, SIP entities that are not operated by the domain
   administrator require an assurance that the sender of a message is
   capable of authenticating themselves to a registrar in their own
   domain.

   Ideally, then, SIP user agents should have some way of proving to
   recipients of SIP requests that their local domain has authenticated
   them.  In the absence of end-user certificates in user agents, it is
   possible to implement 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.  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.  It also requires intermediaries to act
   as proxies, rather than redirecting requests to their destinations
   (redirection lightens loads on SIP intermediaries).



Peterson & Jennings      Expires March 30, 2005                 [Page 4]

Internet-Draft                SIP Identity                September 2004


   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 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
   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 the Identity header in order for
      identity to be added to a request in this fashion.




Peterson & Jennings      Expires March 30, 2005                 [Page 5]

Internet-Draft                SIP Identity                September 2004


   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
   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



Peterson & Jennings      Expires March 30, 2005                 [Page 6]

Internet-Draft                SIP Identity                September 2004


   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, redirect 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
   or redirect 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.

   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.

   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 MAY handle 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



Peterson & Jennings      Expires March 30, 2005                 [Page 7]

Internet-Draft                SIP Identity                September 2004


   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 or redirect 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 assertion of a particular username in the From
   header field of a SIP message is a matter of local policy for the
   authorization service which depends greatly on the manner in which
   authentication is performed.  A RECOMMENDED policy is as follows: the
   username asserted during Digest authentication MUST correspond
   exactly to the username in the From header field of the SIP message.
   However, there are many cases in which a user might manage multiple
   accounts in the same administrative domain.  Accordingly, provided
   the authentication service is aware of the relationships between
   these accounts, it might allow a user providing credentials for one
   account to assert a username associated with another account
   controlled by the user name.  Furthermore, 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



Peterson & Jennings      Expires March 30, 2005                 [Page 8]

Internet-Draft                SIP Identity                September 2004


   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.

7.  Verifying Identity

   When a user agent or proxy server receives a SIP message containing
   an Identity header, it can 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.  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.

   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



Peterson & Jennings      Expires March 30, 2005                 [Page 9]

Internet-Draft                SIP Identity                September 2004


   Identity signatures MUST support all of the operations described
   there.  Any discrepancies or violations MUST be reported to the user.

   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.

   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 requires one important change to existing user agent
   requirements for sending requests: user agents using this mechanism
   to send requests to an authentication service MUST support TLS.
   Because this mechanism does not provide integrity protection for the
   first hop to the authentication service, the UAC MUST send requests
   to an authentication service only over a TLS connection.

   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



Peterson & Jennings      Expires March 30, 2005                [Page 10]

Internet-Draft                SIP Identity                September 2004


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

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.
   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
      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



Peterson & Jennings      Expires March 30, 2005                [Page 11]

Internet-Draft                SIP Identity                September 2004


      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]:

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

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



Peterson & Jennings      Expires March 30, 2005                [Page 12]

Internet-Draft                SIP Identity                September 2004


                                               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.

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






Peterson & Jennings      Expires March 30, 2005                [Page 13]

Internet-Draft                SIP Identity                September 2004


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:







Peterson & Jennings      Expires March 30, 2005                [Page 14]

Internet-Draft                SIP Identity                September 2004


         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



Peterson & Jennings      Expires March 30, 2005                [Page 15]

Internet-Draft                SIP Identity                September 2004


   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+nAkHrH3ntmaxgr01TMxTmtjP7MASwliNRdupRI1vpkXRvZXx1ja9k0nB2sN
                   3W+v1PDsy32MaqZi0M5WfEkXxbgTnPYW0jIoK8HMyY1VT7egt0kk4XrKFCHYWGCl
                   sM9CG4hq+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".











Peterson & Jennings      Expires March 30, 2005                [Page 16]

Internet-Draft                SIP Identity                September 2004


   -----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



Peterson & Jennings      Expires March 30, 2005                [Page 17]

Internet-Draft                SIP Identity                September 2004


   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:








Peterson & Jennings      Expires March 30, 2005                [Page 18]

Internet-Draft                SIP Identity                September 2004


   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+lP8eoiXlrZ
             Ouwb0DicF9GGxA5vw2mCTUxc0XG0KJOhpBnzoXnuPNAZdcZEWsVOQAKj/ERsYR9B
             fxNPazWmJZjGmDoFDbUNamJRjiEPOKn13uAZIcuf9zM=
   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



Peterson & Jennings      Expires March 30, 2005                [Page 19]

Internet-Draft                SIP Identity                September 2004


   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



Peterson & Jennings      Expires March 30, 2005                [Page 20]

Internet-Draft                SIP Identity                September 2004


   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.




Peterson & Jennings      Expires March 30, 2005                [Page 21]

Internet-Draft                SIP Identity                September 2004


   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



Peterson & Jennings      Expires March 30, 2005                [Page 22]

Internet-Draft                SIP Identity                September 2004


   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."

   Users SHOULD NOT provide credentials to an authentication service to
   which they cannot initiate a direct connection, preferably one
   secured by TLS.  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.  If a user cannot connect directly to the desired
   authentication service, the user SHOULD at least use a SIPS URI to
   ensure that mutual TLS authentication will be used to reach the
   remote server.

   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



Peterson & Jennings      Expires March 30, 2005                [Page 23]

Internet-Draft                SIP Identity                September 2004


   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 signers certificate and the identity within the
   authentication token.  Minor discrepancies MAY be characterized as a
   warning.  Additionally, relying parties MAY follow the procedures in
   RFC3263 [4] to look up in the DNS the domain portion of the identity
   in the From header field, and compare the SIP services listed for
   that domain with the subjectAltName of the certificate; this can give
   the relying party a better sense of the canonical SIP services for
   that domain.

   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 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.

   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.





Peterson & Jennings      Expires March 30, 2005                [Page 24]

Internet-Draft                SIP Identity                September 2004


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  Response Code

   This document registers one new SIP response code which is described
   in Section 7.  This response codes 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

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.

   [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.



Peterson & Jennings      Expires March 30, 2005                [Page 25]

Internet-Draft                SIP Identity                September 2004


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",
         draft-ietf-iptel-rfc2806bis-09 (work in progress), June 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

Appendix A.  Acknowledgments

   The authors would like to thank Eric Rescorla, Rohan Mahy, Robert
   Sparks, Jonathan Rosenberg, Mark Watson, Henry Sinnreich, Alan
   Johnston and Patrik Faltstrom for their comments.  The bit-archive
   presented in Appendix B follows the pioneering example of Robert
   Sparks' torture-test draft.




Peterson & Jennings      Expires March 30, 2005                [Page 26]

Internet-Draft                SIP Identity                September 2004


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 . $_;
        }
     }
   }
   print decode_base64($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 --



Peterson & Jennings      Expires March 30, 2005                [Page 27]

Internet-Draft                SIP Identity                September 2004


   H4sICANtWkECA2lkZW50cmVmLnRhcgDsW0vM41hWLmBnUbNCoBELAhJiUKiK33b+
   mUJ9/Xb8SPxOIkCyHT9iO3ZiO7GT5SzQSCyRACE2I2DLis2seUiIzYxASIgFbEEI
   hNiywPn/qupH9d9VoO5ST3eOZDk58ePce+75vnNO7O0mKts6iidPvjiBYRymCGLY
   YyiFkcMexXESGfav5QkCIyiJkyQG409gBCMx5MmIePIe5Ni0fj0aPcn2URvVTVU+
   ctwuT+rquH9WPNs8+QrJ9pX//bbwy9Z/HkZ1+3n7H4FhEscf8z+KINi9/xGUQlCC
   Go4nERR9MoJv/v/C5dlVGF6U9RHLm7YsyCyw+XstpMkyO7FZFuz8BHQyA5Jh44DO
   JPkhzbfitIMZYBgC4Ji1ZjQda6w41zBEvpu5zoU3IA3gIkAcnmU0yUTdU7jb7Fc2
   b2kMuNczvTEzlmazWhpyUJqpZsAd291fROE7cwaZrilqptPxD0pVAL1tw4XBDqZY
   LkhcOOykNNQ1zujmNn/RbNBrF3Dx7nVOB31MmX26mZ9lJfQuZn6WldArM5M13XHG
   aqZUazk9hToweIYxAJesYKDJ4gxUIgPULlpd+hNspytUOB2RnE0gwVWMerZEG2DG
   UTxjjtxWiBDKSwNH4lUqC3sQNUEU2dX+guzpqbT3vKRm/NRKmE7ZtJBWTjRrVu7R
   GldzqwerhMyYramwe5jyd9t5omxEbs7OWZpdZq60KivZ9owLIDgeVvnxBtLP2XnZ
   G2t438BtuMURxLg4XdCDZJgiIGbJxuiSDd9J14k14TnDrHhBjFmHCvX+YCmQfqxP
   wuFCJxmx7+ljNYyXYRLdlSyNjzIGTxiz7Io5xoV+vs2CA9X7zUSWx34YqwdF2EB7
   oek8m1c1kD8snFRjXVfreQ7MrxcalqbNwJsiwGal7/E9nwHjQR86rCPAUCAKMCsS
   8OBXXQPNg79Tjbcc1zEz3tUY7UHXG5oDT2fO4DqWLwxZ2EhJwoNhKXfXA4ZRGoIG
   hnHHdCcl9+M1GSbshJVsr65rxBtuYhFFJAptKPaFutNPgQ106I2guVpugBD2J0Tq
   kOeDaffdoWKOZj7ZWgeunHETBjaM0hLUwyY7Q4Y7WezGfnEEMOGrJucdyG5yjLsl
   Ki3U+Wm5Ogb7Ur9ke99je2yu1nhA6F3ZxLrVtwozg07ydOF1qeWRrlZNQhstilQ/
   btidPlaXi83EUs9YUoqnZduFtQdsWnNXu5x1DNs8BZ5vQrHx4gV0Dw28zr0JF09u
   8hXg/329PeXR+T3yP0KQ1Cf5n6Ru/P+++d+0wGhhyu4Q0COFX32YAwy0N+C8MsAV
   S0s7jQ6mPO7paUC1a1AxrpUrZ3daA36JlWcjSPB02UZI5SFMz44hHNMMqRaxMieV
   cOrHC5N0i0rpvMqpQMiWgT2bXPjtmtwcxizBN4Yoz6tulVvJYTNH9xWUVWxpJpck
   S06gwBETnZEsj+RaNzd6ne0XpR1WdEE74kYJdHUpH/WNoyGaSBsyBwzAQAOzAtAu
   5iJwz3o1XlrzWc+PUYeR/IPmqcZZAgrFVueGoITxvDzGVTg7iPaQUaSCtlrPoSIy
   UljJ0iTs5o5WUQnTHqvWM5wgUCVbVJZbxiQXh2BHsqe0m1lHfXWCT3P9aAeIpRUF
   pPibBMdKH2emuRGd2xI5k0Wc27mCVqtDzrlrFICdaql1nBaIwxoGt6dcQW6TXXkO
   OgVaKN2soZNSGB/HyjQbN344wU6JmNTz0753D12lLQsy8vTJZL+ehIcm9UtO3bX1
   1E8zj+Ugrl+557yoQc6Dy4acqTOQinS4Pu9c1iJmMSxQAnD7fQ8zibmQulmxPTsJ
   OcezxWoMrAQK1QVZXqY+Rnfe2sjI2jwncSyupWXACLsxPSRTDJjt1nxM8JalkGPC
   3Wh2oR+OAdhksw7yGc1ZIQ48Ln2VJME5XnmOfBhnnEyPzVjNFYOWYDk+Rf2pyj3U
   ijtroSD5JYx51uAmc0gzPLRPupa203QXKayBTIgY9fKiMdmCWIhnaexz0cE4y0k2
   9xcFe7EzHpGx8Wzv2PXZhLrpxN7gA8/Wbc2eEEbnhisbbCLJF4FWhVYe8KmbaoAH
   FXvmglYSFnxGIcH4IKLoKsQhayEwIKiwmTgf72oG1pYTsZvVY4yLk4sm+U4/abfl
   oh4fEe5DnvzUsPoS4/8x+Jzh/631H4G9rP9wlMRh5Ir/OInd8P894//CYVSZ/Sj0
   i7EGYJG1DqIlBxg3VCzAcMBQLumAY5mt8dmUAD3GCe9KCdBjnPCulAA9ygmvo/OT
   g77Vw7d6+FYP3+rhr0s9/Jr/g21R9dsvov37Vv6HSfRV/Yej1LX+IygKv/H/l6H/
   S3ED3xXe2/jO+RQicSCNkV8xiWp7xSVE77cuFPOeu1wvdA9JNpPP9oE4xf1PoBH0
   Eo4W1zTkAY400YEfYEhj8CVnA1jj8oHO+E67JIjuVlcd8koHvVR2LvcmWAocsF6B
   JQsj+xDTHzaJ2WvGPTtx0DAeie/WrO/1p+hqtg3Uh5Mqm+ELweVXHxlKYzOFPnNY
   ZmleWKa8vIFz/D3OZQboktVAZisedGfeHOaYPwWdYfLT1DQpVd+itIKfiPMp4SBF
   EKOxwE/RfXpCURkQ/WReFgIj9yWywSSkV3oX3p8X81oALMlWOM3sgC8PSJvJYXlA
   oTCNk4lTbw7rdbc8HEvygKIZaY131BkwUTs+XdpTmtrTM7zFx9v8OBQ2M0osgD8d
   Ers9MKDp3MZopZ+eBIcFHQ+AP2cuNsucM7C5py0D54XEcIq1qfoWtt5NArj3wgNl
   W6WhSRpEdXQXg3tyyBjJ6UIcPHow1Y1TTzE9TTB69gJm1xtADFjZoHBtzTQ6Prlf
   ZTLftbq/1C8PG9MNi+f1OmGGdWLsiibA0v01O4HeSE9e+veangzutR+ykogdksOO
   e7DUZoDTDcNlJhewge7HafK8kAEnSWfbl+vhuF6mabBkmrVFDIlS+MmUhn9IaQbC
   tI7xTBoQJvLTdZHzRNOY5trcTKJa3fMRenRiqbTn582CUeb5KXVF3K1xv6qrRcFT



Peterson & Jennings      Expires March 30, 2005                [Page 28]

Internet-Draft                SIP Identity                September 2004


   yUyB/AnKCNOgSjvgWCyREQesPB1TJ6yU6XK92qGlmuuYDFLWIaumc5kZ6FW/FB02
   M4mmtyApFlOxOag7mzdQVGrqVq1IWl5tu7hdhuo6iPYEnJjukgxvbd6vXf/3Jf9/
   Ae3f/0v/9yX/kwSO3Pj/y9f/5TjZ5AeVNT0xDG+jgiA1DaoGXb2dlMoYaJWzpnDH
   xvydOAlWCVdCEk1GjsNnYuxiYbzka2FplTJNNg5QKyXLOnHFVFvn6GvpOarXZ2HM
   Ta0W3RclEx3I+ADVgb/Yq2RwkoFAYuRkjo5FYYGplpqRioVLxpDT+w5To6vLOWc4
   mFjEXS2htPvR/u9aNWdClxReqKzW+0xXCNwmpA0QF8i6q9CLiGXhykMdez3hk6Pn
   BZR0aQI9Pq4ve0j0wkTqjnMepcuVZCSsAlDSP8Vz8RgFgnRBPRUILGazczfTLjPe
   87u2l0OqWk5Pl60tQIiTowoehgp62cyKhYyTsZTNahntl8ray3tXz9e0GmnNPsyd
   w7UryR1SDrYqdVmZYt5AKRWc9TXQzGFwa3svFVvqKCTT+Vis1LY3dH5Se+Rs4YZu
   vmdPDV0tFs5xjHDURirPwnYOpdF5F2FEe+3/Iga/Lc9TXDFb+7iYePz5vHJyMy7a
   VVaDJcKwFLY8YlMi1LvsVOoddZChGN0Izo7wc3Gb5VOnVZGD3idjP2eS2TIP8q0x
   A8uhAl/meWzWoTTHg0zmwgY7NuMxs1xA5ybnyXWUjAU5b9dRzYo1u3KbSd03bCUF
   AnqCZ46VbYOaJ8o1vfEIjCHnibk3ZgxQ4hqaXoT6oMMnexJFB9cA/hZOJom6RlsB
   H2t7PZXUzlL03MoJXbJ6H5k6VXc62SZN5Jcx00P9fEOyKcXvdrpV00BYEPczPV8O
   iVIU9rpMIFU8PZIJbqOybYaSvTqzB0+dk+4BeAUHiWSUjclJOqY54xzPFE0PYo0V
   szXVr1ls2+jaTtgGomGr6x+//u8r/P/8279v7//C6Ov+L4ERV/zHsBv+/3j0fx+n
   BOgxTnhXSoAe44R3pQToUU74f/d/b/XwrR6+1cO3evirUg9/yP/n6Hnol1W5Df3i
   vfL/9XHfe/5HUZwiSOzK/yhx6/++F2m2+7ugCj54mf5Fvb/bF9Hzqk7urj/5xTaM
   Pnj1bMCrX8Nqd+fTeICHFBmRJIXAdyiGjJgVf2enx18bochIiIIROjh6hOB3yPSO
   QEaiZt/dPb21j77E8X//Zdt+zun/W/s/CIy/fv8DJdH75/8J6hb/70OGmB19GOhD
   voc9/5RoH1nyYoI+h59C7ta/e/VtYqvWCJkOn56jz/FvB7VfhumLyzQV8UAp/Sbd
   NMhwiub3z4Sq7vx609yNqEEj1AOAjJgqGH3ncfz59W+3fvLCJ0OaCOOnkF3djcDV
   yIdzHgWmh9MGq2gMRigKfwpxfhvdjT4TmJ5CrF8Uz2TubvRxXBt+sKLD3eglvD2F
   5JchcjdqGamulb1WzQlnk6nFCiWkXDKxdcYRW7+jt6QfsTy3dE/Jidwv5FCIQsPb
   pG3ESGycCk+h0WspXV3ML/uoprmtA8sKvi8TbwwuW0Q428JuMdfGimCehM3O0DVB
   9uuhPrrARIlTyIFSDPWj1xLr1YnSqGaHar4dnI29jLNpR/mBO45njSYIzgWWE55Q
   qPDF6MMBPZPLeJjitG33zd1k8qY/Jte/hof5qMp2OOOZGpVJm96N4Bukf2Xwfxc1
   jZ9E7/n/fxgfdK/x/3rcgP7krf9zw//PB//fFdq/lsj2sfhvUh/5Au7xtviHKfJ1
   /Udi1/yPQG7P/74fsSSAfOtjtf+vvhjhcUjH0+uLuSEVhxGJbig/osko9ukYi4cd
   uaERLIJvzP8Vi/9tUkab9x7/6Mv3/+/jn4Sv8U+Q5C3+34f81B/+4j/8/e/96Q9+
   f/7Lf/ONn/3u9/q/+6efOX3np3/iz773g/Bvf/NX/uV3/pn+g/8Mu/Bf5//1b7/0
   W+t/vPv3n/8h/Uc/96Pf/stvVH+x/a6m/saPDn/+rd/9/p9887+/f/6P7/zwm8e/
   5tZ/pX/wk3/8P1zh/wL433bOJsRxMo7DXkSslgVXXEWQjq6oBOybj7dJC1XSph9p
   J2mafqU9maZpm6RNsk0/B0Xw5EGF9bSKX5dd8KQisp4EDzuyrqiIggrKehUPIjqL
   e7Kdcd06OzOdg627y/ucCm1DG/L7t+/vecmzR3MWPPPwqc1TkVv9H/jNev6ltSPM
   23/8ePG+tQtvXiJRAq+7/D+u29pq8w/pf/JPg+39PyQEKP+rgIVOG+8XKm69qxQn
   SoZrq3EjbRaJsUxI7CjWcwxYZfqNDBZfJzeMKk2IZBXrSIzuGEqnV/PlBqM64Awt
   GU6lxiwcjohuvFgaa0BJgWwm13Zj9oaj2ANJZGsNrZaoeOVcns2awYTsVeVwzNcc
   i5K6UelmamaqyzlJrl4S1W5GNo2ElMvaODlga7w2aIY3hCj6w7G0/Bv20OgvRwEt
   yj8E5N/5h2C6Zpvln0b972o4WPIcYId2+R8Sp3AYDvBimS/uZYHICCAigNy2QIv7
   hsgwOl19O9GSp/fYAMGEAUNRkAjNP+TFAC9Rgf0O4fd50YLueYZjBwqc5Pdp0YXv
   6EfBbNXfjaqDhuEEqDBOEwG5KAXZsjR7Qo32+m5XdSMgIMWFUpCZXrw3eEmwO//L
   UEAL9//O+Z+d+39NrymU/5WwE9jANUG/kvMDmr/9crRnEcjsNHiLOr8r3eBSmr5/
   zai9esn9TNHc8NqpCVWtH5n/dPudiifmhZEMgq6VU43BpOZKLgtSOsZ2yiKpB9lk
   yNGwJBsaDQwJn5QwXs6aWDfZba4rmO50kobWZ7x5ybNeSDNtJ6k2CySuOZWe6nZK
   ZIJo0XKiHvca7aZiZFze44nCpNXB8q0mpWUb4sAw6lVhlMKV+WNVTX1oZ+heMNOO
   240W0FJKKAMzHAdHWi2fcpOm53Jme8haWnRfX7THl98ljIoTd3pyVdednjG1Px3L
   Qa/hXtu64hTt9/l96Afg/5n/y1BAi/0/edX/0Nv7fyh0/x80/2+K+Y/GH+IGmv/L



Peterson & Jennings      Expires March 30, 2005                [Page 29]

Internet-Draft                SIP Identity                September 2004


   UIAL938C/Or6n5rNfxiCEM3/VbDt/3Z3P49FAxDQON0EdaCGQ7gK8eZ0kGp0o0Fr
   DFRn+3RVoqEDQkXpv8nyvwQFeHj/N80/vb3+J2jk/1bCHQ8yl93jD7x36Y3vXt30
   /f4NiYuW9uT7t2Rvr8Nj9um74EevHEtuVT5Rvt8SHrnN8v/gvPP1r79ceOYF80z9
   y8/LT/PHzn149LWte74iHsU+/uz8i6Xc2bfO8sd/2vj07rVvladOv/tF7P7nzT+f
   q65vvsz9fKd0b/2k/NDrR4iTFy/nPfH8byiB12P+/1sFeHj/B6cvnfX/NIBo/89K
   iE94CrNZK91Lk3a/q45bPYAXhXGx2zclWmALo44hyo2BK/P40LUUeVhTxriphi1g
   xwhP9JEVbIhLnDchCUE9UTOAACvNhKWM662iLVUrwOSdLJMWJlW8XKT1Vh9YFqX0
   ssl4ulpJxTs+TwjHU1T7BFbN1IqCWpBzjlCKtQ2rzJt2nplYOhcqKWJu0uSR/0Mg
   EAgEAoFAIBAIBAKBQCAQiEPzF7It6CQAeAAA
   -- 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-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:



Peterson & Jennings      Expires March 30, 2005                [Page 30]

Internet-Draft                SIP Identity                September 2004


      - 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













































Peterson & Jennings      Expires March 30, 2005                [Page 31]

Internet-Draft                SIP Identity                September 2004


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Peterson & Jennings      Expires March 30, 2005                [Page 32]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/