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Versions: 00 01 02 03 RFC 3893

SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: August 2, 2003                                    February 2003


              SIP Authenticated Identity Body (AIB) Format
                     draft-ietf-sip-authid-body-01

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 2, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   RFC3261 introduces the concept of adding an S/MIME body to a SIP
   request or response in order to provide reference integrity over its
   headers.  This document provides a more specific mechanism to derive
   integrity and authentication properties from an 'authenticated
   identity body', a digitally-signed SIP message or message fragment.
   A standard format for such bodies (known as Authenticated Identity
   Bodies, or AIBs) is given in this document.  Some considerations for
   the processing of AIBs by recipients of SIP messages with such bodies
   are also given.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  AIB Format . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Example of a Request with AIB  . . . . . . . . . . . . . . . .  5
   4.  Special Cases of INVITE Requests . . . . . . . . . . . . . . .  6
   5.  Identity in non-INVITE Requests  . . . . . . . . . . . . . . .  6
   6.  Identity in Responses  . . . . . . . . . . . . . . . . . . . .  7
   7.  Receiving an AIB . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Encryption of Identity . . . . . . . . . . . . . . . . . . . .  8
   9.  Example of Encryption  . . . . . . . . . . . . . . . . . . . .  8
   10. Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
       Normative References . . . . . . . . . . . . . . . . . . . . . 10
       Informative References . . . . . . . . . . . . . . . . . . . . 10
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11


































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1. Introduction

   Section 23.4 of RFC3261 [1] describes an integrity mechanism that
   relies on signing tunneled 'message/sip' MIME bodies within SIP
   requests.  The purpose of this mechanism is to replicate the headers
   of a SIP request within a body carried in that request in order to
   provide a digital signature over these headers.  The signature on
   this body also provides authentication.

   The core requirement that motivates this mechanism is the problem of
   providing a cryptographically verifiable identity within a SIP
   request.  The baseline SIP protocol allows a user agent to express
   the identity of its user in any of a number of headers.  The primary
   place for identity information asserted by the sender of a request is
   the From header.  The From header field contains a URI (like
   'sip:alice@atlanta.com') and an optional display-name (like "Alice")
   that identifies the originator of the request.  A user may have many
   identities that are used in different contexts.

   Typically, this URI is an address-of-record that can be dereferenced
   in order to contact the originator of the request; specifically, it
   is usually the same address-of-record under which a user registers
   their devices in order to receive incoming requests.  This address-
   of-record is assigned and maintained by the administrator of the SIP
   service in the domain identified by the host portion of the address-
   of-record.  However, the From field of a request can usually be set
   arbitrarily by the user of a SIP user agent; the From header of a
   message provides no internal assurance that the originating user can
   legitimately claim the given identity.  Nevertheless, many SIP user
   agents will obligingly display the contents of the From field as the
   identity of the originator of a received request (as a sort of
   'Caller-ID' function), much as email implementations display the From
   field as the sender's identity

   In order to provide the recipient of a SIP message with greater
   assurance of the identity of the sender, a cryptographic signature
   can be provided over the headers of the SIP request, which allows the
   signer to assert a verifiable identity.  Unfortunately, a signature
   over the From header alone is insufficient because it could be cut-
   and-pasted into a replay or forwarding attack, and more headers are
   therefore needed to correlated a signature with a request.  RFC3261
   therefore recommends copying all of the headers from the request into
   a signed MIME body; however, SIP messages can also be large, and many
   of the headers in a SIP message would not be relevant to determining
   the identity of the sender or assuring reference integrity with the
   request, and moreover some headers may change in transit.  It is
   therefore desirable to find a happy medium - to provide a way of
   signing just enough headers that the identity of the sender can be



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   ascertained and correlated with the request.  'message/sipfrag' [3]
   provides a way for a subset of SIP headers to be included in a MIME
   body; the AIB format described in Section 2 is based on 'message/
   sipfrag'.

   For reasons of end-to-end privacy, it may also be desirable to
   encrypt AIBs; procedures for this encryption are given in Section 8.

2. AIB Format

   As a way of sharing authenticated identity among parties in the
   network, a special type of MIME body format, the Authenticated
   Identity Body (AIB) format, is defined in this section.  AIBs allow a
   party in a SIP transaction to cryptographically sign the headers that
   assert the identity of the originator of a message, and provide some
   other headers necessary for reference integrity.

   An AIB is a MIME body of type 'message/sip' or 'message/sipfrag' (see
   [3]).  This body MUST have a Content-Disposition disposition-type of
   'aib', a new value defined in this document specifically for
   authenticated identity bodies.  The Content-Disposition header SHOULD
   also contain a 'handling' parameter indicating that this MIME body is
   optional (i.e.  if this mechanism is not supported by the user agent
   server, it can still attempt to process the request).

   AIBs using the 'message/sipfrag' MIME type MUST contain the following
   headers when providing identity for an INVITE request: From, Date and
   Call-ID; they SHOULD also contain the To, Contact and CSeq header.
   AIBs MAY contain any other headers that help to uniquely identify the
   transaction or provide related reference integrity.  An example of
   the AIB format for an INVITE is:

   Content-Type: message/sipfrag
   Content-Disposition: aib; handling=optional

   From: Alice <sip:alice@atlanta.com>
   To: Bob <sip:bob@biloxi.com>
   Contact: <sip:alice@pc33.atlanta.com>
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE

   Unsigned AIBs MUST NOT be honored by any recipients.  After the AIB
   has been signed, it SHOULD be added it to any existing MIME bodies in
   the request (such as SDP), if necessary by transitioning the
   outermost MIME body to a 'multipart/mixed' format.





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3. Example of a Request with AIB

   The following shows a full SIP INVITE request with an AIB:

   INVITE sip:bob@biloxi.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8
   To: Bob <sip:bob@biloxi.com>
   From: Alice <sip:alice@atlanta.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.com>
   Content-Type: multipart/mixed; boundary=unique-boundary-1

   --unique-boundary-1

   Content-Type: application/sdp
   Content-Length: 147

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

   --unique-boundary-1
   Content-Type: multipart/signed;
     protocol="application/pkcs7-signature";
     micalg=sha1; boundary=boundary42
   Content-Length: 608

   --boundary42
   Content-Type: message/sipfrag
   Content-Disposition: aib; handling=optional

   From: Alice <sip:alice@atlanta.com>
   To: Bob <sip:bob@biloxi.com>
   Contact: <sip:alice@pc33.atlanta.com>
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE

   --boundary42
   Content-Type: application/pkcs7-signature; name=smime.p7s
   Content-Transfer-Encoding: base64



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   Content-Disposition: attachment; filename=smime.p7s;
      handling=required

   ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
   4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
   n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
   7GhIGfHfYT64VQbnj756

   --boundary42--

   --unique-boundary-1--



4. Special Cases of INVITE Requests

   There are special-case uses of the INVITE method in which some SIP
   messages are exchanged before an INVITE is sent, and the identity of
   a party from the prior exchange needs to be carried in the subsequent
   INVITE.

   The use of the REFER [4] method, for example, has a requirement for
   the recipient of an INVITE to ascertain the identity of the referrer
   who caused the INVITE to be sent.  In this instance, the From header
   of the INVITE would indicate the referee, and whereas separate header
   would indicated the referrer.

   Third-party call control (3PCC [5]) has an even more complicated
   identity problem.  A central controller INVITEs one party, gathers
   identity information (and session context) from that party, and then
   uses this information to INVITE another party.  Ideally, the
   controller would also have a way to share a cryptographic identity
   signature given by the first party INVITEd by the controller to the
   second party invited by the controller.

   In both of these cases, the Call-ID and CSeq of the original request
   (3PCC INVITE or REFER) will not correpond with that of the request in
   by the subsequent INVITE, nor would the To and From.  In both the
   REFER case and the 3PCC case, the Call-ID and CSeq cannot be used to
   determine reference integrity, and it is therefore much harder to
   correlated an AIB to a subsequent INVITE request.  Some other special
   headers MAY be used to provide reference integrity between the
   headers in an AIB with the headers of a 3PCC or REFER-generated
   INVITE, but this usage is outside of the scope of this document.

5. Identity in non-INVITE Requests

   The requirements for populating an AIB in requests within a dialog



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   generally parallel those of the INVITE: From, Call-ID and Date are
   REQUIRED.

   Some non-INVITE requests, however, may have different identity
   requirements.  New methods should identify any special identity
   requirements in the Security Considerations of their specification.

6. Identity in Responses

   Many of the practices described in the preceding sections can be
   applied to responses as well as requests.  Note that a new set of
   headers must be generated to populate the AIB in a response.  The
   From header field of the AIB in the response to an INVITE SHOULD
   correspond to the address-of-record of the responder, NOT to the From
   header field received in the request.  The To header field of the
   request MUST NOT be included.  A new Date header field and Contact
   header field should be generated for the AIB in a response.  The
   Call-ID and CSeq should, however, be copied from the request.

   Generally, the To header field of the request will correspond to the
   address-of-record of the responder.  In some architectures where
   redirection is used, however, this need not be the case.  Some
   recipients of response AIBs may consider it a cause for security
   concern if the To header field of the request is not the same as the
   address-of-record in the From header field of the AIB in a response.

7. Receiving an AIB

   When a user agent receives a request containing an AIB, it should
   verify the signature, including validating the certificate of the
   signer, and compare the identity of the signer (the subjectAltName)
   with, in the INVITE case, the From header field of the request (for
   non-INVITE requests, other headers may be used).  The two should
   correspond exactly; if they do not, the user agent should report this
   condition to its user before proceeding.  User agents may distinguish
   between plausibly minor variations (the difference between
   'atlanta.com' and 'sip.atlanta.com') and major variations
   ('atlanta.com' vs.  'evil.tv') when reporting these discrepancies in
   order to give the user some idea of how to handle this situation.
   Similar comparison of the Call-ID header is necessary for INVITE
   requests.  The freshness of the Date header should also be evaluated,
   following the guidance in RFC3261.

   When the originating user agent of a request receives a response
   containing an AIB, it SHOULD compare the identity in the To header
   field of the AIB of the response with the original value of the To
   header field in the request.  If these represent different
   identities, the user agent SHOULD render the identity in the AIB of



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   the response to its user.  Note that a discrepancy in these identity
   fields is not necessary an indication of a security breach; normal
   retargeting may simply have directed the request to a different final
   destination.

8. Encryption of Identity

   Many SIP entities that support the use of S/MIME for signatures also
   support S/MIME encryption, as described in RFC3261 Section 23.4.3.

   While encryption of AIBs entails that only the holder of a specific
   key can decrypt the body, that single key could be distributed
   throughout a network of hosts that exist under common policies.  The
   security of the AIB is therefore predicated on the secure
   distribution of the key.  However, for some networks (in which there
   are federations of trusted hosts under a common policy), the
   widespread distribution of a decryption key could be appropriate.
   Some telephone networks, for example, might require this model.

   When an AIB is encrypted, the AIB SHOULD always be encrypted before
   it is signed.  Note that this means that the recipients of the
   request, even if they are unable to inspect the AIBF, will still be
   able to see who signed that body (although it will not necessarily be
   obvious that the body contains an AIB).

9. Example of Encryption

   The following is an example of an encrypted and signed AIB (without
   any of the preceding SIP headers).  In a rendition of this body sent
   over the wire, the text wrapped in asterisks would be in ciphertext.





















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   Content-Type: multipart/signed;
     protocol="application/pkcs7-signature";
     micalg=sha1; boundary=boundary42
   Content-Length: 568

   --boundary42

   Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
     name=smime.p7m
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7m
     handling=required
   Content-Length: 231

   ***********************************************************
   * Content-Type: message/sipfrag                           *
   * Content-Disposition: aib; handling=optional             *
   *                                                         *
   * From: sip:alice@atlanta.com                             *
   * Call-ID: a84b4c76e66710                                 *
   * Date: Thu, 21 Feb 2002 13:02:03 GMT                     *
   ***********************************************************

   --boundary42

   Content-Type: application/pkcs7-signature; name=smime.p7s
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7s;
      handling=required

   ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
   4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
   n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
   7GhIGfHfYT64VQbnj756

   --boundary42--


10. Security Considerations

   This document recommends the inclusion of the Contact, CSeq and To
   headers in AIBs when 'message/sipfrag' is used.  If these headers are
   omitted, some important security properties of AIB are lost.  For
   example, the Contact header determines how new requests in a dialog
   are routed.  If an attacker were to modify the Contact header of a
   SIP request in transit, and that header were not protected by the
   AIBF, then new requests might not return to the originator of the
   request.



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11. IANA Considerations

   This document defines a new MIME Content-Disposition disposition-type
   value of 'aib'.  This value is reserved for MIME bodies that contain
   an authenticated identity, as described in section Section 2.

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, May 2002.

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

   [3]  Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
        September 2002.

Informative References

   [4]  Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07
        (work in progress), November 2002.

   [5]  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
        "Best Current Practices for Third-Party Call Control in the
        Session Initiation Protocol", draft-ietf-sipping-3pcc-02 (work
        in progress), June 2002.


Author's Address

   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/










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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

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



















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