[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 RFC 3893

SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: April 28, 2003                                 October 28, 2002


              SIP Authenticated Identity Body (AIB) Format
                     draft-ietf-sip-authid-body-00

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
   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 April 28, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  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.






Peterson                 Expires April 28, 2003                 [Page 1]


Internet-Draft                  SIP AIBF                    October 2002


Table of Contents

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





































Peterson                 Expires April 28, 2003                 [Page 2]


Internet-Draft                  SIP AIBF                    October 2002


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 the request in order to
   provide a digital signature over these headers.

   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 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 (using the SIP REGISTER method) 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 (which may have any of a
   number of relationships with the end user).  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 this 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).

   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.  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.  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 ascertained.  'message/
   sipfrag' [3] allows a subset of SIP headers to be included in a MIME
   body; the AIB format described in Section 2 is based on 'message/
   sipfrag'.




Peterson                 Expires April 28, 2003                 [Page 3]


Internet-Draft                  SIP AIBF                    October 2002


   For reasons of end-to-end privacy, it may also be desirable to
   encrypt AIBs; procedures for this encryption are given in Section 6.

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.

   AIBs using the 'message/sipfrag' MIME type MUST contain the following
   headers: 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 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.

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



Peterson                 Expires April 28, 2003                 [Page 4]


Internet-Draft                  SIP AIBF                    October 2002


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

   ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
   4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
   n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
   7GhIGfHfYT64VQbnj756




Peterson                 Expires April 28, 2003                 [Page 5]


Internet-Draft                  SIP AIBF                    October 2002


   --boundary42--

   --unique-boundary-1--



4. 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 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.

5. 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 the From header field of the request.  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.

   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
   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.




Peterson                 Expires April 28, 2003                 [Page 6]


Internet-Draft                  SIP AIBF                    October 2002


6. Encryption of Identity

   Many SIP entities that support the use of S/MIME for signatures will
   also support S/MIME encryption, as described in RFC3261 Section 
   23.4.3.  Encryption of a body prevents any parties other those that
   hold the decryption key from inspecting the body.

   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 AIBF 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).

7. 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 encrypted.
























Peterson                 Expires April 28, 2003                 [Page 7]


Internet-Draft                  SIP AIBF                    October 2002


   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--


8. 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.



Peterson                 Expires April 28, 2003                 [Page 8]


Internet-Draft                  SIP AIBF                    October 2002


9. 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", draft-ietf-
        sip-sipfrag-00 (work in progress), September 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/




















Peterson                 Expires April 28, 2003                 [Page 9]


Internet-Draft                  SIP AIBF                    October 2002


Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.

Acknowledgement

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



















Peterson                 Expires April 28, 2003                [Page 10]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/