Network Working Group                                        J. Peterson
Request for Comments: 3893                                       NeuStar
Category: Standards Track                                 September 2004


                   Session Initiation Protocol (SIP)
                Authenticated Identity Body (AIB) Format

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   RFC 3261 introduces the concept of adding an S/MIME body to a Session
   Initiation Protocol (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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Requirements Notation. . . . . . . . . . . . . . . . . .  3
   2.  AIB Format . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Example of a Request with AIB  . . . . . . . . . . . . . . . .  5
   4.  AIBs for Identifying Third-Parties . . . . . . . . . . . . . .  6
   5.  Identity in non-INVITE Requests  . . . . . . . . . . . . . . .  7
   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  . . . . . . . . . . . . . . . . . . . . . 11
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       12.1. Normative References . . . . . . . . . . . . . . . . . . 11
       12.2. Informative References . . . . . . . . . . . . . . . . . 11
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   14. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
   15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

1.  Introduction

   Section 23.4 of RFC 3261 [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 the tunneled 'message/sip'
   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@example.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 de-referenced
   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
   identification 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 correlate a signature with a request.  RFC 3261
   therefore recommends copying all of the headers from the request into
   a signed MIME body; however, SIP messages can be large, and many of
   the headers in a SIP message would not be relevant in determining the
   identity of the sender or assuring reference integrity with the
   request, and moreover some headers may change in transit for
   perfectly valid reasons.  Thus, this large tunneled 'message/sip'
   body will almost necessarily be at variance with the headers in a
   request when it is received by the UAS, and the burden in on the UAS
   to determine which header changes were legitimate, and which were
   security violations.  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 and correlated with the
   request.  'message/sipfrag' [4] provides a way for a subset of SIP
   headers to be included in a MIME body; the Authenticated Identity
   Body (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.

   This document proposes that the AIB format should be used instead of
   the existing tunneled 'message/sip' mechanism described in RFC 3261,
   section 23.4, in order to provide the identity of the caller; if
   integrity over other, unrelated headers is required, then the
   'message/sip' mechanism should be used.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

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/sipfrag' - for more
   information on constructing sipfrags, including examples, see [4].
   This MIME body MUST have a Content-Disposition [3] 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,
   Call-ID, and Contact; they SHOULD also contain the To and CSeq
   header.  The security properties of these headers, and circumstances
   in which they should be used, are described in Section 10.  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@example.com>
   To: Bob <sip:bob@example.net>
   Contact: <sip:alice@pc33.example.com>
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE

   Unsigned AIBs MUST be treated by any recipients according to the
   rules set out in Section 7 for AIBs that do not validate.  After the
   AIB has been signed, it SHOULD be added to 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@example.net SIP/2.0
   Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
   To: Bob <sip:bob@example.net>
   From: Alice <sip:alice@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.example.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 example.com
   s=Session SDP
   c=IN IP4 pc33.example.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@example.com>
   To: Bob <sip:bob@example.net>
   Contact: <sip:alice@pc33.example.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

   --boundary42--

   --unique-boundary-1--

4.  AIBs for Identifying Third-Parties

   There are special-case uses of the INVITE method in which some SIP
   messages are exchanged with a third party before an INVITE is sent,
   and in which the identity of the third party needs to be carried in
   the subsequent INVITE.  The details of addressing identity in such
   contexts are outside the scope of this document.  At a high level, it
   is possible that identity information for a third party might be
   carried in a supplemental AIB.  The presence of a supplemental AIB
   within a message would not preclude the appearance of a 'regular' AIB
   as specified in this document.

   Example cases in which supplemental AIBs might appear include:

      The use of the REFER [5] 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.

      Third-party call control (3PCC [6]) 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) would not correspond with that of the request
   in by the subsequent INVITE, nor would the To or From.  In both the
   REFER case and the 3PCC case, the Call-ID and CSeq cannot be used to
   guarantee reference integrity, and it is therefore much harder to
   correlate an AIB to a subsequent INVITE request.

   Thus, in these cases some other headers might be used to provide
   reference integrity between the headers in a supplemental AIB with
   the headers of a 3PCC or REFER-generated INVITE, but this usage is

   outside of the scope of this document.  In order for AIBs to be used
   in these third-party contexts, further specification work is required
   to determine which additional headers, if any, need to be included in
   an AIB in a specific third-party case, and how to differentiate the
   primary AIB in a message from a third-party AIB.

5.  Identity in non-INVITE Requests

   The requirements for populating an AIB in requests within a dialog
   generally parallel those of the INVITE: From, Call-ID, Date, and
   Contact header fields are REQUIRED.

   Some non-INVITE requests, however, may have different identity
   requirements.  New SIP methods or extensions that leverage AIB
   security MUST 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 MUST
   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 re-
   targeting 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 MUST
   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 domain portion of the URI in the From
   header field of the request (for non-INVITE requests, other headers
   MAY be subject to this comparison).  The two should correspond
   exactly; if they do not, the user agent MUST report this condition to
   its user before proceeding.  User agents MAY distinguish between
   plausibly minor variations (the difference between 'example.com' and
   'sip.example.com') and major variations ('example.com' vs.

   'example.org') when reporting these discrepancies in order to give
   the user some idea of how to handle this situation.  Analysis and
   comparison of the Date, Call-ID, and Contact header fields as
   described in Section 10 MUST also be performed.  Any discrepancies or
   violations MUST be reported to the user.

   When the originating user agent of a request receives a response
   containing an AIB, it SHOULD compare the identity in the From 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 necessarily an indication of a security breach; normal
   re-targeting may simply have directed the request to a different
   final destination.  Implementors therefore may consider it
   unnecessary to alert the user of a security violation in this case.

8.  Encryption of Identity

   Many SIP entities that support the use of S/MIME for signatures also
   support S/MIME encryption, as described in RFC 3261, 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 be encrypted before it is
   signed.  Implementations MUST still accept AIBs that have been signed
   and then encrypted.

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.

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

   --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@example.com                             *
   * Call-ID: a84b4c76e66710                                 *
   * Contact: sip:alice@device21.example.com                 *
   * 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

   The purpose of an AIB is to provide an identity for the sender of a
   SIP message.  This identity is held in the From header field of an
   AIB.  While other headers are also included, they are provided solely
   to assist in detection of replays and cut-and-paste attacks leveraged
   to impersonate the caller.  The contents of the From header field of
   a valid AIB are suitable for display as a "Caller ID" for the sender
   of the SIP message.

   This document mandates the inclusion of the Contact, Date, Call-ID,
   and From header fields within an AIB, and recommends the inclusion of
   CSeq and To header fields, when 'message/sipfrag' is used to
   represent the identity of a request's sender.  If these headers are
   omitted, some important security properties of AIB are lost.  In
   general, the considerations related to the inclusion of various
   headers in an AIB are the same as those given in RFC 3261 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; were this header to be excluded, the creator of the AIB
   essentially would not be asserting an identity at all.  The Date and
   Contact headers provide reference integrity and replay protection, as
   described in RFC 3261, Section 23.4.2.  Implementations of this
   specification MUST follow the rules for acceptance of the Date header
   field in tunneled 'message/sip' requests described in RFC 3261,
   Section 23.4.2; this ensures that outdated AIBs will not be replayed
   (the suggested 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 AIBs, and MUST remember those
   Call-IDs for at least the duration of a single Date interval (i.e.,
   3600 seconds).  Accordingly, if an AIB is replayed within the Date
   interval, receivers will recognize that it is invalid because of a
   Call-ID duplication; if an AIB is replayed after the Date interval,
   receivers will recognize that it is invalid because the Date is
   stale.  The Contact header field is included to tie the AIB to a
   particular device instance that generated the request.  Were an
   active attacker to intercept a request containing an AIB, and cut-
   and-paste the AIB into their own request (reusing the From, Contact,
   Date, and Call-ID fields that appear in the AIB), 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.

   The To and CSeq header fields provide properties that are generally
   useful, but not for all possible applications of AIBs.  If a new AIB
   is issued each time a new SIP transaction is initiated in a dialog,
   the CSeq header field provides a valuable property (replay protection
   for this particular transaction).  If, however, one AIB is used for
   an entire dialog, subsequent transactions in the dialog would use the
   same AIB that appeared in the INVITE transaction.  Using a single AIB
   for an entire dialog reduces the load on the generator of the AIB.
   The To header field usually designates the original URI that the
   caller intended to reach, and therefore it may vary from the
   Request-URI if re-targeting occurs at some point in the network.
   Accordingly, including the To header field in the AIB helps to
   identify cut-and-paste attacks in which an AIB sent to a particular
   destination is re-used to impersonate the sender to a different
   destination.  However, the inclusion of the To header field probably
   would not make sense for many third-party AIB cases (as described in
   Section 4), nor is its inclusion necessary for responses.

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.

12.  References

12.1.  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", BCP 14, RFC 2119, March 1997.

        [3]  Troost, R., Dorner, S., and K. Moore, "Communicating
        Presentation Information in Internet Messages: The Content-
        Disposition Header Field", RFC 2183, August 1997.

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

12.2.  Informative References

   [5]  Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
        Mechanism", RFC 3892, September 2004.

   [6]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
        "Best Current Practices for Third Party Call Control (3pcc) in
        the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
        2004.

13.  Acknowledgements

   The author would like to thank Robert Sparks, Jonathan Rosenberg,
   Mary Watson, and Eric Rescorla for their comments.  Rohan Mahy also
   provided some valuable guidance.

14.  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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