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PROPOSED STANDARD
Network Working Group                                        J. PetersonRequest for Comments: 3893                                       NeuStarCategory: Standards Track                                 September 2004Session Initiation Protocol (SIP)Authenticated Identity Body (AIB) FormatStatus 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).AbstractRFC 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.Peterson                    Standards Track                     [Page 1]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Notation. . . . . . . . . . . . . . . . . .32.  AIB Format . . . . . . . . . . . . . . . . . . . . . . . . . .43.  Example of a Request with AIB  . . . . . . . . . . . . . . . .54.  AIBs for Identifying Third-Parties . . . . . . . . . . . . . .65.  Identity in non-INVITE Requests  . . . . . . . . . . . . . . .76.  Identity in Responses  . . . . . . . . . . . . . . . . . . . .77.  Receiving an AIB . . . . . . . . . . . . . . . . . . . . . . .78.  Encryption of Identity . . . . . . . . . . . . . . . . . . . .89.  Example of Encryption  . . . . . . . . . . . . . . . . . . . .810. Security Considerations  . . . . . . . . . . . . . . . . . . .911. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .1112. References . . . . . . . . . . . . . . . . . . . . . . . . . .1112.1. Normative References . . . . . . . . . . . . . . . . . .1112.2. Informative References . . . . . . . . . . . . . . . . .1113. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .1114. Author's Address . . . . . . . . . . . . . . . . . . . . . . .1215. Full Copyright Statement . . . . . . . . . . . . . . . . . . .131.  IntroductionSection 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 setPeterson                    Standards Track                     [Page 2]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   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 inSection 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 inSection 8.   This document proposes that the AIB format should be used instead of   the existing tunneled 'message/sip' mechanism described inRFC 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 inBCP 14,RFC 2119 [2].Peterson                    Standards Track                     [Page 3]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 20042.  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 inSection 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 inSection 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.Peterson                    Standards Track                     [Page 4]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 20043.  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: base64Peterson                    Standards Track                     [Page 5]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   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 isPeterson                    Standards Track                     [Page 6]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   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.Peterson                    Standards Track                     [Page 7]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   '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 inSection 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 inRFC 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   --boundary42Peterson                    Standards Track                     [Page 8]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   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 inRFC 3261 forPeterson                    Standards Track                     [Page 9]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 2004   including headers in tunneled 'message/sip' MIME bodies (seeSection23 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 inRFC 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 inRFC 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 inSection 4), nor is its inclusion necessary for responses.Peterson                    Standards Track                    [Page 10]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 200411.  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 sectionSection 2.12.  References12.1.  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.Peterson                    Standards Track                    [Page 11]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 200414.  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                    Standards Track                    [Page 12]

RFC 3893      SIP Authenticated Identity Body (AIB) FormatSeptember 200415.  Full Copyright Statement   Copyright (C) The Internet Society (2004).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/S HE   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.Intellectual Property   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 IETF's procedures with respect to rights in IETF Documents can   be found inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Peterson                    Standards Track                    [Page 13]

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