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Versions: 00 01 02 draft-wing-rtcweb-identity-media

Network Working Group                                            D. Wing
Internet-Draft                                             Cisco Systems
Intended status:  Standards Track                              H. Kaplan
Expires:  August 26, 2008                                    Acme Packet
                                                       February 23, 2008


                     SIP Identity using Media Path
                    draft-wing-sip-identity-media-02

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on August 26, 2008.

Abstract

   This document defines a new SIP identity mechanism which operates
   through SBCs and B2BUAs.  This new identity mechanism creates a
   signature over certain SIP headers and certain SDP lines.  When the
   SIP body contains SDP, both the SIP signaling path and the media path
   are used to perform the identity function; when the SIP body contains
   non-SDP body parts, they are signed in their entirety.








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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Identity Media Signature . . . . . . . . . . . . . . . . .  6
     3.2.  Authentication Service . . . . . . . . . . . . . . . . . .  7
     3.3.  Validation . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Proof of Identity Techniques . . . . . . . . . . . . . . . . .  7
     4.1.  TLS  . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  SRTP after DTLS optional . . . . . . . . . . . . . . .  8
     4.3.  ICE  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       4.3.1.  ICE Public Key SDP Attribute . . . . . . . . . . . . .  9
       4.3.2.  New STUN attributes  . . . . . . . . . . . . . . . . .  9
     4.4.  HIP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  ZRTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     6.1.  Device Disclosure  . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Modification of SDP  . . . . . . . . . . . . . . . . . . . 12
   7.  Operational Differences from RFC4474 . . . . . . . . . . . . . 12
   8.  Limitations  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.2.  ICE  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.3.  Request without SDP  . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     12.2. Informational References . . . . . . . . . . . . . . . . . 19
   Appendix A.  ToDo List . . . . . . . . . . . . . . . . . . . . . . 19
   Appendix B.  Changes From Previous Versions  . . . . . . . . . . . 19
     B.1.  Changes from 00 to 01  . . . . . . . . . . . . . . . . . . 19
     B.2.  Changes from 01 to 02  . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21













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1.  Introduction

   SIP Identity [RFC4474] provides cryptographic identity for SIP
   requests.  It provides this protection by signing certain SIP header
   fields (Contact, Date, Call-ID, CSeq, To, and From) and the SIP
   message body.  The SIP message body typically contains the SDP.
   However, as discussed in [I-D.wing-sip-identity-analysis], RFC4474
   does not work well if intermediate domains have B2BUAs or SBCs.  As
   of this writing, most service providers utilize SBCs at network
   ingress and at network egress.

   The mechanism described in this document provides cryptographic
   assurance of the endpoint's identity, and works through most B2BUAs
   and through most SBCs.

   The mechanism described in this document signs only certain SDP
   attributes, and not all the same SIP headers.  The remote endpoint is
   expected to validate the signature over the SIP headers and specified
   SDP attributes, to initiate a proof of possession test over the media
   path, which proves the session has been established with the "From:"
   party in the SIP header.  Mechanisms to perform this proof of
   possession are shown using DTLS and using a small extension to ICE
   [I-D.ietf-mmusic-ice].  This mechanism is also extensible, in order
   to be usable by future mechanisms which need signed SDP attributes

   Readers of this document are expected to be familiar with RFC4474,
   "Enhancements for Authenticated Identity Management in the Session
   Initiation Protocol (SIP)", which defines the Identity and Identity-
   Info header fields.  A future version of this document will have less
   reliance on RFC4474.


2.  Notational Conventions

   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 [RFC2119].


3.  Operation

   The operation of SIP-Identity-Media is similar to RFC4474 and uses
   authentication service proxies much like RFC4474.  The basic steps
   are:

   o  A new header, Identity-Media, is created containing the names of
      certain SDP attributes from SDP bodyparts, and containing a hash
      of non-SDP bodyparts.



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   o  Several SIP headers and the Identity-Media header are all signed
      (as detailed in Section 3.1), and the result is placed in
      Identity-Media-Signature.

   o  The receiving domain validates the signature, and if the request
      is an invitation to establish a media channel, performs a proof of
      identity validation using DTLS, TLS, ICE, HIP, or ZRTP over the
      media path.

   The following figure shows how the Authentication Service and the
   media validation is performed.  The figure assumes the endpoints
   themselves perform the media validation.

                                 :  Service   :
                Enterprise-A     : Provider-1 :    Enterprise-B
                                 :            :
                          Auth.  :  B2BUA or  :  Auth.
             Endpoint-A  Service :    SBC     : Service  Endpoint-B
                 |          |    :     |      :   |         |
          1.     |--INVITE->|    :     |      :   |         |
          2.     |        sign   :     |      :   |         |
          3.     |          |-INVITE-->|-INVITE-->|         |
          4.     |          |    :     |      : validate    |
          5.     |          |    :     |      :   |-------->|
          6.     |<=====TLS, DTLS, ICE, HIP, or ZRTP=======>|
          7.     |          |    :     |      :   |     validated
          8.     |          |    :     |      :   |     ring phone
                 |          |    :     |      :   |         |
                                 :            :

                          Figure 1: Message Flow

   Step 1:  Originating endpoint prepares to send an INVITE and chooses
            the identity-challenge technique it supports, and indicates
            that in the SDP it generates.  Described in this document
            are identity challenges for TLS, DTLS, ICE, HIP, and ZRTP.
            It then sends the INVITE to its local SIP proxy.

   Step 2:  Originating endpoint's authentication service creates a new
            header, Identity-Media, containing certain attribute names
            from the SDP (e.g., "a=fingerprint", "a=ice-pub-key").  The
            authentication service then creates a signature over certain
            SIP headers (e.g., From, To) and this new Identity-Media
            header.  The resulting signature is inserted into the new
            Identity-Media-Signature header.  An Identity-Info header is
            added, pointing to this domain's certificate.  The INVITE,
            with these additional headers, is forwarded to the next
            administrative domain.



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            [NOTE:  alternatively, we could allow the UAC to create the
            Identity- Media header with the attributes it wants signed,
            then have the auth server sign them and insert the signature
            header - this would be more flexible]

   Step 3:  The next administrative domain has an SBC (or B2BUA).  The
            SBC modifies or rewrites certain SDP fields.  Most typically
            an SBC will modify the "m" and "c" lines.  These
            modifications do not break the signature, so long as the SBC
            doesn't remove the headers Identity-Media, Identity-Media-
            Signature, or Identity-Info, and do not remove or alter the
            signed attributes from the SDP.

   Step 4:  The terminating endpoint's authentication service receives
            the INVITE.  It validates that the signature contained in
            the Identity-Media-Signature header, and validates that the
            signing certificate is owned by the originating domain from
            step 2.  This validation is done by using the certificate
            pointed to in the Identity-Info header, which MUST match the
            domain in the From:  address.

   Step 5:  If the validation was successful, the terminating endpoint's
            authentication service forwards the INVITE to the endpoint.

   Step 6:  The terminating endpoint chooses a compatible identity-
            challenge technique from the INVITE (TLS, DTLS, ICE, HIP, or
            ZRTP), and performs that challenge.  Described in this
            document are identity challenges for TLS, DTLS, ICE, HIP,
            and ZRTP.

   Step 7:  All of the identity challenges (TLS, DTLS, ICE, HIP, and
            ZRTP) cause the exchange of either a certificate or a public
            key in the media path.  The terminating endpoint compares
            the certificate or public key with the fingerprint in the
            (signed) Identity-Media header (originally created in step
            2).  If they match, the terminating endpoint completes the
            identity challenge exchange.  After completion, the
            originating endpoint has proven (to the terminating
            endpoint) that the originating endpoint knows the private
            key associated with the certificate (or public key) signed
            in step 2.  The terminating endpoint has now validated the
            identity of the originating endpoint.

   Step 8:  The terminating endpoint can reliably and honestly indicate
            calling party information ("caller-id") and ring the phone.






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3.1.  Identity Media Signature

   In RFC4474, a signature is formed over some SIP headers and over the
   entire body (which most typically contains SDP).  In this
   specification, some SIP headers are signed but only specific SDP
   attributes that provide cryptographic identity are signed (e.g.,
   "a=fingerprint" and its value).  The specific SDP attributes that are
   signed depends on which cryptographic identity technique(s) is used;
   see section Section 4.

   The SIP headers that are signed, for the signature placed into the
   Identity-Media-Signature header are:

   o  The AoR of the UA sending the message, or addr-spec of the From
      header field (referred to occasionally here as the 'identity
      field').

   o  The addr-spec component of the To header field, which is the AoR
      to which the request is being sent.

   o  The SIP method.

   o  [NOTE:  Contact, CSeq and Call-Id not included]

   o  The Date header field, with exactly one space each for each SP and
      the weekday and month items case set as shown in the BNF in
      RFC3261.  RFC3261 specifies that the BNF for weekday and month is
      a choice amongst a set of tokens.  The RFC2234 rules for the BNF
      specify that tokens are case sensitive.  However, when used to
      construct the canonical string defined here, the first letter of
      each week and month MUST be capitalized, and the remaining two
      letters must be lowercase.  This matches the capitalization
      provided in the definition of each token.  All requests that use
      the Identity-Media mechanism MUST contain a Date header.

   o  The Identity-Media header field value.

   The hash is formed of these elements:

      digest-string = addr-spec "|" addr-spec "|"
                      Method "|" SIP-date "|"
                      attrib-bodyhash-list

   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.

   The Identity-Info header points to where the authentication service's
   certificate can be retrieved from.



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3.2.  Authentication Service

   The authentication service examines the SIP message body to build the
   Identity-Media header.  For each message body found, in the order
   found:

   o  if the body part is application/sdp, the authentication service
      retrieves only the cryptographic attributes from the SDP (as
      described in Section 4), and appends that information to the
      Identity-Media header.

   o  otherwise, for all other body parts, the body part is hashed using
      SHA-1, and the first 96 bytes are appended to the Identity-Media
      header using "BPH=".

   For example, A SIP request with three bodyparts:  text/plain,
   application/sdp, and image/jpg, the Identity-Media attribute would
   contain a bodypart hash of the text/plain part, certain SDP attribute
   lines from the application/sdp bodypart (a=fingerprint in this
   example), and a bodypart hash of the image/jpg bodypart:

     Identity-Media: BPH="e32je3j23cjek3dz","a=fingerprint",
       BPH="8fj289r3i892381c"

   This Identity-Media header, along with the headers and portions of
   headers described in Section 3.1 are all signed by the authentication
   service.  The resulting signature is placed on the new Identity-
   Media-Signature header.

3.3.  Validation

   The validation service can be performed by the remote endpoint itself
   or by a device acting on behalf of the endpoint.  The validation
   service first checks the signature in the Identity-Media-Signature
   field.  If this is valid, the endpoint (or its validation service
   operating on its behalf) then initiates a DTLS, TLS, ICE, HIP, or
   ZRTP identity proof (Section 4).  This causes the originating
   endpoint to prove possession of its private key that corresponds to
   the certificate (or public key) that was signed by the remote
   domain's authentication service.


4.  Proof of Identity Techniques

   Five techniques are described below, TLS, DTLS, ICE, HIP, and ZRTP.
   Each provides a means to cryptographically prove the identity signed
   by the authentication service in SIP is the same as the identity on
   the media path.



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   Each of these techniques work similarly -- each technique causes
   unique information to appear in the SDP -- a certificate fingerprint
   (DTLS, TLS), public key (ICE), or hash (ZRTP).  The authentication
   service creates a new Identity-Media header and places into that
   header those SDP attribute names associated with that technique.  The
   authentication service then creates a signature over specific SIP
   headers (see Section 3.1), and places that signature into the new
   Identity-Media-Signature header.  The SIP request is then sent
   outside of the originating domain.

   The receiving domain validates the Identity-Media-Signature.  If
   successful, the SIP request is forwarded to the end system.  The end
   system initiates a TLS, DTLS, ICE, HIP, or ZRTP session and validates
   that the (signed) certificate fingerprint presented in the SIP
   signaling matches the certificate presented in the TLS, DTLS, ICE,
   HIP, or ZRTP exchange.  If they match, and the TLS, DTLS, ICE, HIP,
   or ZRTP exchange completes successfully, the local endpoint has
   validated the identity of the remote endpoint.

   Note:  Due to SIP forking, the calling party may receive many
   identity challenges, each incurring a public key operation to prove
   identity.  Mechanisms to deal with this are for future study.

      Discussion point:  It is anticipated that, during the course of
      standardization, a subset of these five techniques will be chosen
      as mandatory to implement for the purpose of establishing
      identity.

4.1.  TLS

   TLS uses the "fingerprint" attribute to provide a hash of the
   certificate in the SDP.  The fingerprint attribute is defined by
   [RFC4572] for TLS.

4.2.  DTLS

   DTLS uses the same "fingerprint" attribute originally described for
   TLS.  The syntax is described in [I-D.ietf-sip-dtls-srtp-framework].

4.2.1.  SRTP after DTLS optional

   [[Discussion Point:  Is there interest in having identity without
   SRTP??]]








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   DTLS is only necessary to prove identity with DTLS; SRTP [RFC3711]
   does not need to be used afterwards.  Obviously, using SRTP provides
   significant benefits over continuing to use RTP, because an attacker
   can inject bogus RTP after a successful validation of identity which
   is quite undesirable.  The SDP for doing RTP after a DTLS exchange
   might be signaled in SDP by using "RTP/AVP" rather than "RTP/SAVP"
   (lines folded for readability):

       v=0
       o=- 25678 753849 IN IP4 192.0.2.1
       s=
       c=IN IP4 192.0.2.1
       t=0 0
       m=audio 3456 RTP/AVP 0 18
       a=fingerprint:SHA-1
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

   Of course, it would be desirable to more clearly indicate this
   somehow in SDP.  The example above collides with non-standard, but
   deployed, "best-effort" media encryption mechanisms.  SDP Capability
   Negotiation [I-D.ietf-mmusic-sdp-capability-negotiation] might be a
   useful consideration for this functionality.

4.3.  ICE

   ICE doesn't have inherent support for public/private keys.  If public
   keys were sent with other ICE attributes, there can be a real risk of
   an ICE connectivity check exceeding the MTU.  ICE lacks a mechanism
   to fragment such large messages.  It is also bandwidth inefficient to
   send multiple ICE connectivity checks containing public keys, either
   as retransmissions or with multiple candidates.  Thus, for ICE, the
   public key is sent in SDP and the public key's fingerprint is
   exchanged on the media path -- opposite of TLS, DTLS, HIP, and ZRTP.

4.3.1.  ICE Public Key SDP Attribute

   The offerer includes its public key, which it will use for the
   subsequent PK-CHALLENGE and PK-RESPONSE, in its SDP.  The syntax is a
   BASE64-encoded version of the endpoint's public key.

   The new attribute is called "ice-pub-key", which may appear on the
   session level, media level, or both.

4.3.2.  New STUN attributes

   Two new STUN attributes are defined to carry the plaintext challenge
   and the encrypted response.




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4.3.2.1.  PK-CHALLENGE

   This is sent in a STUN Binding Request, and contains two fields:  the
   fingerprint of the public key exchanged in the SDP, and the public
   key challange.  The fingerprint is included so that the remote peer
   can choose the correct key, in the event it used different public
   keys.  The public key challange field are the bits to be encrypted by
   the remote peer's private key.  Up to 256 bits can be included in the
   challenge.

   The PK-CHALLENGE MUST be the same for each candidate address that is
   being tested for connectivity.  If this requirement is not followed,
   the peer will incur a public key operation for every ICE connectivity
   check, which is not reasonable or necessary.

   When the remote peer receives a STUN Binding Request containing this
   attribute, the contents of the PK-CHALLENGE are encrypted using the
   private key associated with the public key's fingerprint, and the
   result is sent in the PK-RESPONSE attribute of the Binding Response.

4.3.2.2.  PK-RESPONSE

   This is sent in a STUN Binding Response from the offerer to the
   answerer, and contains the encrypted result of the PK-CHALLENGE.

4.4.  HIP

   In [I-D.tschofenig-hiprg-host-identities], a new attribute "key-
   mgmt:host-identity-tag" is defined which contains the hash of the
   public key used in the subsequent HIP exchange.  This can be utilized
   and signed exactly like the "fingerprint" attribute for TLS or DTLS.

4.5.  ZRTP

   In [I-D.zimmermann-avt-zrtp], a new attribute "zrtp-hello-hash" is
   defined which contains a hashed value of the ZRTP Hello packet.  The
   entire ZRTP exchange is protected as described in Section 10 of
   [I-D.zimmermann-avt-zrtp].  After the ZRTP exchange has completed,
   the remote party's identity is proven to match the identity signed
   via SIP-Identity-Media.











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5.  ABNF

   The following figure shows the syntax of the new SIP header fields
   using ABNF [RFC5234]

      identity-media        = "Identity-Media" HCOLON
                              attrib-bodyhash-list
      attrib-bodyhash-list  = attrib-bodyhash *(COMMA attrib-bodyhash)
      attrib-bodyhash       = quoted-attrib | quoted-bodyparthash
      quoted-attribute      = DQUOTE attribute DQUOTE  ; SDP "a=" line
      quoted-bodyhash       = "BPH" EQUAL DQUOTE bodyparthash DQUOTE
      bodyparthash          = 32HEXDIG

      identity-media-sig    = "Identity-Media-Signature" HCOLON
                              signature
      signature             = DQUOT 32HEXDIG DQUOT

      Identity-Info = "Identity-Info" HCOLON ident-info
                       *( SEMI ident-info-params )
      ident-info = LAQUOT absoluteURI RAQUOT
      ident-info-params = ident-info-alg / ident-info-extension
      ident-info-alg = "alg" EQUAL token
      ident-info-extension = generic-param

                    Figure 2: ABNF for new SIP headers

   The following figure shows the syntax of the new SDP attribute
   containing the ICE public key.  This is used only by endpoints
   implementing the ICE proof of identity technique (Section 4.3).

     ice-pub-key        = token    ; BASE64 encoded public key

                   Figure 3: ABNF for new SDP attribute


6.  Security Considerations

   [[some of RFC4474's security considerations also apply.]]

6.1.  Device Disclosure

   Although the mechanism described in this paper allows SBCs to be used
   with a cryptographic identity scheme, it does expose the identity of
   the user's certificate.  If a unique certificate is installed on each
   user's device, the remote party will be able to discern which device
   is terminating the call.  This problem is more pronounced when SIP
   retargeting occurs in conjunction with Connected Identity [RFC4916].




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   If this isn't desired, there are two solutions:

   o  All devices under the control of the user will need to have the
      same certificate (and associated private key) installed on them.

   o  The device needs to manufacture a new self-signed certificate (or
      public key) for each call, and populate the appropriate SDP
      attributes with that certificate (or public key).  This is
      possible because the identity service described in this paper does
      not require the same certificate or public key to be used on every
      call.

6.2.  Modification of SDP

   One issue with only signing specific SDP attributes is that a man in
   the middle can modify the un-signed SDP for nefarious purposes,
   beyond simply changing m=/c= lines.  In particular, an attacker could
   set the c= connection line used for DTLS-SRTP fingerprint to 0.0.0.0
   and the m= media line to port 0, essentially disabling that offered
   media session.  The attacker could also add a set of c=/m= lines for
   non-SRTP media, and thus make a non-SRTP offer with a perfectly valid
   identity signature.  Or an attacker could insert SDP capability
   negotiation attributes to create a best-effort type SRTP offer, with
   SRTP (rather than RTP) being the lowest preference.

   This draft prevents such downgrade attacks by requiring the called UA
   use DTLS-SRTP, HIP, ICE, or TLS on the media path to establish
   identity.  Thus, an attacker performing the attacks described above
   will not successfully fool the called UA because the (intended)
   victim will use DTLS-SRTP (or HIP, ICE, or TLS) on the media path,
   and the attacker does not possess the private key of the legitimate
   caller.


7.  Operational Differences from RFC4474

   RFC4474 imposes one public key operation for the authentication
   service and one for validation.  If Connected Identity [RFC4916] is
   used, only one additional public key operation is necessary for the
   header signature validation; the expense of the DTLS, TLS, or ICE
   public key operation has already been incurred by both parties and is
   not repeated.

   RFC4474 includes the Contact URI in the signed headers.  That is not
   required by this mechanism because it adds no security property, and
   will fail validation when crossing SBCs and B2BUA's.  It is of
   dubious security value because Via/Record-Route can be inserted for
   response interception regardless, and some requests don't contain a



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   Contact anyway (e.g., MESSAGE).  It does not provide any replay/
   copy-paste protection either, for the same reasons.

   RFC4474 includes the CSeq in the signed headers.  That is not
   required by this mechanism because it adds little security, and will
   fail validation when crossing SBCs and B2BUA's in some cases.  It is
   of little security value because it provides no protection from cut-
   paste attack for different targets, and although it would prevent
   replay attack within the same session, since the media key-related
   SDP portions are signed anyway, replaying the request will not do
   anything useful.

   RFC4474 includes the Call-Id in the signed headers.  That is not
   required by this mechanism because it adds little security, and will
   fail validation when crossing SBCs and B2BUA's in some cases.  It is
   of little security value because it provides no protection from cut-
   paste attack for different targets, and although it would prevent
   replay attack for the same target, since the media key-related SDP
   portions are signed anyway, replaying the request will not do
   anything useful.

   The mechanism described in this document has the following advantages
   over RFC4474:

   o  Only the edge network needs to create signatures on SIP requests
      -- not every intervening SBC,

   o  The original cryptographically-provable identity is preserved
      across any number of SBCs, B2BUA's, etc.

   o  SBCs, B2BUA's, and other "middle-boxes" in intermediate domains do
      not need to be upgraded or changed in order for the originating
      and terminating domains to use this new mechanism.


8.  Limitations

   For the identity procedure described in this document to function,
   every device -- including Session Border Controllers -- on the path
   MUST permit DTLS, TLS, ICE, HIP, or ZRTP messages to be exchanged in
   the media path.  Further, those devices MUST NOT interfere with the
   signed SDP attributes or the new SIP headers necessary for Identity
   Media to operate.

   For the technique described in this document to function, all on-path
   SIP elements -- SBCs, B2BUAs, and SIP proxies -- MUST NOT interfere
   with the signed headers.  The identity mechanism described in this
   document is not harmed if on-path SIP elements alter the SDP (e.g.,



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   by deleting non-signed attributes, connection addresses, etc.).


9.  Examples

9.1.  DTLS

   This example shows how two a=fingerprint lines in SDP would populate
   the Identity-Media SIP header field.  The following is an example of
   an INVITE created by the endpoint.

   (lines folded for readability)

      INVITE sip:bob@biloxi.example.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=- 6418913922105372816 2105372818 IN IP4 192.0.2.1
      s=example2
      c=IN IP4 192.0.2.1
      t=0 0
      m=audio 54113 RTP/SAVP 0
      a=fingerprint:SHA-1
        4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
      m=video 54115 RTP/SAVP 0
      a=fingerprint:SHA-1
        4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

                        Figure 4: Example with DTLS













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   The SIP proxy performing the Media Identity authentication service
   would then insert the following three SIP headers into the message.
   The Identity-Media header contains all of the SDP attribute lines
   that are signed and the Identity-Media header contains the signature
   of all of the relevant SIP headers and of the Identity-Media header.
   Lines are folded for readability:

     Identity-Info: <https://atlanta.example.com/atlanta.cer>
        ;alg=rsa-sha1
     Identity-Media: "a=fingerprint","a=fingerprint"
     Identity-Media-Signature:
      "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
       ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
       FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="

         Figure 5: SIP Headers Inserted by Authentication Service

9.2.  ICE

   With ICE, the public key is exchanged in the signaling path (in SDP)
   rather than in the media path (as is done with TLS, DTLS, HIP, and
   ZRTP).





























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   This is the INVITE as it left the SIP user agent (lines folded for
   readability):

      INVITE sip:bob@biloxi.example.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=- 6418913922105372816 2105372818 IN IP4 192.0.2.1
      s=example2
      c=IN IP4 192.0.2.1
      t=0 0
      a=ice-pwd:asd88fgpdd777uzjYhagZg
      a=ice-ufrag:8hhY
      a=pub-key:ejfiwj289ceucuezeceEJFjefkcjeiquiefekureickejfeefe
        uirujejfecejejejkfeJJCEIUQQIEFJCQUCJCEQUURIE09dnjkeefjek
      m=audio 54113 RTP/AVP 0
      a=candidate:1 1 UDP 2130706431 192.0.2.1 54113 typ host

                        Figure 6: Example with ICE

   The SIP proxy performing the Media Identity authentication service
   would then insert the following three SIP headers into the message.
   The Identity-Media header contains the ICE public key attribute and
   the Identity-Media header contains the signature of all of the
   relevant SIP headers and of the Identity-Media header (lines are
   folded for readability):

     Identity-Info: <https://atlanta.example.com/atlanta.cer>
       ;alg=rsa-sha1
     Identity-Media: "a=pub-key"
     Identity-Media-Signature:
      "jjsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qc
       efQBbHC00VMZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0Ssjcd
       VcunyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Jcqe="

           Figure 7: Headers Inserted by Authentication Service






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9.3.  Request without SDP

   This example shows how a SIP request without SDP is signed.

   Message as sent by the UAC (lines folded for readability)

      MESSAGE sip:user2@example.com SIP/2.0
      Via: SIP/2.0/TCP user1pc.example.com;branch=z9hG4bK776sgdkse
      Max-Forwards: 70
      From: sip:user1@example.com;tag=49583
      To: sip:user2@example.com
      Call-ID: asd88asd77a@1.2.3.4
      CSeq: 1 MESSAGE
      Content-Type: text/plain
      Content-Length: 18

      Watson, come here.

                       Figure 8: Example with no SDP

   The authentication service would add the following headers to the
   above message:

     Identity-Info: <https://atlanta.example.com/atlanta.cer>
       ;alg=rsa-sha1
     Identity-Media:
       BPH="MZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxA"
     Identity-Media-Signature:
      "diOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qcjjsR
       bHC00VMZr2k+t6efQBVmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB09JcVc
       unyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVnqeSsjcd="

                          Figure 9: added headers


10.  Acknowledgements

   The mechanism described in this paper is derived from Jon Peterson
   and Cullen Jennings' [RFC4474], which was formerly a document of the
   SIP working group.

   Thanks to Hans Persson for his suggestions which improved this
   document.


11.  IANA Considerations

   This document will add new IANA registrations for its new STUN



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   attributes.

   [[This section will be completed in a later version of this
   document.]]


12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [I-D.ietf-sip-dtls-srtp-framework]
              Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing an SRTP Security Context using DTLS",
              draft-ietf-sip-dtls-srtp-framework-00 (work in progress),
              November 2007.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

   [I-D.tschofenig-hiprg-host-identities]
              Tschofenig, H., "Interaction between SIP and HIP",
              draft-tschofenig-hiprg-host-identities-05 (work in
              progress), June 2007.

   [I-D.zimmermann-avt-zrtp]
              Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Secure RTP",
              draft-zimmermann-avt-zrtp-05 (work in progress),
              February 2008.




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   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

12.2.  Informational References

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
              progress), December 2007.

   [I-D.wing-sip-identity-analysis]
              Wing, D. and H. Kaplan, "An Analysis of SIP Identity with
              SIP Back-to-Back User Agents and Session Border
              Controllers", draft-wing-sip-identity-analysis-00 (work in
              progress), January 2008.


Appendix A.  ToDo List

   o  Add Table-2 of RFC3261

   o  re-use RFC4474 response code for failures, or invent new ones?

   o  describe what occurs if both SIP-Identity-Media and SIP-Identity
      are both used?


Appendix B.  Changes From Previous Versions

B.1.  Changes from 00 to 01

   o  Removed "Contact" header from signature.  SBCs need to change it.

   o  Removed "Call ID" header from signature.  This header often
      contains an IP address, so many SBCs change it.

   o  Removed "CSeq" header from signature.  This header is sometimes
      changed by SBCs and B2BUA's.

   o  include SDP attribute names in Identity-Media signature.  This
      allows any attribute to be signed.

   o  Old "Identity-Fingerprints" header renamed to "Identity-Media",
      and only attribute names are listed in it, not attribute values.




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   o  Old "Identity-Media" header renamed to "Identity-Media-Signature".

   o  Described how to sign SIP requests without an SDP body part, and
      with a mix of SDP and non-SDP bodyparts.

B.2.  Changes from 01 to 02

   o  Describe how modification of SDP is prevented (section 7.2).

   o  Moved B2BUA and SBC analysis to separate document,
      [I-D.wing-sip-identity-analysis].

   o  Added ZRTP as another authentication technique.


Authors' Addresses

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  dwing@cisco.com


   Hadriel Kaplan
   Acme Packet
   71 Third Ave.
   Burlington, MA  01803
   USA

   Phone:
   Fax:
   Email:  hkaplan@acmepacket.com
   URI:















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