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SIPPING                                                           K. Ono
Internet-Draft                                              S. Tachimoto
Expires: April 22, 2005                                  NTT Corporation
                                                        October 22, 2004


    End-to-middle Security in the Session Initiation Protocol (SIP)
                draft-ono-sipping-end2middle-security-03

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   Some services provided by intermediaries depend on the ability to
   inspect the message bodies in the Session Initiation Protocol (SIP).
   When sensitive information is included in these bodies, a SIP User
   Agent (UA) needs to protect it from all intermediaries except for
   certain selected ones.  This document proposes a mechanism for
   securing information passed between an end user and a selected
   intermediary using S/MIME.  It also proposes mechanisms for notifying



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   the UA that an intermediary needs to inspect an S/MIME-secured
   message body, and that the message body needs to be transmitted
   securely.

Conventions used in this document

   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 RFC-2119 [1].

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Generating S/MIME-secured message body . . . . . . . . . . .   3
   2.1  Generating S/MIME CMS EnvelopedData  . . . . . . . . . . . .   3
   2.2  Generating S/MIME CMS SignedData . . . . . . . . . . . . . .   4
   3.   Indicating the Target Content  . . . . . . . . . . . . . . .   5
   4.   Notification of the Proxy Server's Policies  . . . . . . . .   6
   5.   Behavior of UAs and Proxy Servers  . . . . . . . . . . . . .   7
   5.1  UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.2  UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.3  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . .   8
   6.   Proxy-Required-Body Header Field Use . . . . . . . . . . . .   9
   7.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.1  Example of Request for End-to-Middle Confidentiality . . . .  10
   7.2  Example of Request for End-to-Middle Integrity . . . . . . .  12
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  12
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  13
   10.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  13
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  13
   11.1 Normative References . . . . . . . . . . . . . . . . . . . .  13
   11.2 Informative References . . . . . . . . . . . . . . . . . . .  14
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  14
        Intellectual Property and Copyright Statements . . . . . . .  15

















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1.  Introduction

   When a SIP [2] UA requires services provided by intermediaries that
   depend on the message bodies in request/response messages, end-to-end
   confidentiality will currently have to be disabled.  This problem is
   pointed out in Section 23 of [2].  Since such an intermediary is not
   always adjacent to the UA, this situation requires security between
   the UA and the intermediary for the message bodies.  We call this
   "end-to-middle security", where by "end" we mean a UA and by "middle"
   we mean a specific intermediary, typically a proxy server.

   This document describes proposed mechanisms for providing data
   confidentiality and integrity for end-to-middle security to meet the
   requirements discussed in [3].  Since the major requirement is to
   have little impact on the standardized end-to-end security
   mechanisms, the proposed mechanisms are based on S/MIME [4].  The
   mechanisms consist of generating S/MIME-secured message body and
   indicating the target message body for a selected proxy server.  In
   addition, it also includes mechanisms for notifying the UA that an
   intermediary needs to inspect an S/MIME-secured message body, and
   that the message body needs to be transmitted securely.

2.  Generating S/MIME-secured message body

2.1  Generating S/MIME CMS EnvelopedData

   For end-to-middle confidentiality, a UA MUST generate S/MIME CMS [5]
   EnvelopedData, whose recipient list is specified in the
   "recipientInfos" field.  The structure of the S/MIME CMS
   EnvelopedData contains data encrypted with a content-encryption-key
   (CEK) and the CEK encrypted with different key-encryption-keys
   (KEKs), one for each recipient as specified in [5].  The KEKs are the
   public keys of each recipient or the shared keys between the UA and
   each recipient.

   If the data is encrypted only for a selected proxy server, the
   recipient list contains only the proxy server.  If there is encrypted
   data destined for multiple proxy servers, the recipient list contains
   the targeted proxy servers.  If there is encrypted data destined for
   each proxy server, the recipient list of each piece of encrypted data
   contains the targeted proxy server.  In order to concatenate multiple
   pieces of encrypted data, the UAC MUST generate a multipart MIME
   body.

   Since proxy servers are prohibited from deleting any body, the
   encrypted data for the proxy server is transmitted to the user agent
   server (UAS) but the UAS will be unable to decrypt it.  In order to
   avoid causing unnecessary error conditions in the UAS, the user agent



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   client (UAC) MUST set the value "optional" in the handling parameter
   of the "Content-Disposition" MIME header for the message body.

   If the multipart MIME body consists of encrypted MIME bodies with the
   value "optional", the "Content-Disposition" MIME header of the
   multipart MIME body MUST also contain the value "optional" in the
   handling parameter.  The UAS SHOULD NOT try to decrypt encrypted data
   that has the value "optional".

   If the multipart MIME body contains a body with the value "required"
   and another body with the value "optional", the multipart MIME body
   SHOULD have the value "required" in the handling parameter of the
   "Content-Disposition" MIME header.

   If the encrypted data is meant to be shared with the UAS and selected
   proxy servers, the recipient list SHOULD be addressed to the UAS and
   the selected proxy servers.  The UAC SHOULD set the value "required"
   in the handling parameter of the "Content-Disposition" MIME header
   for the message body.  The UAS MUST try to decrypt the encrypted data
   that has the value "required" in the handling parameter.  If the
   handling parameter is not set, the default behavior is the same as
   for setting the value "required", as specified in [2].

   If a piece of encrypted data is destined for a selected proxy server
   and another piece of encrypted data for the UAS, the recipient of
   each piece of encrypted data is each respective entity.  In this
   case, the UAC MUST generate a multipart MIME body to concatenate the
   two.

      For example, a UA uses this mechanism when keying materials, such
      as keys for use by Secure RTP (SRTP), are included in the SDP[9].
      One CMS EnvelopedData contains SDP that includes keying materials
      of an SRTP stream only for the UA.  The other CMS EnvelopedData
      contains an SDP that does not include the keying materials of the
      SRTP stream only for a selected proxy server that needs to view
      SDP (i.e., for a firewall traversal service).

2.2  Generating S/MIME CMS SignedData

   For end-to-end data integrity, a UA generates S/MIME CMS SignedData
   that can be verified by any entity that knows the public key of the
   UA.  For end-to-middle data integrity, a UA MUST generate S/MIME CMS
   SignedData in the same way as for end-to-end data integrity.

      Note: Even if the handling parameter of the signature of the whole
      message body is set to the value "optional", the UAS SHOULD
      validate the signature of the whole message body since the
      "Content-Disposition" might be modified by a malicious entity.



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3.  Indicating the Target Content

   A UA needs a way to indicate content that they expect to be viewed by
   a proxy server, in order for the proxy server to easily determine
   whether to process MIME bodies and if so, which one.  To meet this
   requirement, the UAs SHOULD set a label to indicate a selected proxy
   server and the target content.  This document defines a new SIP
   header, "Proxy-Required-Body", for the label.

      Open Issue: There are four options for the label: a new SIP
      header, a new parameter of an existing SIP header, a new MIME
      header, or a new parameter for an existing MIME header.

      1) Using a new SIP header, "Proxy-Required-Body", to indicate the
      selected proxy server and the target body.  The new SIP header
      contains one or more "content-id" parameter(s) for setting the
      "Content-ID" MIME header into the target body.  When a message has
      no target body destined for a proxy server, it is easier for this
      option to determine that fact, as this only needs to look at the
      SIP header.
      To protect the integrity of the label in the SIP headers, UAs need
      to generate an application/sipfrag body and attach a digital
      signature for the whole body to protect the data integrity of the
      label.  Generating the application/sipfrag body would required an
      additional multipart MIME structure.  The validation cost for
      integrity protection of these headers reduces the merit of using a
      new SIP header.
      2) Using a new parameter of the Route header, "content-id".  Since
      a proxy server is allowed to modify the Route header, there is no
      way to protect the data integrity of the label against other proxy
      servers along the signaling path.
      3) Using a new MIME header, "Content-Target", as described in the
      last version of this draft.  When a message has a target body
      destined for a proxy server, this shows better performance than
      using a SIP header in searching the target body.  This requires
      only the inspection of the MIME header, while using a SIP header
      requires the inspection of the SIP header "Content-ID" MIME
      header.  All that the UA needs to do to protect the integrity of
      the label is to attach a digital signature for the whole body.
      This is simpler than using a SIP header.
      4) Using a new parameter of "Content-Disposition" MIME header,
      "required-entity".  This MIME header is ambiguous to indicate the
      target entity.

   When a UA labels the encrypted data, it SHOULD set the
   "Proxy-Required-Body" SIP header that contains the address of the
   server and "content-id" parameter indicating the S/MIME CMS
   EnvelopedData.  When a UA labels the data with signature, it SHOULD



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   set the "Proxy-Required-Body" SIP header that contains the address of
   the server and "content-id" parameter indicating the S/MIME CMS
   SignedData.  When a proxy server receives a message, it SHOULD
   inspect the "Proxy-Required-Body" SIP headers.

   A UA SHOULD generate a digital signature of the whole message body
   including an application/sipfrag body that contains the new SIP
   header in order to protect the integrity of the label.  The proxy
   server SHOULD validate the signature of the whole message body to
   check the integrity of the indication, even when the "content-id"
   parameter contained in the "Proxy-Required-Body" SIP header is not
   set for the whole message body.

      Open Issue: Should the target proxy server remove the label before
      forwarding the message?

   If a UA already knows that a selected proxy server needs to inspect a
   message body, the UA MAY NOT set such label.  The proxy server MAY
   view a message body independently of the label, in order to inspect
   the target body using "recipientinfo" in CMS EnvelopedData or
   "Content-Type" MIME header.

4.  Notification of the Proxy Server's Policies

   A notification mechanism for the proxy server's policies is needed
   when a UA does not know the policies of the proxy server in a
   signaling path and the proxy server has its own policies for
   providing some services.  There are two ways in which a UA can learn
   the policies of the proxy server.  The UA MAY learn them by getting a
   policy package from a policy server[10].  When a proxy server needs
   to inspect the message body contained in the response, it needs to
   learn the policies from a policy server before sending the response.
   Alternatively, the UA MAY learn them by receiving a response with an
   error code.

   When the proxy server receives a request in which it cannot view the
   message body that has to be read in order to proceed, the proxy
   server MUST send a response with an error code.  If the request
   contains encrypted data, the error code SHOULD be 493
   (Undecipherable), accompanied by a proxy's public key certificate and
   required Content-Type.

   When the proxy server receives a request whose sending condition
   cannot be accepted, the proxy server MUST also send a response with
   an error code.  If a digital signature is not attached to the request
   and it required for an integrity check, the error code SHOULD be 403
   (Forbidden) accompanied by a required Content-Type that is
   "multipart/signed".



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      Open Issues:
         Is it better to define a new error code for requiring a
         signature attachment?
         How should the error message indicate the Content-Type to which
         a signature needs to be attached? Can these Content-Types be
         nested such as "Content-Type: multipart/signed" for
         "Content-Type: application/sdp"?
         When proxy servers require both disclosure and an integrity
         check, how should it be described?

   When the UAC receives one of the above error codes, it needs to
   authenticate the proxy server.  Therefore, the error code SHOULD
   contain the digital signature of the proxy server.

   In the worst case, this notification mechanism requires two messages
   for each proxy server in the signaling path to establish a session
   between the UAs.  In addition, it requires validation procedures
   using the digital signatures for all proxy servers.  Since this
   causes an increase in the delay before session establishment, it is
   recommended that a UA learns in advance the policies of as many proxy
   servers as they can.

5.  Behavior of UAs and Proxy Servers

   We describe here an example of the behavior of UAs and proxy servers
   in a model in which a proxy server that provides a logging service
   for instant messages exists in a message path as shown in Figure 1.

       +-----+     +-----+     +-----+     +-----+
       | C   |-----| C   |-----| *   |-----| C   |
       +-----+     +-----+     +-----+     +-----+
        UA #1      Proxy #1    Proxy #2     UA #2
                   w/Logging function

   C: Content that UA #1 allows the entity to inspect
   *: Content that UA #1 prevents the entity from inspecting

                      Figure 1: Deployment example


5.1  UAC Behavior

   When a UAC sends a MESSAGE [11] request including encrypted message
   content for end-to-end and end-to-middle confidentiality, it MUST use
   S/MIME CMS EnvelopedData to encrypt them.  In this example, UA #1 is
   assumed to know the services and the public key of Proxy #1.  UA #1
   MUST use S/MIME CMS EnvelopedData body for UA #2 and Proxy #1.  UA #1
   SHOULD specify the hostname of Proxy #1 and Content-ID of the S/MIME
   CMS EnvelopedData to be decrypted in the "Proxy-Required-Body" SIP
   header.



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   When the UAC sends a request and needs end-to-end and end-to-middle
   integrity for the message body, it MUST use S/MIME CMS SignedData to
   attach a digital signature.  In this example, UA #1 MUST use the CMS
   SignedData body of the contents.  UA #1 SHOULD specify the hostname
   of Proxy#1 and Content-ID of the CMS SignedData to be validated in
   the "Proxy-Required-Body" SIP header.

   When the UAC sends multiple requests to the same UAS, the CEK reuse
   mechanism is beneficial in letting UAs efficiently encrypt/decrypt
   data.  The CEK reuse mechanism is described in [6][7].  The UAC
   SHOULD use the "unprotectedAttrs" field to stipulate reuse of the CEK
   and indicate its identifier.  When the UAC reuses the CEK in the
   previous request as the KEK, it generates CMS EnvelopedData with the
   type "KEKRecipientInfo" of "RecipientInfo" attribute.

5.2  UAS Behavior

   When a UAS sends a response to the request with this mechanism, the
   use of the same type of S/MIME CMS data is recommended.  For example,
   if the UAS receives an INVITE request in which the SDP is encrypted
   by using CMS EnvelopedData, the response is RECOMMENDED to be a "200
   OK" containing the encrypted SDP based on the CMS EnvelopedData.  In
   the above example, however, the response of the MESSAGE request does
   not need to use the same type of S/MIME CMS data since the response
   does not contain the message content.

   In particular, when the CMS EnvelopedData body of the request
   contains the "unprotectedAttrs" attribute specifying reuse of the
   CEK, the UAS SHOULD keep the CEK with the identifier specified in the
   "unprotectedAttrs" attribute.

   When the UAS receives a request that uses S/MIME, it decrypts and/or
   validates the S/MIME bodies as usual.

   Even when the UAS receives a request without this mechanism, the UAS
   may need end-to-end and end-to-middle confidentiality of the message
   bodies and/or headers in the response.  In this case, the UAS MUST
   use CMS EnvelopedData to encrypt them.  When the UAS sends a response
   and needs end-to-end and end-to-middle integrity of the message
   bodies and/or headers, it MUST use CMS SignedData to attach a digital
   signature.  This is the same way in which the UAC normally operates
   with this mechanism.

5.3  Proxy Behavior

   When a proxy server supporting this mechanism receives a message, it
   MUST inspect the "Proxy-Required-Body" SIP header.  If the SIP header
   includes the processing server's own hostname, the proxy server MUST



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   inspect the specified body in the "content-id" parameter.

   When the specified body is CMS EnvelopedData, the proxy server MUST
   inspect it and try to decrypt the "recipientInfos" field.  If the
   proxy server fails to decrypt that, it SHOULD cancel the subsequent
   procedure and respond with a 493 (Undecipherable) response if it is a
   request, otherwise any existing dialog MAY be terminated.  If the
   proxy server succeeds in this decryption, it MUST inspect the
   "unprotectedAttrs" field of the CMS EnvelopedData body.  If the
   attribute gives the key's identifier, the proxy server MUST keep the
   CEK with its identifier until the lifetime of the CEK has expired.
   If it receives subsequent messages within the lifetime, it MUST try
   to decrypt the type "KEKRecipientInfo" of the "RecipientInfo"
   attribute by using this CEK.

   When the specified content is CMS SignedData, the proxy server MUST
   inspect it and validate the digital signature.  If the verification
   fails, the proxy server SHOULD reject the subsequent procedure and
   respond with a 403 (Forbidden) response if the message is a request,
   otherwise any existing dialog MAY be terminated.

   When the proxy server forwards the request, it modifies the routing
   headers normally.  It does not need to modify the message body.

   If a proxy does not support this mechanism and receives a message
   with the "Proxy-Required-Body" SIP header, the proxy MUST ignore the
   header and operate as usual.

6.  Proxy-Required-Body Header Field Use

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC-2234 [8].  The new header
   "Proxy-Required-Body" is defined as a SIP header.

   Proxy-Required-Body   = "Proxy-Required-Body" HCOLON required-proxy SEMI target-body
   required-proxy        = host
   target-body           = cid-param *(COMMA cid-param)
   cid-param             = "cid" EQUAL content-id
   content-id            = LDQUOT dot-atom "@" (dot-atom / host) RDQUOT
   dot-atom              = atom *( "." atom )
   atom                  = 1*( alphanum / "-" / "!" / "%" / "*" /
                           "_" / "+" / "'" / "`" / "~"   )

   Information about the use of headers in relation to SIP methods and
   proxy processing is summarized in Table 1.






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   Header field         where   proxy  ACK BYE CAN INV OPT REG
   ------------------------------------------------------------------
   Proxy-Required-Body  R        adr    o   o   o   o   o   o
   Proxy-Required-Body  200-699  adr    -   o   -   o   o   o
   Proxy-Required-Body  1xx      adr    -   -   -   o   -   -

   Header field         where   proxy  SUB NOT PRK IFO UPD MSG
   -----------------------------------------------------------------
   Proxy-Required-Body  R        adr    o   o   -   o   o   o
   Proxy-Required-Body  200-699  adr    o   o   -   o   o   o

   Table 1: Summary of header field use

   The "where" column gives the request and response types in which the
   header field can be used.  The values in the "where" column are as
   follows:
      *  R: The header field may appear in requests
      *  200-699: A numeral range indicates response codes with which
         the header field can be used.
      *  a: A proxy can add or concatenate the header field if it is not
         present.
      *  r: A proxy must be able to read the header field, so it cannot
         be encrypted.
      *  d: A proxy can delete a header field value.
      *  o: The header field is optional.

7.  Examples

   The following examples illustrate the use of the mechanism defined in
   the previous section.

7.1  Example of Request for End-to-Middle Confidentiality

   In the following example, a UA needs the message content in a MESSAGE
   request to be confidential and it allows a selected proxy server to
   view the message content.  It also needs to protect the label of the
   target content.  In addition, it needs to reuse the CEK in the
   subsequent request messages.  In the example encrypted message below,
   the text with the box of asterisks ("*") is encrypted:

   MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com


   MESSAGE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   Route: <sip:ss1.atlanta.example.com;lr>
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl



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   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 MESSAGE
   Date: Fri, 20 June 2003 13:02:03 GMT
   Proxy-Required-Body: ss1.atlanta.example.com;cid=1234@atlanta.example.com
   Content-Type: multipart/signed;protocol="application/pkcs7-signature";
            micalg=sha1;boundary=boundary1
   Content-Length: ...

   --boundary1

   Content-Type: application/pkcs7-mime;smime-type=enveloped-data;
                 name=smime.p7m
   Content-Transfer-Encoding: binary
   Content-ID: 1234@atlanta.example.com
   Content-Disposition: attachment;filename=smime.p7m;handling=required
   Content-Length: ...

   ******************************************************************
   * (encryptedContentInfo)                                         *
   * Content-Type: text/plain                                       *
   * Content-Length: ...                                            *
   *                                                                *
   * Hello.                                                         *
   * This is confidential.                                          *
   *                                                                *
   * (recipientInfos)                                               *
   * RecipientInfo[0] for ss1.atlanta.example.com public key        *
   * RecipientInfo[1] for Bob's public key                          *
   *                                                                *
   * (unprotectedAttrs)                                             *
   *  CEKReference                                                  *
   ******************************************************************

   --boundary1--
   Content-Type: application/pkcs7-signature; name=smime.p7s
   Content-Transfer-Encoding: binary
   Content-Disposition: attachment;
   filename=smime.p7s;handling=required

   [ binary data ]

   --boundary1--








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7.2  Example of Request for End-to-Middle Integrity

   In the following example, a UA needs the integrity of the message
   content in a MESSAGE request to be validated by a selected proxy
   server before it views the message content.  It also needs to protect
   the label of the target content.

   MESSAGE alice@atlanta.example.com --> ss1.atlanta.example.com


   MESSAGE sip:bob@biloxi.example.com SIP/2.0
   Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   Route: <sip:ss1.atlanta.example.com;lr>
   From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
   To: Bob <sip:bob@biloxi.example.com>
   Call-ID: 3848276298220188511@atlanta.example.com
   CSeq: 1 MESSAGE
   Date: Fri, 20 June 2003 13:02:03 GMT
   Proxy-Required-Body: ss1.atlanta.example.com;cid=1234@atlanta.example.com
   Content-Type: multipart/signed;protocol="application/pkcs7-signature";
            micalg=sha1;boundary=boundary1
   Content-Length: ...

   --boundary1

   Content-Type: text/plain
   Content-Length: ...

   Hello.
   This is protected with the signature.

   --boundary1--
   Content-Type: application/pkcs7-signature; name=smime.p7s
   Content-Transfer-Encoding: binary
   Content-ID:1234@atlanta.example.com
   Content-Disposition: attachment;filename=smime.p7s;handling=required

   [ binary data ]

   --boundary1--



8.  Security Considerations

   This proposal allows a UA to encrypt data for multiple recipients,
   such as multiple proxy servers, or the UAS and proxy servers.  When a



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   proxy server or the UAS receives an encrypted data, the encrypted
   data may be decrypted an modified at another entity on the recipient
   list.  If the encrypted data is meant to be shared with multiple
   recipients, the UAC SHOULD generate S/MIME CMS SignedData for each
   piece of data before encryption or for the whole message body.

9.  IANA Considerations

   This document requires a new "Proxy-Required-Body" SIP header.

10.  Acknowledgments

   Thanks to Rohan Mahy and Cullen Jennings for their initial support of
   this concept and to many people for useful comments, especially Jon
   Peterson, Jonathan Rosenberg, Eric Burger.

11.  References

11.1  Normative References

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

   [2]  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.

   [3]  Ono, K. and S. Tachimoto, "Requirements for end-to-middle
        security in the Session Initiation Protocol (SIP)",
        draft-ietf-sipping-e2m-sec-reqs-04 (work in progress), October
        2004.

   [4]  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
        2633, June 1992.

   [5]  Housley, R., "Cryptographic Message Syntax", RFC 2630, June
        1999.

   [6]  Farrell, S. and S. Turner, "Reuse of CMS Content Encryption
        Keys", RFC 3185, October 2001.

   [7]  Ono, K. and S. Tachimoto, "Key reuse in S/MIME for SIP",
        draft-ono-sipping-keyreuse-smime-00  (work in progress),
        February 2004.

   [8]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.




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11.2  Informative References

   [9]   Andreasen, F., Baugher, M. and D. Wing, "Session Description
         Protocol Security Descriptions for Media Streams",
         draft-ietf-mmusic-sdescriptions-03.txt (work in progress),
         February 2004.

   [10]  Hilt, V., Camarillo, G. and J. Rosenberg, "Profile Data for
         Session Initiation Protocol (SIP) Policies", September 2003.

   [11]  Campbell, Ed., B., Rosenberg, J., Schulzrinne, H., Huitema, C.
         and D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.


Authors' Addresses

   Kumiko Ono
   Network Service Systems Laboratories
   NTT Corporation
   9-11, Midori-Cho 3-Chome
   Musashino-shi, Tokyo  180-8585
   Japan

   EMail: ono.kumiko@lab.ntt.co.jp


   Shinya Tachimoto
   Network Service Systems Laboratories
   NTT Corporation
   9-11, Midori-Cho 3-Chome
   Musashino-shi, Tokyo  180-8585
   Japan

   EMail: tachimoto.shinya@lab.ntt.co.jp
















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