SIPPING                                                           K. Ono
Internet-Draft                                              S. Tachimoto
Expires: December 29, 2004                               NTT Corporation
                                                           June 30, 2004


   Requirements for End-to-Middle Security for the Session Initiation
                             Protocol (SIP)
                   draft-ietf-sipping-e2m-sec-reqs-03

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   patent or other IPR claims of which I am aware have been disclosed,
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   This Internet-Draft will expire on December 29, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   A SIP User Agent (UA) does not always trust all intermediaries in its
   request path to inspect its message bodies and/or headers contained
   in its message.  The UA might want to protect the message bodies and/
   or headers from intermediaries except those that provide services
   based on its content.  This situation requires a mechanism called
   "end-to-middle security" to secure the information passed between the
   UA and intermediaries, which does not interfere with end-to-end
   security.  This document defines a set of requirements for a



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   mechanism to achieve end-to-middle security.

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.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1   Examples of Scenarios  . . . . . . . . . . . . . . . . . .  3
     2.2   Service Examples . . . . . . . . . . . . . . . . . . . . .  5
       2.2.1   Logging Services for Instant Messages  . . . . . . . .  5
       2.2.2   Non-emergency Call Routing Based on the Location
               Object . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.2.3   User Authentication  . . . . . . . . . . . . . . . . .  6
       2.2.4   Media-related Services . . . . . . . . . . . . . . . .  6
   3.  Scope of End-to-Middle Security  . . . . . . . . . . . . . . .  7
   4.  Requirements for a Solution  . . . . . . . . . . . . . . . . .  8
     4.1   General Requirements . . . . . . . . . . . . . . . . . . .  8
     4.2   Requirements for End-to-Middle Confidentiality . . . . . .  8
     4.3   Requirements for End-to-Middle Integrity . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1   Changes from 02.txt  . . . . . . . . . . . . . . . . . . .  9
     7.2   Changes from 01.txt  . . . . . . . . . . . . . . . . . . . 10
     7.3   Changes from 00.txt  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

















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1.  Introduction

   The Session Initiation Protocol (SIP) [2] supports hop-by-hop
   security using Transport Layer Security (TLS) [3] and end-to-end
   security using Secure MIME (S/MIME) [4].  These security mechanisms
   assume that a SIP UA trusts all proxy servers along its request path
   to inspect the message bodies contained in the message, or a SIP UA
   does not trust any proxy servers to do so.

   However, there is a model where trusted and partially-trusted proxy
   servers are mixed along a message path.  The partially-trusted proxy
   servers are only trusted to provide SIP routing, but these proxy
   servers are not trusted by users to inspect its data except routing
   headers.  A hop-by-hop confidentiality service using TLS is not
   suitable for this model.  An end-to-end confidentiality service using
   S/MIME is also not suitable when the intermediaries provide services
   based on reading the message bodies and/or headers.  This problem is
   described in Section 23 of [2].

   In some cases, a UA might want to protect its message bodies and/or
   headers from proxy servers along its request path except from those
   that provides services based on reading its message bodies and/or
   headers.  Conversely, a proxy server might want to view the message
   bodies and/or headers to sufficiently provide these services.  Such
   proxy servers are not always the first hop from the UA.  This
   situation requires a security mechanism to secure message bodies and/
   or headers between the UA and the proxy servers, yet disclosing
   information to those that need it.  We call this "end-to-middle
   security".

2.  Use Cases

2.1  Examples of Scenarios

   We describe here examples of scenarios in which trusted and
   partially-trusted proxy servers both exist in a message path.  These
   situations demonstrate the reasons why end-to-middle security is
   required.

   In the following example, User #1 does not know the security policies
   or services provided by Proxy server #1 (Proxy#1).  User #1 sends a
   MESSAGE [5] request including S/MIME-encrypted message content for
   end-to-end security as shown in Figure 1, while Proxy #1 erases the
   encrypted data in the request or rejects the request base on its
   strict security policy that prohibits the forwarding of unknown data.
   For the MESSAGE request to correctly traverse Proxy #1, the UA will
   need to discover if end-to-end confidentiality will conflict with
   intermediary's services or security policies.



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               Home network
               +---------------------+
               | +-----+     +-----+ |   +-----+     +-----+
   User #1-----| | C   |-----| *   |-----| *   |-----| C   |-----User #2
               | +-----+     +-----+ |   +-----+     +-----+
               | UA #1      Proxy #1 |   Proxy #2     UA #2
               +---------------------+

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

                    Figure 1: Deployment example #1

   In the second example, Proxy server #1 is the home proxy server of
   User #1 using UA #1.  User #1 communicates with User #2 through Proxy
   #1 and Proxy #2 as shown in Figure 2.  Although User #1 already knows
   Proxy #1's security policy which requires the inspection of the
   content of the MESSAGE request, User #1 does not know whether Proxy
   #2 is trustworthy, and thus wants to protect the message bodies in
   the request.  To accomplish this, UA #1 will need to be able to grant
   a trusted intermediary (Proxy #1) to inspect message bodies, while
   preserving their confidentiality from other intermediaries (Proxy
   #2).

   Even if UA #1's request message authorizes a selected proxy server
   (Proxy #1) to inspect the message bodies, UA #1 is unable to
   authorize the same proxy server to inspect the message bodies in
   subsequent MESSAGE requests from UA #2.

               Home network
               +---------------------+
               | +-----+     +-----+ |   +-----+     +-----+
   User #1-----| | C   |-----| C   |-----| *   |-----| C   |----- User #2
               | +-----+     +-----+ |   +-----+     +-----+
               | UA #1      Proxy #1 |   Proxy #2     UA #2
               +---------------------+

   C: Content that UA #1 needs to disclose
   *: Content that UA #1 needs to protect

                    Figure 2: Deployment example #2

   In the third example, User #1 connects UA #1 to a proxy server in a
   visited (potentially insecure) network, e.g., a hotspot service or a
   roaming service.  Since User #1 wants to utilize certain home network
   services, UA #1 connects to a home proxy server, Proxy #1.  However,
   UA #1 must connect to Proxy #1 via the proxy server of the visited
   network (Proxy A), because User #1 must follow the policy of that



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   network.  Proxy A performs access control based on the destination
   addresses of calls.  User #1 only trusts Proxy A to route requests,
   not to inspect the message bodies the requests contain as shown in
   Figure 3.  User #1 trusts Proxy #1 both to route requests and to
   inspect the message bodies for some purpose.

   The same problems as in the second example also exist here.

               Visited network
              +---------------------+
              | +-----+     +-----+ |   +-----+     +-----+     +-----+
   User #1 -- | | C   |-----| *   |-----| C   |-----| *   |-----| C   |
              | +-----+     +-----+ |   +-----+     +-----+     +-----+
              | UA #1       Proxy A |   Proxy #1     Proxy #2    UA #2
              +---------------------+

   C: Content that UA #1 needs to disclose
   *: Content that UA #1 needs to protect

                    Figure 3: Deployment example #3


2.2  Service Examples

   We describe here several services that require end-to-middle
   security.

2.2.1  Logging Services for Instant Messages

   Logging Services are provided by the archiving function, which is
   located in the proxy server, that logs the message content exchanged
   between UAs.  The archiving function could be located at the
   originator network and/or the destination network.  When the content
   of an instant message contains private information, UACs (UA Clients)
   encrypt the content for the UASs (UA Servers).  The archiving
   function needs a way to log the content in a message body in
   bidirectional MESSAGE requests in such a way that the data is
   decipherable.  The archiving function also needs a way to verify the
   data integrity of the content before logging.

   This service might be deployed in financial or health care service
   provider's networks, where archiving communication is required by
   their security policies, as well as other networks.

2.2.2  Non-emergency Call Routing Based on the Location Object

   The Location Object [6] includes private information as well as
   routing information for appropriate proxy servers.  Some proxy



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   servers have the capability to provide location-based routing.  When
   UAs want to employ location-based routing in non-emergency
   situations, the UAs need to connect to the proxy servers with such a
   capability and disclose the location object contained in the message
   body of the INVITE request, while protecting it from other proxy
   servers along the request path.

   The Location Object also needs to be verified for integrity before
   location-based routing is applied.  Sometimes the UAC want to also
   send the Location Object to the UASs.  This is another good example
   of the need for a UAC to simultaneously send secure data to a proxy
   server and to the UAS.

2.2.3  User Authentication

2.2.3.1  User Authentication using the AIBs

   The Authenticated Identity Bodies (AIBs) [7] is a digitally-signed
   data that is used as way to identify users.  Proxy servers that need
   to authenticate a user verify the signature.  When the originator
   needs anonymity, the user identity in the AIB is encrypted before
   being signed.  Proxy servers that authenticate the user need to
   decrypt the body in order to view the user identity in the AIB.  Such
   proxy servers can be located at adjacent and/or non-adjacent to the
   UA.

   The AIB could be included in all request/response messages.  The
   proxy server needs to view it in request messages in order to
   authenticate users.  Another proxy server sometimes needs to view it
   in response messages for user authentication.

2.2.3.2  User Authentication in HTTP Digest Authentication

   User authentication data for HTTP digest authentication includes two
   types of information; potentially private information, such as a user
   name, and information that can be used for "replay-attacks", such as
   the "response" parameter that is created by a calculation using a
   user's password.  The user authentication data can be set only in a
   SIP header of request messages.  This information needs to be
   transmitted securely to servers that authenticate users, located
   either adjacently and/or non-adjacently to the UA.

2.2.4  Media-related Services

   Firewall traversal is an example of services based on media
   information typically in a message body, such as the Session
   Description Protocol (SDP).  A firewall entity that supports the SIP
   protocol, or a midcom [8] agent co-located with a proxy server,



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   controls a firewall based on media information.  The SDP includes the
   address and port information for media streams and/or key parameters
   for Secure RTP (SRTP) [9].  Critical information contained in SDP
   requires UAs to encrypt the SDP for recipient UAs.  If the SDP is
   encrypted for end-to-end confidentiality, the proxy server operating
   as a midcom agent will have no way to provide firewall traversal as
   it can not inspect the SDP.  Therefore, there is a need for proxy
   server to be able to decrypt the SDP, as well as to verify the
   integrity of the SDP.

      [Note: The validity of the use case depends on which mechanism is
      selected for session policies [10] by the SIPPING WG.  If the
      session policy mechanism would require that UAs disclose media
      information to the policy servers using out-of-band messages, such
      as OPTIONS request, end-to-middle security is not required for
      these use cases.  If the session policy mechanism employs in-band
      messages in order for UAs to disclose media information to the
      policy servers co-located with a proxy server, end-to-middle
      security is required.  As the mechanism proposes to place subset
      of SDP into the header to be viewed by proxy servers, such as
      addresses and port numbers of media streams, these information
      need to be secured from entities except the policy servers.]

3.  Scope of End-to-Middle Security

   End-to-middle security consists of user authentication, data
   integrity, and data confidentiality.  However, this document only
   describes requirements for data confidentiality and data integrity,
   since authentication is covered by existing mechanisms such as HTTP
   digest authentication [2], S/MIME Cryptographic Message Syntax (CMS)
   SignedData body [11], or an AIB.

   As for data integrity, the CMS SignedData body can be used for
   verification of the data integrity by any entities.  The CMS
   SignedData body could be used for end-to-middle security at the same
   time for end-to-end security.

   Although a proxy server is able to verify the integrity of the data,
   there is no way for UAs to request a selected proxy server to verify
   a message with the CMS SignedData body.  Therefore some new
   mechanisms are needed to achieve data integrity for end-to-middle
   security.

   This document mainly discusses requirements for data confidentiality
   and the integrity of end-to-middle security.






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4.  Requirements for a Solution

   We describe here requirements for a solution.  The requirements are
   mainly applied during the phase of a dialog creation or sending a
   MESSAGE method.

4.1  General Requirements

   The following are general requirements for end-to-middle
   confidentiality and integrity.

   REQ-GEN-1: The solution SHOULD have little impact on the way a UA
              handles S/MIME-secured messages.
   REQ-GEN-2: It SHOULD have no impact on proxy servers that do not
              provide services based on S/MIME bodies in terms of
              handling the existing SIP headers.
   REQ-GEN-3: It SHOULD have little impact on the standardized mechanism
              of proxy servers in terms of handling message bodies.
   REQ-GEN-4: It SHOULD allow a UA to discover security policies of
              proxy servers.  Security policies imply what data is
              needed to disclose and/or verify in a message.
                 This requirement is necessary when the UA does not know
                 statically which proxy servers or domains need
                 disclosing data and/or verification.

4.2  Requirements for End-to-Middle Confidentiality

   REQ-CONF-1: The solution MUST be enable an encrypted data to be
               shared with the recipient UA and selected proxy servers,
               when a UA wants.
   REQ-CONF-2: It MUST NOT violate end-to-end encryption when the
               encrypted data does not need to be shared with any proxy
               servers.
   REQ-CONF-3: It SHOULD allow a UA to request selected proxy servers to
               view specific message bodies.  The request itself SHOULD
               be secure.
   REQ-CONF-4: It SHOULD allow a UA to request that the recipient UA
               disclose information to the proxy server, which
               requesting UA is disclosing the information to.  The
               request itself SHOULD be secure.

4.3  Requirements for End-to-Middle Integrity

   REQ-INT-1: The solution SHOULD work even when the SIP end-to-end
              integrity service is enabled.






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   REQ-INT-2: It SHOULD allow a UA to request selected proxy servers to
              verify specific message bodies.  The request itself SHOULD
              be secure.
   REQ-INT-3: It SHOULD allow a UA to request the recipient UA to send
              the verification data of the same information that the
              requesting UA is providing to the proxy server.  The
              request itself SHOULD be secure.

5.  Security Considerations

   This document describes the requirements for confidentiality and
   integrity between a UA and a proxy server.  Although this document
   does not cover authentication, it is important in order to prevent
   attacks from malicious users and servers.

   The end-to-middle security requires additional processing on message
   bodies, such as unpacking MIME structure, data decryption, and/or
   signature verification to proxy servers.  Therefore the proxy servers
   that enable end-to-middle security are vulnerable to a
   Denial-of-Services attack.  There is a threat model where a malicious
   user sends many complicated-MIME-structure messages to a proxy
   server, containing user authentication data obtained by
   eavesdropping.  This attack will result in a slow down of the overall
   performance of these proxy servers.  To prevent this attack, user
   authentication mechanism needs protection against replay attack.  Or
   the user authentication always needs to be executed simultaneously
   with protection of data integrity.  In order to prevent an attack,
   the following requirements should be satisfied.

   o  The solution MUST support mutual authentication, data
      confidentiality and data integrity protection between a UA and a
      proxy server.
   o  It SHOULD support protection against a replay attack for user
      authentication.
   o  It SHOULD simultaneously support user authentication and data
      integrity protection.

6.  IANA Considerations

   This document requires no additional considerations.

7.  Changes

7.1  Changes from 02.txt

   o  Changed the text about the use case of SDP-based service in order
      to decrease the dependency on session policies discussion.  The
      title was changed to "media-related service".



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   o  Simplified the "Scope of End-to-Middle Security" section.
   o  Removed some of the text that described detailed information on
      mechanisms in the "Requirements for a Solution" section.
   o  Closed open issues as follows:
      *  Deleted an open issue described in the "General Requirements"
         section, since it is no longer an issue.  The issue was
         concerning the necessity for the proxy server to notify the UAS
         after receiving a response, which is not necessary, because
         proxy servers' security policies or services have no
         dependencies on the information in a response.
      *  Deleted an open issue described in the "Requirements for
         End-to-Middle Confidentiality" section, since it is not an
         issue of requirements, but that of a mechanism.
   o  Changed the last item of the general requirements from
      proxy-driven to UA-driven.
   o  Deleted the text in the requirements that describes the relation
      between the requirements and the service examples.
   o  Added some text in the "Security Consideration" section.
   o  Many editorial correction.

7.2  Changes from 01.txt

   o  Extracted use cases from the Introduction section, and created a
      new section to describe the use cases in more detail.  The use
      cases are also updated.
   o  Deleted a few "may" words from the "Problem with Existing
      Situations" section to avoid confusion with "MAY" as a key word.
   o  Added the relation between the requirements and the service
      examples.
   o  Deleted the redundant requirements for discovery of the
      targeted-middle.  The requirement is described only in the
      "Generic Requirements", not in the "Requirements for End-to-Middle
      Confidentiality/Integrity".
   o  Changed the 4th requirement of end-to-middle confidentiality from
      "MUST" to "SHOULD".
   o  Changed the 3rd requirement of end-to-middle integrity from "MUST"
      to "SHOULD".
   o  Added some text about DoS attack prevention in the "Security
      Consideration" section.

7.3  Changes from 00.txt

   o  Reworked the subsections in Section 4 to clarify the objectives,
      separating end-to-middle confidentiality and integrity.

8.  Acknowledgments

   Thanks to Rohan Mahy and Cullen Jennings for their initial support of



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   this concept, and to Jon Peterson, Gonzalo Camarillo, Sean Olson, and
   Mark Baugher for their helpful comments.

9  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]   Allen, C. and T. Dierks, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.

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

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

   [6]   Cuellar, J., Morris, J., Mulligan, D., Peterson, J. and J.
         Polk, "Geopriv Requirements", RFC 3693, February 2004.

   [7]   Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
         draft-ietf-sip-authid-body-03.txt (work in progress), May 2004.

   [8]   Srisuresh, P., Kuthan, J., Rosenberg, J., Brim, S., Molitor, A.
         and A. Rayhan, "Middlebox communication architecture and
         framework", RFC 3303, August 2002.

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

   [10]  Rosenberg, J., "Requirements for Session Policy for the Session
         Initiation Protocol (SIP)",
         draft-ietf-sipping-session-policy-req-01 (work in progress),
         February 2004.

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








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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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