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Versions: 00 01 02 03 04 05 06 RFC 4189

SIPPING                                                           K. Ono
Internet-Draft                                              S. Tachimoto
Expires: September 15, 2005                              NTT Corporation
                                                          March 14, 2005


   Requirements for End-to-Middle Security for the Session Initiation
                             Protocol (SIP)
                   draft-ietf-sipping-e2m-sec-reqs-06

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 September 15, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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



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   "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
   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
   3.  Scope of End-to-Middle Security  . . . . . . . . . . . . . . .  7
   4.  Requirements for a Solution  . . . . . . . . . . . . . . . . .  7
     4.1   General Requirements . . . . . . . . . . . . . . . . . . .  7
     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  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1   Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2   Informative References . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
       Intellectual Property and Copyright Statements . . . . . . . . 14




















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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 provide 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 rejects the
   request based on its strict security policy that prohibits the
   forwarding of unknown data.






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

   C:   Content that UA #1 allows the entity to inspect
   [C]: 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 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] |-----|  C  |----- User #2
               | +-----+     +-----+ |   +-----+     +-----+
               | UA #1      Proxy #1 |   Proxy #2     UA #2
               +---------------------+

   C:   Content that UA #1 needs to disclose
   [C]: 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
   network.  Proxy A performs access control based on the destination



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   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 the requests and to
   inspect the message bodies.

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

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

   C:   Content that UA #1 needs to disclose
   [C]: 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 networks, health care
   service provider's networks, as well as other networks where
   archiving communication is required by their security policies.

2.2.2  Non-emergency Call Routing Based on the Location Object

   The Location Object [6] includes a person's geographical location
   information that is privacy-sensitive.  Some proxy servers will have
   the capability to provide routing based on the geographical location



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   information.  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 geographical location
   information 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 data integrity by
   the proxy servers before location-based routing is applied.
   Sometimes the UACs want to send the Location Object to the UASs.
   This is another good example presenting the need for UACs to
   simultaneously send secure data to a proxy server and to the UASs.

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 for identifying 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 [8] includes
   potentially private information, such as a user name.  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 in a message body, such as the Session Description
   Protocol (SDP) [9].  A firewall entity that supports the SIP
   protocol, or a midcom [10] agent co-located with a proxy server,
   controls a firewall based on the address and port information of
   media streams in the SDP offer/answer.  The address and port
   information in the SDP needs to be transmitted securely to recipient
   UAs and the proxy server operating as a midcom agent.  Therefore,
   there is a need for a proxy server to be able to decrypt the SDP, as



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   well as to verify the integrity of the SDP.

   When the SDP includes key parameters for Secure RTP (SRTP) [11], the
   key parameters need to be encrypted only for end-to-end
   confidentiality.

3.  Scope of End-to-Middle Security

   End-to-middle security consists of user authentication, data
   integrity, and data confidentiality.  Providing data integrity
   requires authenticating peer who creates the data.  However, this
   document only describes requirements for data confidentiality and
   data integrity, since end-to-middle authentication is covered by
   existing mechanisms such as HTTP Digest authentication, S/MIME
   Cryptographic Message Syntax (CMS) SignedData body [12], or an AIB.

   As for data integrity, the CMS SignedData body can be used for
   verification of the data integrity and authentication of the signer
   by any entities.  The CMS SignedData body can be used for
   end-to-middle security and end-to-end security simultaneously.
   However, a proxy server generally don't verify the data integrity
   using the CMS SignedData body, and there is no way for a UA to
   request the proxy server to verify the message.  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.

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 request.

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 NOT have an impact on proxy servers that do not
              provide services based on S/MIME-secured bodies in terms
              of handling the existing SIP headers.





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   REQ-GEN-3: It SHOULD NOT violate 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 allow encrypted data to be shared with
               the recipient UA and a proxy server, 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 a proxy server to view
               specific message bodies.  The request itself SHOULD be
               secure, namely be authenticated for the UA and be
               verified for the data integrity.

   REQ-CONF-4: It MAY allow a UA to request that the recipient UA
               disclose information to the proxy server, to which the
               requesting UA is initially disclosing information.  The
               request itself SHOULD be secure, namely be authenticated
               for the UA and be verified for the data integrity.

                  This requirement is necessary when a provider
                  operating the proxy server allows its security
                  policies to be revealed to the provider serving the
                  recipient UA.

4.3  Requirements for End-to-Middle Integrity

   This section enumerates the requirements for the end-to-middle
   integrity.  Verifying the data integrity requires seeing if the data
   is created by the authenticated user, not forged by a malicious user.
   Therefore verification of the data integrity requires the user
   authentication.

   REQ-INT-1: The solution SHOULD work even when the SIP end-to-end
              authentication and integrity services are enabled.





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   REQ-INT-2: It SHOULD allow a UA to request a proxy server to verify
              specific message bodies and authenticate the user.  The
              request itself SHOULD be secure, namely be authenticated
              for the UA and be verified for the data integrity.

   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, namely authenticated for
              the UA and be verified for the data integrity.

                 This requirement is necessary when a provider operating
                 the proxy server allows its security policies to be
                 revealed to the provider serving the recipient UA.

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 any requirements for authentication, verifying the
   data integrity requires peer authentication.  Also, peer
   authentication 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.  A threat model is where a malicious user
   sends many complicated-MIME-structure messages to a proxy server,
   containing user authentication data obtained by eavesdropping.
   Another threat model is where a malicious proxy server sends many
   complicated-MIME-structure messages to a proxy server, containing the
   source IP address and the Via header of an adjacent proxy server.
   These attacks will slow down the overall performance of target proxy
   servers.

   To prevent these attacks, user and server authentication mechanism
   needs to be protected against replay attack.  Or the user and server
   authentication always needs to be executed simultaneously with
   protection of data integrity.  In order to prevent these attacks, the
   following requirements should be met.

   o  The solution MUST support mutual authentication, data
      confidentiality and data integrity protection between a UA and a
      proxy server.





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   o  It SHOULD support protection against a replay attack for user
      authentication.

   o  It SHOULD simultaneously support user authentication and data
      integrity protection.

         These last two requirements are met by HTTP Digest
         authentication.

   o  It MUST support mutual authentication, data confidentiality and
      data integrity protection between proxy servers.

   o  It SHOULD support protection against a replay attack for server
      authentication.

   o  It SHOULD simultaneously support server authentication and data
      integrity protection.

         These last three requirements are met by TLS.

6.  IANA Considerations

   This document requires no additional considerations.

7.  Acknowledgments

   The authors would like to thank to Rohan Mahy and Cullen Jennings for
   their initial support of this concept, and to Jon Peterson, Gonzalo
   Camarillo, Sean Olson, Mark Baugher and Mary Barnes and others for
   their reviews and constructive comments.

8.  Changes

   [Note to RFC editor.  Please remove this entire section when this
   draft is published as an RFC.]

   o  Changes from 05.txt

      *  Updated Ascii art in Section 2.1.
      *  Aligned terminology with the reference[6] to Section 2.2.2.
      *  Added more text to Section 3, for note that data integrity is
         not provided without peer authentication.
      *  Added more text to Section 4.3.
      *  Added a threat model by a malicious server to the "Security
         Consideration" section.
      *  Updated references.
      *  More editorial changes.




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   o  Changes from 04.txt

      *  Updated references.
      *  Fixed editorial errors.

   o  Changes from 03.txt

      *  Removed some of the text that described an illegal behavior of
         a proxy server and the scope of session policies in the
         "Examples of Scenarios" section.
      *  Added notes to describe the requirements met by session
         policies in the "Requirements for a Solution" section.
      *  Added a note to describe the requirements met by an existing
         mechanism.
      *  Changed the last requirements of end-to-middle confidentiality
         and integrity from "SHOULD" to "MAY", and added the conditions
         of the requirements.
      *  Categorized references to normative and informative ones.

   o  Changes from 02.txt

      *  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".
      *  Simplified the "Scope of End-to-Middle Security" section.
      *  Removed some of the text that described detailed information on
         mechanisms in the "Requirements for a Solution" section.
      *  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.
      *  Changed the last item of the general requirements from
         proxy-driven to UA-driven.
      *  Deleted the text in the requirements that describes the
         relation between the requirements and the service examples.
      *  Added some text in the "Security Consideration" section.
      *  Many editorial correction.

   o  "Changes from 01.txt"





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      *  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.
      *  Deleted a few "may" words from the "Problem with Existing
         Situations" section to avoid confusion with "MAY" as a key
         word.
      *  Added the relation between the requirements and the service
         examples.
      *  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".
      *  Changed the 4th requirement of end-to-middle confidentiality
         from "MUST" to "SHOULD".
      *  Changed the 3rd requirement of end-to-middle integrity from
         "MUST" to "SHOULD".
      *  Added some text about DoS attack prevention in the "Security
         Consideration" section.

   o  "Changes from 00.txt"

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

9.  References

9.1  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.

9.2  Informative References

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

   [4]   Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Certificate Handling", RFC 3850, July
         2004.

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



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   [6]   Peterson, J., "A Presence-based GEOPRIV Location Object
         Format", Internet-Draft draft-ietf-geopriv-pidf-lo-03,
         September 2004.

   [7]   Peterson, J., "Session Initiation Protocol (SIP) Authenticated
         Identity Body (AIB) Format", RFC 3893, September 2004.

   [8]   Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
         Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
         Basic and Digest Access Authentication", RFC 2617, June 1999.

   [9]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

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

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

   [12]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852,
         July 2004.


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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Copyright Statement

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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
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