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rfc:rfc3853

Network Working Group J. Peterson Request for Comments: 3853 Neustar Updates: 3261 July 2004 Category: Standards Track

             S/MIME Advanced Encryption Standard (AES)
       Requirement for the Session Initiation Protocol (SIP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 RFC 3261 currently specifies 3DES as the mandatory-to-implement
 ciphersuite for implementations of S/MIME in the Session Initiation
 Protocol (SIP).  This document updates the normative guidance of RFC
 3261 to require the Advanced Encryption Standard (AES) for S/MIME.

Table of Contents

 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
 3. S/MIME Ciphersuite Requirements for SIP  . . . . . . . . . . . . 3
 4. Security Considerations  . . . . . . . . . . . . . . . . . . . . 3
 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
    5.1.  Normative References . . . . . . . . . . . . . . . . . . . 4
    5.2.  Informative References . . . . . . . . . . . . . . . . . . 4
 6. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . 4
 7. Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
 8. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 6

Peterson Standards Track [Page 1] RFC 3853 S/MIME AES Requirement for SIP July 2004

1. Introduction

 The Session Initiation Protocol (SIP) specification (RFC 3261 [1])
 currently details optional support (a normative MAY) for the use of
 secure MIME, or S/MIME (RFC 2633 [8]).  Since RFC 3261 was published,
 the S/MIME specification and the underlying Cryptographic Message
 Syntax (CMS, RFC 3369 [3]) have undergone some revision.  Ongoing
 work has identified AES as a algorithm that might be used for content
 encryption in S/MIME.
 The Advanced Encryption Standard (AES [6]) is widely believed to be
 faster than Triple-DES (3DES, which has previously been mandated for
 usage with S/MIME) and to be comparably secure.  AES is also believed
 to have comparatively low memory requirements, which makes it
 suitable for use in mobile or embedded devices, an important use-case
 for SIP.
 As an additional consideration, the SIP specification has a
 recommendation (normative SHOULD) for support of Transport Layer
 Security (TLS, RFC 2246 [7]).  TLS support in SIP requires the usage
 of AES.  That means that currently, implementations that support both
 TLS and S/MIME must support both 3DES and AES.  A similar duplication
 of effort exists with DSS in S/MIME as a digital signature algorithm
 (the mandatory TLS ciphersuite used by SIP requires RSA).  Unifying
 the ciphersuite and signature algorithm requirements for TLS and
 S/MIME would simplify security implementations.
 It is therefore desirable to bring the S/MIME requirement for SIP
 into parity with ongoing work on the S/MIME standard, as well as to
 unify the algorithm requirements for TLS and S/MIME.  To date, S/MIME
 has not yet seen widespread deployment in SIP user agents, and
 therefore the minimum ciphersuite for S/MIME could be updated without
 obsoleting any substantial deployments of S/MIME for SIP (in fact,
 these changes will probably make support for S/MIME easier).  This
 document therefore updates the normative requirements for S/MIME in
 RFC 3261.
 Note that work on these revisions in the S/MIME working group is
 still in progress.  This document will continue to track that work as
 it evolves.  By initiating this process in the SIP WG now, we provide
 an early opportunity for input into the proposed changes and give
 implementers some warning that the S/MIME requirements for SIP are
 potentially changing.

Peterson Standards Track [Page 2] RFC 3853 S/MIME AES Requirement for SIP July 2004

2. Terminology

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
 RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
 described in BCP 14, RFC 2119 [2] and indicate requirement levels for
 compliant SIP implementations.

3. S/MIME Ciphersuite Requirements for SIP

 The following updates the text of RFC 3261 Section 23.3, specifically
 the fifth bullet point.  The text currently reads:
 o  S/MIME implementations MUST at a minimum support SHA1 as a digital
    signature algorithm, and 3DES as an encryption algorithm.  All
    other signature and encryption algorithms MAY be supported.
    Implementations can negotiate support for these algorithms with
    the "SMIMECapabilities" attribute.
 This text is updated with the following:
 S/MIME implementations MUST at a minimum support RSA as a digital
 signature algorithm and SHA1 as a digest algorithm [5], and AES as an
 encryption algorithm (as specified in [4].  For key transport, S/MIME
 implementations MUST support RSA key transport as specified in
 section 4.2.1. of [5].  S/MIME implementations of AES MUST support
 128-bit AES keys, and SHOULD support 192 and 256-bit keys.  Note that
 the S/MIME specification [8] mandates support for 3DES as an
 encryption algorithm, DH for key encryption and DSS as a signature
 algorithm.  In the SIP profile of S/MIME, support for 3DES, DH and
 DSS is RECOMMENDED but not required.  All other signature and
 encryption algorithms MAY be supported.  Implementations can
 negotiate support for algorithms with the "SMIMECapabilities"
 attribute.
 Since SIP is 8-bit clean, all implementations MUST use 8-bit binary
 Content-Transfer-Encoding for S/MIME in SIP.  Implementations MAY
 also be able to receive base-64 Content-Transfer-Encoding.

4. Security Considerations

 The migration of the S/MIME requirement from Triple-DES to AES is not
 known to introduce any new security considerations.

Peterson Standards Track [Page 3] RFC 3853 S/MIME AES Requirement for SIP July 2004

5. References

5.1. Normative References

 [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
      Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
      Session Initiation Protocol", RFC 3261, June 2002.
 [2]  Bradner, S., "Key words for use in RFCs to indicate requirement
      levels", BCP 14, RFC 2119, March 1997.
 [3]  Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369,
      August 2002.
 [4]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
      Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC
      3565, July 2003.
 [5]  Housley, R., "Cryptographic Message Syntax (CMS) Algorithms",
      RFC 3394, August 2002.

5.2. Informative References

 [6]  National Institute of Standards & Technology, "Advanced
      Encryption Standard (AES).", FIPS 197, November 2001.
 [7]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
      2246, January 1999.
 [8]  Ramsdell, B., Ed., "S/MIME Version 3.1 Message Specification",
      RFC 3851, July 2004.

6. Acknowledgments

 Thanks to Rohan Mahy, Gonzalo Camarillo, and Eric Rescorla for review
 of this document.

Peterson Standards Track [Page 4] RFC 3853 S/MIME AES Requirement for SIP July 2004

7. Author's Address

 Jon Peterson
 NeuStar, Inc.
 1800 Sutter St
 Suite 570
 Concord, CA  94520
 US
 Phone: +1 925/363-8720
 EMail: jon.peterson@neustar.biz
 URI:   http://www.neustar.biz/

Peterson Standards Track [Page 5] RFC 3853 S/MIME AES Requirement for SIP July 2004

8. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Peterson Standards Track [Page 6]

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