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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 3853.
Author Jon Peterson
Last updated 2015-10-14 (Latest revision 2003-07-02)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 3853 (Proposed Standard)
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Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Allison J. Mankin
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SIP WG                                                       J. Peterson
Internet-Draft                                                   NeuStar
Expires: December 27, 2003                                 June 28, 2003

                     S/MIME AES Requirement for SIP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on December 27, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   RFC3261 currently specifies 3DES as the required minimum ciphersuite
   for implementations of S/MIME in SIP.  This document updates the
   normative guidance of RFC3261 to require the Advanced Encryption
   Standard (AES) for S/MIME.

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Table of Contents

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

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1. Introduction

   The Session Initiation Protocol (SIP) specification (RFC3261 [1])
   currently details optional support (a normative MAY) for the use of
   secure MIME, or S/MIME (RFC2633 [8]).  Since RFC3261 was published,
   the S/MIME specification and the underlying Cryptographic Message
   Syntax (CMS, RFC3369 [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 make 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, RFC2246 [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

   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.

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as

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   described in RFC2119 [2] and indicate requirement levels for
   compliant SIP implementations.

3. S/MIME Ciphersuite Requirements for SIP

   The following updates the text of RFC3261 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, SHA1 as a digest algorithm, and AES as an
   encryption algorithm (as specified in [4].  For key wrap, S/MIME
   implementations MUST support the AES Key Wrap Algorithm ([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 Triples-DES to AES is
   not known to introduce any new security considerations.

5. IANA Considerations

   This document introduces no considerations for IANA.

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, May 2002.

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   [2]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

   [3]  Housley, R., "Cryptographic Message Syntax", RFC 3369, August

   [4]  Schaad, J. and R. Housley, "Use of the AES Encryption Algorithm
        and RSA-OAEP Key Transport in CMS", draft-ietf-smime-aes-alg-06
        (work in progress), January 2003.

   [5]  Schaad, J. and R. Housley, "Advanced Encryption Standard (AES)
        Key Wrap Algorithm", RFC 3394, Sept 2002.

Informative References

   [6]  National Institute of Standards & Technology, "Advanced
        Encryption Standard (AES).", FIPS 197, Nov 2001.

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

   [8]  Ramsdell, B., "S/MIME Version 3 Message Specification", draft-
        ietf-smime-rfc2633bis-03 (work in progress), January 2003.

Author's Address

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St
   Suite 570
   Concord, CA  94520

   Phone: +1 925/363-8720

Appendix A. Acknowledgments

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

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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   or assist in its implementation may be prepared, copied, published
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   This document and the information contained herein is provided on an


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   Internet Society.

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