SIP WG J. Peterson Internet-Draft NeuStar Expires: December 27, 2003 June 28, 2003 S/MIME AES Requirement for SIP draft-ietf-sip-smime-aes-01 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- Drafts. 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:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 27, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract 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. Peterson Expires December 27, 2003 [Page 1] Internet-Draft S/MIME AES Requirement for SIP June 2003 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 Peterson Expires December 27, 2003 [Page 2] Internet-Draft S/MIME AES Requirement for SIP June 2003 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 RFC3261. 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", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as Peterson Expires December 27, 2003 [Page 3] Internet-Draft S/MIME AES Requirement for SIP June 2003 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. Peterson Expires December 27, 2003 [Page 4] Internet-Draft S/MIME AES Requirement for SIP June 2003 [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 2002. [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 US Phone: +1 925/363-8720 EMail: jon.peterson@neustar.biz URI: http://www.neustar.biz/ Appendix A. Acknowledgments Thanks to Rohan Mahy, Gonzalo Camarillo and Eric Rescorla for review of this document. Peterson Expires December 27, 2003 [Page 5] Internet-Draft S/MIME AES Requirement for SIP June 2003 Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. 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