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)
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Copyright (C) The Internet Society (2004).
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.
The Session Initiation Protocol (SIP) specification (RFC 3261 ) currently details optional support (a normative MAY) for the use of secure MIME, or S/MIME (RFC 2633 ). Since RFC 3261 was published, the S/MIME specification and the underlying Cryptographic Message Syntax (CMS, RFC 3369 ) 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 ) 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 ). 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.
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  and indicate requirement levels for compliant SIP implementations.
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 , and AES as an encryption algorithm (as specified in . For key transport, S/MIME implementations MUST support RSA key transport as specified in section 4.2.1. of . 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  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.
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