Network Working Group R. Housley Request for Comments: 4853 Vigil Security Updates: 3852 April 2007 Category: Standards Track
Cryptographic Message Syntax (CMS) Multiple Signer Clarification
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 IETF Trust (2007).
Abstract
This document updates the Cryptographic Message Syntax (CMS), which is published in RFC 3852. This document clarifies the proper handling of the SignedData protected content type when more than one digital signature is present.
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1. Introduction
This document updates the Cryptographic Message Syntax [CMS]. The
CMS SignedData protected content type allows multiple digital
signatures, but the specification is unclear about the appropriate
processing by a recipient of such a signed content. This document
provides replacement text for a few paragraphs, making it clear that
the protected content is validly signed by a given signer, if any of
the digital signatures from that signer are valid.
This property is especially important in two cases. First, when the recipients do not all implement the same digital signature algorithm, a signer can sign the content with several different digital signature algorithms so that each of the recipients can find an acceptable signature. For example, if some recipients support RSA and some recipients support ECDSA, then the signer can generate two signatures, one with RSA and one with ECDSA, so that each recipient will be able to validate one of the signatures. Second, when a community is transitioning one-way hash functions or digital signature algorithms, a signer can sign the content with the older and the newer signature algorithms so that each recipient can find an acceptable signature, regardless of their state in the transition. For example, consider a transition from RSA with SHA-1 to RSA with SHA-256. The signer can generate two signatures, one with SHA-1 and one with SHA-256, so that each recipient will be able to validate at least one of the RSA signatures.
2. Terminology
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 [STDWORDS].
3. Update to RFC 3852, Section 5: Signed-data Content Type
RFC 3852, section 5, the next to the last paragraph says:
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This block of text is replaced with:
4. Update to RFC 3852, Section 5.1: SignedData Type
RFC 3852, section 5.1, the next to the last paragraph says:
This block of text is replaced with:
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6. Security Considerations
The replacement text will reduce the likelihood of interoperability errors during the transition from MD5 and SHA-1 to stronger one-way hash functions, or to better signature algorithms.
7. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA
EMail: housley@vigilsec.com
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