S/MIME Working Group R. Housley (Vigil Security) Updates: 3852 (once approved) April 2006 Expires October 2006 Cryptographic Message Syntax (CMS) Multiple Signer Clarification Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html 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. Housley [Page 1] INTERNET DRAFT CMS Multiple Signer Clarification April 2006 1. Introduction This document updates the Cryptographic Message Syntax (CMS) [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 valid if any of the digital signatures is valid. This property is especially important in two cases. First, when the recipients do not all implement the same digital signature algorithm, the 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 signature. Second, when a community is transitioning one-way hash functions or digital signature algorithms, the 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: | A recipient independently computes the message digest. This message | digest and the signer's public key are used to verify the signature | value. The signer's public key is referenced either by an issuer | distinguished name along with an issuer-specific serial number or by | a subject key identifier that uniquely identifies the certificate | containing the public key. The signer's certificate can be included | in the SignedData certificates field. Housley [Page 2] INTERNET DRAFT CMS Multiple Signer Clarification April 2006 This block of text is replaced with: | A recipient independently computes the message digest. This message | digest and the signer's public key are used to verify the signature | value. The signer's public key is referenced either by an issuer | distinguished name along with an issuer-specific serial number or by | a subject key identifier that uniquely identifies the certificate | containing the public key. The signer's certificate can be included | in the SignedData certificates field. | | When more than one signature is present, the successful validation | of any one of these signatures ought to be treated as a successful | validation of the signed-data content type. The primary reason | is that signers may include separate signatures for different | communities of recipients. For example, the signed-data content | type might include signatures generated with the RSA signature | algorithm and with the ECDSA signature algorithm. This allows | recipients to verify one algorithm or the other. 4. Update to RFC 3852, Section 5.1: SignedData Type RFC 3852, section 5.1, the next to the last paragraph says: | signerInfos is a collection of per-signer information. There MAY | be any number of elements in the collection, including zero. The | details of the SignerInfo type are discussed in section 5.3. | Since each signer can employ a digital signature technique and | future specifications could update the syntax, all implementations | MUST gracefully handle unimplemented versions of SignerInfo. | Further, since all implementations will not support every possible | signature algorithm, all implementations MUST gracefully handle | unimplemented signature algorithms when they are encountered. This block of text is replaced with: | signerInfos is a collection of per-signer information. There MAY | be any number of elements in the collection, including zero. When | the collection represents more than one signature, the successful | validation of any one of these collection members ought to be | treated as a successful validation of the signed-data content | type. The details of the SignerInfo type are discussed in | section 5.3. Since each signer can employ a digital signature | technique and future specifications could update the syntax, all | implementations MUST gracefully handle unimplemented versions of | SignerInfo. Further, since all implementations will not support | every possible signature algorithm, all implementations MUST | gracefully handle unimplemented signature algorithms when they | are encountered. Housley [Page 3] INTERNET DRAFT CMS Multiple Signer Clarification April 2006 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. 7. Normative References [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. [STDWORDS] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 8. IANA Considerations None. Please remove this section prior to publication as an RFC. Authors' Addresses Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: housley(at)vigilsec.com Housley [Page 4] INTERNET DRAFT CMS Multiple Signer Clarification April 2006 Copyright Statement Copyright (C) The Internet Society (2006). 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. 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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Housley [Page 5]