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Network Working Group                                         R. HousleyRequest for Comments: 4853                                Vigil SecurityUpdates:3852                                                 April 2007Category: Standards TrackCryptographic Message Syntax (CMS)Multiple Signer ClarificationStatus 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 inRFC 3852.  This document clarifies the proper   handling of the SignedData protected content type when more than one   digital signature is present.Housley                     Standards Track                     [Page 1]

RFC 4853           CMS Multiple Signer Clarification          April 20071.  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 inRFC 2119 [STDWORDS].3.  Update toRFC 3852, Section 5: Signed-data Content TypeRFC 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                     Standards Track                     [Page 2]

RFC 4853           CMS Multiple Signer Clarification          April 2007   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 one signature associated with a given signer is usually treated|  as a successful signature by that signer.  However, there are some|  application environments where other rules are needed.  An|  application that employs a rule other than one valid signature for|  each signer must specify those rules.  Also, where simple matching of|  the signer identifier is not sufficient to determine whether the|  signatures were generated by the same signer, the application|  specification must describe how to determine which signatures were|  generated by the same signer.  Support of different communities of|  recipients is the primary reason that signers choose to include more|  than one signature.  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 the signature associated with one algorithm or the other.4.  Update toRFC 3852, Section 5.1: SignedData TypeRFC 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 insection 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 one of signature from a given signer ought to be|     treated as a successful signature by that signer.  However,|     there are some application environments where other rules areHousley                     Standards Track                     [Page 3]

RFC 4853           CMS Multiple Signer Clarification          April 2007|     needed.  The details of the SignerInfo type are discussed in|section 5.3.  Since each signer can employ a different 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.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)",RFC3852, 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.comHousley                     Standards Track                     [Page 4]

RFC 4853           CMS Multiple Signer Clarification          April 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Housley                     Standards Track                     [Page 5]