Multiple Signatures in Cryptographic Message Syntax (CMS)
RFC 5752

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Internet Engineering Task Force (IETF)                         S. Turner
Request for Comments: 5752                                          IECA
Category: Standards Track                                      J. Schaad
ISSN: 2070-1721                                             Soaring Hawk
                                                            January 2010

       Multiple Signatures in Cryptographic Message Syntax (CMS)

Abstract

   Cryptographic Message Syntax (CMS) SignedData includes the SignerInfo
   structure to convey per-signer information.  SignedData supports
   multiple signers and multiple signature algorithms per signer with
   multiple SignerInfo structures.  If a signer attaches more than one
   SignerInfo, there are concerns that an attacker could perform a
   downgrade attack by removing the SignerInfo(s) with the 'strong'
   algorithm(s).  This document defines the multiple-signatures
   attribute, its generation rules, and its processing rules to allow
   signers to convey multiple SignerInfo objects while protecting
   against downgrade attacks.  Additionally, this attribute may assist
   during periods of algorithm migration.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5752.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must

Turner & Schaad              Standards Track                    [Page 1]
RFC 5752              Multiple Signatures in S/MIME         January 2010

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Rationale .......................................................3
      2.1. Attribute Design Requirements ..............................4
   3. Multiple Signature Indication ...................................5
   4. Message Generation and Processing ...............................6
      4.1. SignedData Type ............................................6
      4.2. EncapsulatedContentInfo Type ...............................7
      4.3. SignerInfo Type ............................................7
      4.4. Message Digest Calculation Process .........................7
           4.4.1. multiple-signatures Signed Attribute Generation .....7
           4.4.2. Message Digest Calculation Process ..................7
      4.5. Signature Generation Process ...............................8
      4.6. Signature Verification Process .............................8
   5. Signature Evaluation Processing .................................8
      5.1. Evaluation of a SignerInfo Object ..........................9
      5.2. Evaluation of a SignerInfo Set .............................9
      5.3. Evaluation of a SignedData Set ............................10
   6. Security Considerations ........................................11
   7. References .....................................................11
      7.1. Normative References ......................................11
      7.2. Informative References ....................................12
   Appendix A. ASN.1 Module...........................................13