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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 3852 (ref. 'CMS') (Obsoleted by RFC 5652) Summary: 4 errors (**), 0 flaws (~~), 2 warnings (==), 8 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 S/MIME Working Group R. Housley 3 Updates: 3852 (once approved) Vigil Security 4 Expires August 2007 February 2007 6 Cryptographic Message Syntax (CMS) 7 Multiple Signer Clarification 8 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/1id-abstracts.html 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html 33 Abstract 35 This document updates the Cryptographic Message Syntax (CMS), which 36 is published in RFC 3852. This document clarifies the proper 37 handling of the SignedData protected content type when more than one 38 digital signature is present. 40 1. Introduction 42 This document updates the Cryptographic Message Syntax (CMS) [CMS]. 43 The CMS SignedData protected content type allows multiple digital 44 signatures, but the specification is unclear about the appropriate 45 processing by a recipient of such a signed content. This document 46 provides replacement text for a few paragraphs, making it clear that 47 the protected content is validly signed by a given signer, if any of 48 the digital signatures from that signer is valid. 50 This property is especially important in two cases. First, when the 51 recipients do not all implement the same digital signature algorithm, 52 a signer can sign the content with several different digital 53 signature algorithms so that each of the recipients can find an 54 acceptable signature. For example, if some recipients support RSA 55 and some recipients support ECDSA, then the signer can generate two 56 signatures, one with RSA and one with ECDSA, so that each recipient 57 will be able to validate one of the signatures. Second, when a 58 community is transitioning one-way hash functions or digital 59 signature algorithms, a signer can sign the content with the older 60 and the newer signature algorithms so that each recipient can find an 61 acceptable signature, regardless of their state in the transition. 62 For example, consider a transition from RSA with SHA-1 to RSA with 63 SHA-256. The signer can generate two signatures, one with SHA-1 and 64 one with SHA-256, so that each recipient will be able to validate at 65 least one of the RSA signatures. 67 2. Terminology 69 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 70 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 71 document are to be interpreted as described in RFC 2119 [STDWORDS]. 73 3. Update to RFC 3852, Section 5: Signed-data Content Type 75 RFC 3852, section 5, the next to the last paragraph says: 77 | A recipient independently computes the message digest. This message 78 | digest and the signer's public key are used to verify the signature 79 | value. The signer's public key is referenced either by an issuer 80 | distinguished name along with an issuer-specific serial number or by 81 | a subject key identifier that uniquely identifies the certificate 82 | containing the public key. The signer's certificate can be included 83 | in the SignedData certificates field. 85 This block of text is replaced with: 87 | A recipient independently computes the message digest. This message 88 | digest and the signer's public key are used to verify the signature 89 | value. The signer's public key is referenced either by an issuer 90 | distinguished name along with an issuer-specific serial number or by 91 | a subject key identifier that uniquely identifies the certificate 92 | containing the public key. The signer's certificate can be included 93 | in the SignedData certificates field. 94 | 95 | When more than one signature is present, the successful validation 96 | of one signature associated with a given signer is usually treated 97 | as a successful signature by that signer. However, there are some 98 | application environments where other rules are needed. An 99 | application that employs a rule other than one valid signature for 100 | each signer must specify those rules. Also, where simple matching of 101 | the signer identifier is not sufficient to determine whether the 102 | signatures were generated by the same signer, the application 103 | specification must describe how to determine which signatures were 104 | generated by the same signer. Support of different communities of 105 | recipients is the primary reason that signers choose to include more 106 | than one signature. For example, the signed-data content type might 107 | include signatures generated with the RSA signature algorithm and 108 | with the ECDSA signature algorithm. This allows recipients to 109 | verify the signature associated with one algorithm or the other. 111 4. Update to RFC 3852, Section 5.1: SignedData Type 113 RFC 3852, section 5.1, the next to the last paragraph says: 115 | signerInfos is a collection of per-signer information. There MAY 116 | be any number of elements in the collection, including zero. The 117 | details of the SignerInfo type are discussed in section 5.3. 118 | Since each signer can employ a digital signature technique and 119 | future specifications could update the syntax, all implementations 120 | MUST gracefully handle unimplemented versions of SignerInfo. 121 | Further, since all implementations will not support every possible 122 | signature algorithm, all implementations MUST gracefully handle 123 | unimplemented signature algorithms when they are encountered. 125 This block of text is replaced with: 127 | signerInfos is a collection of per-signer information. There MAY 128 | be any number of elements in the collection, including zero. When 129 | the collection represents more than one signature, the successful 130 | validation of one of signature from a given signer ought to be 131 | treated as a successful signature by that signer. However, 132 | there are some application environments where other rules are 133 | needed. The details of the SignerInfo type are discussed in 134 | section 5.3. Since each signer can employ a different digital 135 | signature technique and future specifications could update the 136 | syntax, all implementations MUST gracefully handle unimplemented 137 | versions of SignerInfo. Further, since all implementations will 138 | not support every possible signature algorithm, all 139 | implementations MUST gracefully handle unimplemented signature 140 | algorithms when they are encountered. 142 6. Security Considerations 144 The replacement text will reduce the likelihood of interoperability 145 errors during the transition from MD5 and SHA-1 to stronger one-way 146 hash functions, or to better signature algorithms. 148 7. Normative References 150 [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", 151 RFC 3852, July 2004. 153 [STDWORDS] S. Bradner, "Key words for use in RFCs to Indicate 154 Requirement Levels", BCP 14, RFC 2119, March 1997. 156 8. IANA Considerations 158 None. Please remove this section prior to publication as an RFC. 160 Authors' Addresses 162 Russell Housley 163 Vigil Security, LLC 164 918 Spring Knoll Drive 165 Herndon, VA 20170 166 USA 168 EMail: housley(at)vigilsec.com 170 Copyright Statement 172 Copyright (C) The IETF Trust (2007). 174 This document is subject to the rights, licenses and restrictions 175 contained in BCP 78, and except as set forth therein, the authors 176 retain all their rights. 178 This document and the information contained herein are provided on an 179 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 180 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 181 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 182 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 183 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 184 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 186 The IETF takes no position regarding the validity or scope of any 187 Intellectual Property Rights or other rights that might be claimed to 188 pertain to the implementation or use of the technology described in 189 this document or the extent to which any license under such rights 190 might or might not be available; nor does it represent that it has 191 made any independent effort to identify any such rights. 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