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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: 7 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 (Vigil Security) 3 Updates: 3852 (once approved) July 2006 4 Expires January 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 a "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 valid if any of the digital signatures is 48 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 the 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 signature. Second, when a 58 community is transitioning one-way hash functions or digital 59 signature algorithms, the 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 of signature associated with each signer is usually treated 97 | as a successful validation of the signed-data content type. However, 98 | there are some application environments where other rules are needed. 99 | An 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, then 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 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 each signer ought to be 131 | treated as a successful validation of the signed-data content 132 | type. However, there are some application environments where 133 | other rules are needed. The details of the SignerInfo type are 134 | discussed in section 5.3. Since each signer can employ a 135 | different digital signature technique and future specifications 136 | could update the syntax, all implementations MUST gracefully 137 | handle unimplemented versions of SignerInfo. Further, since all 138 | implementations will not support every possible signature 139 | algorithm, all implementations MUST gracefully handle 140 | unimplemented signature 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 Internet Society (2006). 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 AND THE INTERNET 181 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 182 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 183 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. Information 192 on the procedures with respect to rights in RFC documents can be 193 found in BCP 78 and BCP 79. 195 Copies of IPR disclosures made to the IETF Secretariat and any 196 assurances of licenses to be made available, or the result of an 197 attempt made to obtain a general license or permission for the use of 198 such proprietary rights by implementers or users of this 199 specification can be obtained from the IETF on-line IPR repository at 200 http://www.ietf.org/ipr. 202 The IETF invites any interested party to bring to its attention any 203 copyrights, patents or patent applications, or other proprietary 204 rights that may cover technology that may be required to implement 205 this standard. Please address the information to the IETF at 206 ietf-ipr@ietf.org.