Network Working Group R. Housley Internet Draft Vigil Security expires in six months July 2004 Protecting Multiple Contents with the Cryptographic Message Syntax (CMS) Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3667. 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 as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on 21 December 2004. Abstract This document describes a convention for using the Cryptographic Message Syntax (CMS) to protect more than one content. 1 Introduction This document describes a convention for using the Cryptographic Message Syntax (CMS) [CMS] to more than one content. The content collection content type is used to transfer one or more contents, each identified by a content type. When CMS is used with MIME [MSG], there is no need to use this Housley [Page 1] INTERNET DRAFT July 2004 specification. In this processing environment, MIME multipart [MIME] provides a straightforward and widely deployed mechanism for carrying more than one content, each associated with a MIME type. CMS is not always used with MIME. Sometimes CMS is used in an exclusively ASN.1 [ASN1] environment. In this case, the content collection content type is used to gather more than one content, each with an object identifier to provide the content type. 1.1 Example This section provides one simple example to motivate the need for the content collection content type. Consider an art collector that wants to sell one of his pieces, an ancient Greek urn, called an amphora. The collector wants to compose a digitally signed offer for sale. It includes three parts. The first part contains the owner's offer for sale, including the asking price. The second part contains a high-quality image of the amphora. The final part contains an appraisal from a well-respected ceramics expert. The final part is digitally signed by the expert. Figure 1 illustrates the structure, and the CMS SignedData content type is used for the two digital signatures. 1.2 Terminology In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as described in [STDWORDS]. Housley [Page 2] INTERNET DRAFT July 2004 +---------------------------------------------------------+ | | | ContentInfo | | | | +-----------------------------------------------------+ | | | | | | | SignedData | | | | | | | | +-------------------------------------------------+ | | | | | | | | | | | ContentCollection | | | | | | | | | | | | +-----------+ +-----------+ +-----------------+ | | | | | | | | | | | | | | | | | | | Owner's | | Image | | SignedData | | | | | | | | Offer to | | of the | | | | | | | | | | Sell the | | Amphora | | +-------------+ | | | | | | | | Amphora | | | | | | | | | | | | | | | | | | | Appraisal | | | | | | | | | | | | | | of Ceramics | | | | | | | | | | | | | | Expert | | | | | | | | | | | | | | | | | | | | | | | | | | | +-------------+ | | | | | | | | | | | | | | | | | | | +-----------+ +-----------+ +-----------------+ | | | | | | | | | | | +-------------------------------------------------+ | | | | | | | +-----------------------------------------------------+ | | | +---------------------------------------------------------+ Figure 1. Sample use of the ContentCollection Content Type. 2 Content Collection Content Type The content collection content type is used to transfer one or more contents, each identified by a content type. The syntax accommodates contents with varying levels of protection. For example, a content collection could include CMS protection content types as well as unprotected content types. A content collection is expected to be encapsulated in one or more CMS protecting content types, but this is not required by this specification. Housley [Page 3] INTERNET DRAFT July 2004 The following object identifier names the content collection content type: id-ct-contentCollection OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 19 } The content collection content has the following syntax: ContentCollection ::= SEQUENCE OF ContentInfo The ContentCollection contains a sequence of ContentInfo, one for each content in the collection. The ContentInfo structure is defined in CMS. The contentType object identifier within the ContentInfo indicates the type of the associated content. Implementations of this specification SHOULD be prepared to handle object identifiers for the SignedData, EncryptedData, EnvelopedData, and AuthenticatedData content types as specified in [CMS]. Implementations of this specification SHOULD also be prepared to handle the object identifier for the CompressedData content type as specified in [COMPRESS]. 3 References This section provides normative and informative references. 3.1 Normative References ASN1 CCITT. Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1). 1988. COMPRESS Gutmann, P. Compressed Data Content Type for Cryptographic Message Syntax (CMS). RFC 3274. June 2002. CMS Housley, R. Cryptographic Message Syntax. RFC 3852. July 2004. STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate Requirement Levels. RFC 2119. March 1997. 3.2 Informative References MIME Freed, N., and N. Borenstein. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. RFC 2045, November 1996. Housley [Page 4] INTERNET DRAFT July 2004 4 Security Considerations This specification does not introduce any new security considerations beyond those already discussed in [CMS]. 5 Author Address Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA housley@vigilsec.com Appendix A: ASN.1 Module The ASN.1 module contained in this appendix defines the structures that are needed to implement this specification. It is expected to be used in conjunction with the ASN.1 modules in [CMS] and [COMPRESS]. ContentCollectionModule { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) 26 } DEFINITIONS IMPLICIT TAGS ::= BEGIN IMPORTS ContentInfo FROM CryptographicMessageSyntax -- [CMS] { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) }; -- Content Collection Content Type and Object Identifier id-ct-contentCollection OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 19 } ContentCollection ::= SEQUENCE OF ContentInfo END Housley [Page 5] INTERNET DRAFT July 2004 Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. 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