Network Working Group R. Housley Internet Draft Vigil Security expires in six months August 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, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html IPR Statement By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Abstract This document describes a convention for using the Cryptographic Message Syntax (CMS) to protect more than one content. If desired, attributes can be associated with the content. Housley [Page 1] INTERNET DRAFT August 2004 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. If desired, the content with attributes content type can be used to associate arbitrary attributes with the content. When CMS is used with MIME [MSG], there is no need to use this 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 Content Collection 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 Content with Attributes Example This section provides one simple example to motivate the need for the content with attributes content type. Consider the same art collector as in the previous example. In stead of providing a single image of the amphora, the collector provides several images. To aid potential buyers, the collector attaches several attributes to each image. The attributes provide information about the resolution of the image, the date the image was taken, the photographer, and so on. Figure 2 illustrates the collection of images, showing only two images, each with three attributes. This entire image content collection could be carried instead of the single image shown in Figure 1, allowing it to be covered by the Housley [Page 2] INTERNET DRAFT August 2004 collector's digital signature. +---------------------------------------------------------+ | | | 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. 1.3 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 3] INTERNET DRAFT August 2004 +----------------------------------------------------------+ | | | ContentCollection | | | | +-------------------------+ +-------------------------+ | | | | | | | | | ContentWithAttributes | | ContentWithAttributes | | | | | | | | | | +---------------------+ | | +---------------------+ | | | | | | | | | | | | | | | First Image of | | | | Second Image of | | | | | | the Amphora | | | | the Amphora | | | | | | | | | | | | | | | | | | | | | | | | | +---------------------+ | | +---------------------+ | | | | | | | | | | +---------------+ | | +---------------+ | | | | | | | | | | | | | | | Attribute 1 | | | | Attribute 1 | | | | | | +--+ | | | +--+ | | | | +-+-------------+ | | | +-+-------------+ | | | | | | Attribute 2 | | | | Attribute 2 | | | | | | +--+ | | | +--+ | | | | +-+--------------+ | | | +-+--------------+ | | | | | | Attribute 3 | | | | Attribute 3 | | | | | | | | | | | | | | | +-----------------+ | | +-----------------+ | | | | | | | | | +-------------------------+ +-------------------------+ | | | +----------------------------------------------------------+ Figure 2. Sample use of the ContentWithAttributes 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 4] INTERNET DRAFT August 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 Content With Attributes Content Type The content with attributes content type is used to transfer a single content, which is identified by a content type, and a collection of attributes associated with that content. The syntax accommodates an arbitrary number of attributes; however, there must be at least on attribute. The following object identifier names the content with attributes content type: id-ct-contentWithAttrs OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 20 } The content with attributes content has the following syntax: ContentWithAttributes ::= SEQUENCE { content ContentInfo, attrs SEQUENCE SIZE (1..MAX) OF Attribute } The ContentWithAttributes contains a sequence of a single ContentInfo, followed by a sequence of attributes. The ContentInfo structure is defined in CMS. The contentType object identifier within the ContentInfo indicates the type of the content. The Attribute stricture was originally defined in X.501 [X501], and the Housley [Page 5] INTERNET DRAFT August 2004 definition is repeated in CMS. 4 References This section provides normative and informative references. 4.1 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. 4.2 Informative References MIME Freed, N., and N. Borenstein. Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. RFC 2045, November 1996. X501 CCITT. Recommendation X.501: The Directory -- Models. 1988. 4 Security Considerations This specification does not introduce any new security considerations beyond those already discussed in [CMS]. 5 IANA Considerations No IANA actions are needed. Housley [Page 6] INTERNET DRAFT August 2004 6 IPR Considerations 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 in BCP 78 and BCP 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 at http://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. 7 Author's Address Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA housley@vigilsec.com Housley [Page 7] INTERNET DRAFT August 2004 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 Attribute, 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 -- Content With Attributes Content Type and Object Identifier id-ct-contentWithAttrs OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 20 } ContentWithAttributes ::= SEQUENCE { content ContentInfo, attrs SEQUENCE SIZE (1..MAX) OF Attribute } END Housley [Page 8] INTERNET DRAFT August 2004 Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. 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