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rfc:rfc4073

Network Working Group R. Housley Request for Comments: 4073 Vigil Security Category: Standards Track May 2005

               Protecting Multiple Contents with the
                 Cryptographic Message Syntax (CMS)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document describes a convention for using the Cryptographic
 Message Syntax (CMS) to protect a content collection.  If desired,
 attributes can be associated with the content.

1. Introduction

 This document describes a convention for using the Cryptographic
 Message Syntax (CMS) [CMS] to protect a content collection.  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.
 The convention described in this document is not needed when CMS is
 used with MIME [MSG].  MIME multipart [MIME] provides a
 straightforward and widely deployed mechanism for carrying more than
 one content item, each associated with a MIME type.
 However, 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 item,
 each with an object identifier to specify the content type.
 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 Standards Track [Page 1] RFC 4073 Protecting Multiple Contents with the CMS May 2005

1.1. Content Collection Example

 This section provides one simple example to illustrate the need for
 the content-collection content type.  Consider an art collector who
 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.
 +---------------------------------------------------------+
 |                                                         |
 | 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

Housley Standards Track [Page 2] RFC 4073 Protecting Multiple Contents with the CMS May 2005

1.2. Content with Attributes Example

 This section provides one simple example to illustrate the need for
 the content-with-attributes content type.  Consider the art collector
 from the previous example.  Instead 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 collector's digital signature.
 +----------------------------------------------------------+
 |                                                          |
 | 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

Housley Standards Track [Page 3] RFC 4073 Protecting Multiple Contents with the CMS May 2005

2. Content Collection Content Type

 The content-collection content type is used to transfer a collection
 of content items, 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.
 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 SIZE (1..MAX) 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 one
 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 }

Housley Standards Track [Page 4] RFC 4073 Protecting Multiple Contents with the CMS May 2005

 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
 item 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
 structure was originally defined in X.501 [X501], and the definition
 is repeated in CMS.

4. Security Considerations

 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.  As a result, implementations
 MUST be prepared to handle multiple levels of encapsulation.
 The security considerations discussed in [CMS] are relevant when CMS
 is used to protect more than one content by making use of the content
 collection content type or content with attributes content type.

5. References

5.1. 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 (CMS)", RFC
             3852, July 2004.
 [STDWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

Housley Standards Track [Page 5] RFC 4073 Protecting Multiple Contents with the CMS May 2005

5.2. Informative References

 [MIME]      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
             Extensions (MIME) Part One: Format of Internet Message
             Bodies", RFC 2045, November 1996.
 [MSG]       Ramsdell, B., "Secure/Multipurpose Internet Mail
             Extensions (S/MIME) Version 3.1 Message Specification",
             RFC 3851, July 2004.
 [X501]      CCITT.  Recommendation X.501: The Directory -- Models.
             1988.

Housley Standards Track [Page 6] RFC 4073 Protecting Multiple Contents with the CMS May 2005

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 CryptographicMessageSyntax2004 -- [CMS]
       { iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };
  1. - 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 SIZE (1..MAX) OF ContentInfo
  1. - 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 Standards Track [Page 7] RFC 4073 Protecting Multiple Contents with the CMS May 2005

Author's Address

 Russell Housley
 Vigil Security, LLC
 918 Spring Knoll Drive
 Herndon, VA 20170
 USA
 EMail: housley@vigilsec.com

Housley Standards Track [Page 8] RFC 4073 Protecting Multiple Contents with the CMS May 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Housley Standards Track [Page 9]

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