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RFC 4073 - Protecting Multiple Contents with the Cryptographic M


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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].

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

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

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 }

   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.

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.

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) };

   -- 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

   -- 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

Author's Address

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com

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