Network Working Group R. Housley
Internet Draft Vigil Security
expires in six months July 2004
Protecting Multiple Contents with the
Cryptographic Message Syntax (CMS)
<draft-housley-contentcollection-01.txt>
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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
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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].
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+---------------------------------------------------------+
| |
| 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.
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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.
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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
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