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-02.txt>
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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.
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
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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
collector's digital signature.
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].
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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.
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+----------------------------------------------------------+
| |
| 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.
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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 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
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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 Author Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
housley@vigilsec.com
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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
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