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].
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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
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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
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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 }
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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.
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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.
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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) };
-- 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
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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
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RFC 4073 Protecting Multiple Contents with the CMS May 2005
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Acknowledgement
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Internet Society.
Housley Standards Track [Page 9]
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