CMS Multiple Contents: Protecting Multiple Contents with the Cryptographic Message Syntax (CMS)
Two CMS content types for bundling several contents together, optionally with attributes, under one layer of protection.
Housley, R., "Protecting Multiple Contents with the Cryptographic Message Syntax (CMS)", RFC 4073, DOI 10.17487/RFC4073, May 2005, <https://www.rfc-editor.org/info/rfc4073>.
@misc{rfc4073,
series = {Request for Comments},
number = 4073,
howpublished = {RFC 4073},
publisher = {RFC Editor},
doi = {10.17487/RFC4073},
url = {https://www.rfc-editor.org/info/rfc4073},
author = {R. Housley},
title = {{Protecting Multiple Contents with the Cryptographic Message Syntax (CMS)}},
pagetotal = 9,
year = 2005,
month = may,
} RFC text
Contents
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
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.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Why Collections Are Needed
RFC 4073 describes a convention for using the Cryptographic Message Syntax (RFC 3852) to protect a content collection. The content-collection content type transfers one or more contents, each identified by a content type. The content-with-attributes content type can attach arbitrary attributes to a content.
The convention is not needed when CMS is used with MIME, because MIME multipart (RFC 2045) already carries more than one content item, each with a MIME type. It is aimed at cases where CMS is used in an exclusively ASN.1 environment, where each item is identified by an object identifier instead.
Email already has a way to put many attachments in one message. This document is for the other situations, where there is no such thing and you still want to bundle things up.
§1.1The Amphora Examples
The RFC illustrates the need with an art collector selling an ancient Greek urn called an amphora. The signed offer has three parts: the offer for sale with the asking price, a high-quality image, and an appraisal from a ceramics expert. The expert signs the appraisal, and the collector signs the whole offer, so two SignedData layers are used.
A second example shows why attributes help. If the collector provides several images, each can carry attributes such as resolution, the date the image was taken and the photographer. The whole image collection can then replace the single image and be covered by the collector's signature.
A seller packs an offer, photos and an expert's report into one box and signs it. Each photo can have a sticky note saying who took it and when.
- ContentInfo
- SignedData the collector's signature
- ContentCollection
- Owner's offer to sell the amphora
- Image of the amphora
- SignedData contains the expert's appraisal
- ContentCollection
- SignedData the collector's signature
§2The Content Collection Type
The content-collection type is named by the object identifier id-ct-contentCollection, whose final arc is 19. Its syntax is ContentCollection ::= SEQUENCE SIZE (1..MAX) OF ContentInfo, so it holds at least one ContentInfo, one for each content. The contentType inside each ContentInfo says what that content is.
Contents can have varying levels of protection. A collection could mix CMS protection content types with unprotected ones, and it is expected, but not required, to be encapsulated in one or more CMS protecting content types.
Implementations SHOULD be prepared to handle the object identifiers for SignedData, EncryptedData, EnvelopedData and AuthenticatedData from CMS, and for CompressedData from RFC 3274.
The collection is a list of wrapped items, with at least one in it. Each item says what it is, and some may be locked while others are not.
§3The Content-with-Attributes Type
The content-with-attributes type carries a single content, identified by a content type, together with a collection of attributes about it. Its object identifier is id-ct-contentWithAttrs, whose final arc is 20.
The syntax is a SEQUENCE of content (a ContentInfo) and attrs (a SEQUENCE SIZE (1..MAX) OF Attribute). Any number of attributes is allowed, but there must be at least one. The Attribute structure was originally defined in X.501 and is repeated in CMS.
This is one item with a list of sticky notes. You must stick on at least one note, and you can add as many as you like.
§4Security Considerations
Because a collection can contain contents with different levels of protection, and is only expected to be wrapped in CMS protection, implementations MUST be prepared to handle multiple levels of encapsulation.
The security considerations in RFC 3852 are relevant whenever CMS protects more than one content through either of these two content types. The RFC adds no further security rules of its own.
Boxes can sit inside boxes, so a program has to be ready to open several layers. Everything that makes plain CMS safe still applies.