Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 8769 August Cellars
Category: Informational March 2020
ISSN: 2070-1721
Cryptographic Message Syntax (CMS) Content Types for Concise Binary
Object Representation (CBOR)
Abstract
Concise Binary Object Representation (CBOR) is becoming a widely used
method of doing content encoding. The Cryptographic Message Syntax
(CMS) is still a widely used method of doing message-based security.
This document defines a set of content types for CMS that hold CBOR
content.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8769.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. CBOR Content Type
3. CBOR Sequence Content Type
4. ASN.1 Module
5. IANA Considerations
6. Security Considerations
7. Normative References
Author's Address
1. Introduction
Concise Binary Object Representation (CBOR) [CBOR] is a compact self-
describing binary encoding formation that is starting to be used in
many different applications. One of the primary uses of CBOR is in
the Internet of Things, the constrained nature of which means that
having minimal size of encodings becomes very important. The
Cryptographic Message Syntax (CMS) [CMS] is still one of the most
common methods for providing message-based security, although in many
cases, the CBOR Object Signing and Encryption (COSE) [COSE] message-
based security system is starting to be used. Given that CBOR is
going to be transported using CMS, it makes sense to define CMS
content types for the purpose of denoting that the embedded content
is CBOR. This document defines two new content types: CBOR content
type and CBOR Sequence content type [CBOR-SEQ].
2. CBOR Content Type
[CBOR] defines an encoded CBOR item. This section defines a new
content type for wrapping an encoded CBOR item in a CMS object.
The following object identifier identifies the CBOR content type:
id-ct-cbor OBJECT IDENTIFIER ::= { iso(1) member-body(2) usa(840)
rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 44 }
The CBOR content type is intended to refer to a single object encoded
using the CBOR encoding format [CBOR]. Nothing is stated about the
specific CBOR object that is included. CBOR can always be decoded to
a tree, as the encoding is self descriptive.
The CBOR content type is intended to be encapsulated in the signed
data and auth-enveloped data, but it can be included in any CMS
wrapper. It cannot be predicted whether the compressed CMS
encapsulation will provide compression, because the content may be
binary rather than text.
[RFC7193] defined an optional parameter, "innerContent", to allow for
identification of what the inner content is for an application/cms
media type. This document defines the string "cbor" as a new value
that can be placed in this parameter when a CBOR content type is
used.
3. CBOR Sequence Content Type
[CBOR-SEQ] defines a CBOR Sequence as a concatenation of zero or more
CBOR objects. This section defines a new content type for wrapping a
CBOR Sequence in a CMS object.
The following object identifier identifies the CBOR Sequence content
type:
id-ct-cborSequence OBJECT IDENTIFIER ::= { iso(1) member-body(2)
usa(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1)
45 }
The CBOR Sequence content type is intended to refer to a sequence of
objects encoded using the CBOR encoding format. The objects are
concatenated without any markers delimiting the individual CBOR
objects. Nothing is stated about the specific CBOR objects that are
included. CBOR can always be decoded to a tree, because the encoding
is self descriptive.
The CBOR Sequence content type is intended to be encapsulated in the
signed data and auth-enveloped data, but it can be included in any
CMS wrapper. It cannot be predicted whether the compressed CMS
encapsulation will provide compression, because the content may be
binary rather than text.
[RFC7193] defined an optional parameter, "innerContent", to allow for
identification of what the inner content is for an application/cms
media type. This document defines the string "cborSequence" as a new
value that can be placed in this parameter when a CBOR Sequence
content type is used.
4. ASN.1 Module
CborContentTypes { iso(1) member-body(2) usa(840)
rsadsi(113549) pkcs(1) pkcs9(9) smime(16) modules(0)
id-mod-cbor-2019(71) }
DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
CONTENT-TYPE
FROM CryptographicMessageSyntax-2010
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }
;
id-ct-cbor OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1)
44 }
id-ct-cborSequence OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1)
45 }
-- Content is encoded directly and does not have any ASN.1
-- structure
ct-Cbor CONTENT-TYPE ::= { IDENTIFIED BY id-ct-cbor }
-- Content is encoded directly and does not have any ASN.1
-- structure
ct-CborSequence CONTENT-TYPE ::= {
IDENTIFIED BY id-ct-cborSequence
}
END
5. IANA Considerations
IANA has registered the following in the "SMI Security for S/MIME
Module Identifier (1.2.840.113549.1.9.16.0)" subregistry within the
SMI Numbers registry:
+---------+------------------+------------+
| Decimal | Description | References |
+=========+==================+============+
| 71 | id-mod-cbor-2019 | RFC 8769 |
+---------+------------------+------------+
Table 1
IANA has registered the following in the "SMI Security for S/MIME CMS
Content Type (1.2.840.113549.1.9.16.1)" subregistry within the SMI
Numbers registry:
+---------+--------------------+------------+
| Decimal | Description | References |
+=========+====================+============+
| 44 | id-ct-cbor | RFC 8769 |
+---------+--------------------+------------+
| 45 | id-ct-cborSequence | RFC 8769 |
+---------+--------------------+------------+
Table 2
IANA has registered the following in the "CMS Inner Content Types"
subregistry within the "MIME Media Type Sub-Parameter Registries":
+--------------+----------------------------+-----------+
| Name | Object Identifier | Reference |
+==============+============================+===========+
| cbor | 1.2.840.113549.1.9.16.1.44 | RFC 8769 |
+--------------+----------------------------+-----------+
| cborSequence | 1.2.840.113549.1.9.16.1.45 | RFC 8769 |
+--------------+----------------------------+-----------+
Table 3
6. Security Considerations
This document only provides identification for content types; it does
not introduce any new security issues by itself. The new content
types mean that id-data does not need to be used to identify these
content types; they can therefore reduce confusion.
7. Normative References
[CBOR] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[CBOR-SEQ] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[COSE] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC7193] Turner, S., Housley, R., and J. Schaad, "The application/
cms Media Type", RFC 7193, DOI 10.17487/RFC7193, April
2014, <https://www.rfc-editor.org/info/rfc7193>.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
