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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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 Provisions Relating to IETF Documents
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 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

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