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INFORMATIONAL

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.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   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


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