CBOR Object Signing and Encryption (COSE) "typ" (type) Header Parameter
RFC 9596



Internet Engineering Task Force (IETF)                        M.B. Jones
Request for Comments: 9596                        Self-Issued Consulting
Category: Standards Track                                      O. Steele
ISSN: 2070-1721                                                Transmute
                                                               June 2024

CBOR Object Signing and Encryption (COSE) "typ" (type) Header Parameter

Abstract

   This specification adds the equivalent of the JSON Object Signing and
   Encryption (JOSE) "typ" (type) header parameter to CBOR Object
   Signing and Encryption (COSE).  This enables the benefits of explicit
   typing (as defined in RFC 8725, "JSON Web Token Best Current
   Practices") to be brought to COSE objects.  The syntax of the COSE
   type header parameter value is the same as the existing COSE content
   type header parameter.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc9596.

Copyright Notice

   Copyright (c) 2024 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Notation and Conventions
   2.  COSE "typ" (type) Header Parameter
   3.  Security Considerations
   4.  IANA Considerations
     4.1.  COSE Header Parameter Registrations
   5.  References
     5.1.  Normative References
     5.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   CBOR Object Signing and Encryption (COSE) [RFC9052] defines header
   parameters that parallel many of those defined by the JSON Object
   Signing and Encryption (JOSE) specifications [RFC7515] [RFC7516].
   However, one way in which COSE does not provide equivalent
   functionality to JOSE is that it does not define an equivalent of the
   "typ" (type) header parameter, which is used for declaring the type
   of the entire JOSE data structure.  The security benefits of having
   "typ" (type) are described in Section 3.11 of [RFC8725], which
   recommends its use for "explicit typing" -- using "typ" values to
   distinguish between different kinds of JSON Web Tokens (JWTs)
   [RFC7519].

   This specification adds the equivalent of the JOSE "typ" (type)
   header parameter to COSE so that the benefits of explicit typing can
   be brought to COSE objects.  The syntax of the COSE type header
   parameter value is the same as the existing COSE content type header
   parameter, allowing both unsigned integers as registered in the "CoAP
   Content-Formats" registry [CoAP.ContentFormats] and string media type
   values [MediaTypes] to be used.

   The term "COSE object" is used as defined in [RFC9052].  An example
   of a COSE object is a COSE_Sign1 structure, as described in
   Section 4.2 of [RFC9052].

1.1.  Requirements Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  COSE "typ" (type) Header Parameter

   The "typ" (type) header parameter is used by COSE applications to
   declare the type of this complete COSE object, as compared to the
   content type header parameter, which declares the type of the COSE
   object payload.  This is intended for use by the application when
   more than one kind of COSE object could be present in an application
   data structure that can contain a COSE object; the application can
   use this value to disambiguate among the different kinds of COSE
   objects that might be present.  It will typically not be used by
   applications when the kind of COSE object is already known.  Use of
   this header parameter is OPTIONAL.

   The syntax of this header parameter value is the same as the content
   type header parameter defined in Section 3.1 of [RFC9052]; it is
   either an unsigned integer as registered in the "CoAP Content-
   Formats" registry [CoAP.ContentFormats] or a string content type
   value.  Content type values have a media type name [MediaTypes] and
   MAY include media type parameters.

   The "typ" (type) header parameter is ignored by COSE implementations
   (libraries implementing [RFC9052] and this specification), other than
   being passed through to applications using those implementations.
   Any processing of this parameter is performed by the COSE application
   using application-specific processing rules.  For instance, an
   application might verify that the "typ" value is a particular
   application-chosen media type and reject the data structure if it is
   not.

   The "typ" parameter MUST NOT be present in unprotected headers.

   The "typ" parameter does not describe the content of unprotected
   headers.  Changes to unprotected headers do not change the type of
   the COSE object.

3.  Security Considerations

   The case for explicit typing of COSE objects is equivalent to the
   case made for explicit typing in Section 3.11 of [RFC8725]: Explicit
   typing can prevent confusion between different kinds of COSE objects.

   COSE applications employing explicit typing should reject COSE
   objects with a type header parameter value different than values that
   they expect in that application context.  They should also reject
   COSE objects without a type header parameter when one is expected.

4.  IANA Considerations

4.1.  COSE Header Parameter Registrations

   IANA has registered the following value in the IANA "COSE Header
   Parameters" registry [COSE.HeaderParameters].

    +======+=====+======+=======================+===========+=========+
    |Name  |Label|Value | Value Registry        |Description|Reference|
    |      |     |Type  |                       |           |         |
    +======+=====+======+=======================+===========+=========+
    |typ   |16   |uint /| [CoAP.ContentFormats] |Content    |Section 2|
    |(type)|     |tstr  | or [MediaTypes]       |type of the|of RFC   |
    |      |     |      | registry              |complete   |9596     |
    |      |     |      |                       |COSE object|         |
    +------+-----+------+-----------------------+-----------+---------+

                                  Table 1

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

5.2.  Informative References

   [CoAP.ContentFormats]
              IANA, "CoAP Content-Formats",
              <https://www.iana.org/assignments/core-parameters>.

   [COSE.HeaderParameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose>.

   [MediaTypes]
              IANA, "Media Types",
              <https://www.iana.org/assignments/media-types>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <https://www.rfc-editor.org/info/rfc7516>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

Acknowledgements

   We would like to thank Henk Birkholz, Carsten Bormann, Susan Hares,
   Dan Harkins, Murray Kucherawy, Marco Tiloca, Gunter Van de Velde,
   Éric Vyncke, and Dale Worley for their valuable contributions to this
   specification.

Authors' Addresses

   Michael B. Jones
   Self-Issued Consulting
   Email: [email protected]
   URI:   https://self-issued.info/

   Orie Steele
   Transmute
   Email: [email protected]