RFC9596: CBOR Object Signing and Encryption (COSE) "typ" (type) Header Parameter
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Related keywords:
(Explicit Typing)
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.
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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: michael_b_jones@hotmail.com
URI: https://self-issued.info/
Orie Steele
Transmute
Email: orie@transmute.industries