Internet-Draft | COSE "typ" (type) Header Parameter | November 2023 |
Jones & Steele | Expires 9 May 2024 | [Page] |
This specification adds the equivalent of the JOSE typ
(type) header parameter to COSE
so that the benefits of explicit typing,
as defined in the JSON Web Token Best Current Practices BCP,
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.¶
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CBOR Object Signing and Encryption (COSE) [RFC9052] defines header parameters
that parallel many of those defined by the JSON Object Signing and Encryption (JOSE)
[RFC7515] [RFC7516] specifications.
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
the JSON Web Token Best Current Practices [RFC8725],
which recommends its use for "explicit typing" --
using typ
values to distinguish between different kinds of objects.¶
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 integer CoAP Content-Formats [IANA.CoAP.ContentFormats] values
and string Media Type [IANA.MediaTypes] values to be used.¶
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.¶
The typ
(type) header parameter
is used by COSE applications to declare the
type of this complete COSE object.
This is intended for use by the application when
more than one kind of 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 objects that might be present.
It will typically not be used by applications when
the kind of object is already known.
This parameter is ignored by COSE implementations;
any processing of this parameter is performed by the COSE application.
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 integer CoAP Content-Formats [IANA.CoAP.ContentFormats] value or a string Content Type value. Content Type values have a Media Type name [IANA.MediaTypes] and MAY include Media Type parameters.¶
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.¶
The case for explicit typing of COSE objects is equivalent to the case made for explicit typing in JSON Web Token Best Current Practices [RFC8725]; explicit typing can prevent confusion between different kinds of 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.¶
This section registers the following value in the IANA "COSE Header Parameters" registry [IANA.COSE.HeaderParameters].¶
Name: typ (type)¶
Label: TBD (requested assignment 14)¶
Value Type: uint / tstr¶
Value Registry: [IANA.CoAP.ContentFormats] or [IANA.MediaTypes]¶
Description: Type of the complete COSE object¶
[[ to be removed by the RFC Editor before publication as an RFC ]]¶
-01¶
Added language about media type parameters.¶
-00¶
Initial working group version based on draft-jones-cose-typ-header-parameter-01.¶
We would like to thank Carsten Bormann for his valuable contributions to this specification.¶