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SEC cose Explicit Typing 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.

Introduction CBOR Object Signing and Encryption (COSE) defines header parameters that parallel many of those defined by the JSON Object Signing and Encryption (JOSE) 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 , which recommends its use for "explicit typing" -- using "typ" values to distinguish between different kinds of JSON Web Tokens (JWTs) . 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 and string media type values to be used. The term "COSE object" is used as defined in . An example of a COSE object is a COSE_Sign1 structure, as described in .

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 when, and only when, they appear in all capitals, as shown here.

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 ; it is either an unsigned integer as registered in the "CoAP Content-Formats" registry or a string content type value. Content type values have a media type name and MAY include media type parameters. The "typ" (type) header parameter is ignored by COSE implementations (libraries implementing 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.

Security Considerations The case for explicit typing of COSE objects is equivalent to the case made for explicit typing in : 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.

IANA Considerations

COSE Header Parameter Registrations IANA has registered the following value in the IANA "COSE Header Parameters" registry .

Name	Label	Value Type	Value Registry	Description	Reference
typ (type)	16	uint / tstr	or registry	Content type of the complete COSE object	of RFC 9596

References Normative References Informative References IANA IANA IANA

Acknowledgements We would like to thank , , , , , , , , and for their valuable contributions to this specification.