[OAUTH-WG] Explicit typing of SD-JWTs (was SD-JWT architecture feedback)

[Michael Jones <[e-mail]>]

Actually, the JWT BCP (which we were both authors of) does not recommend using a single media type.  Rather, it recommends using a specific media type suffix in the “typ” values<https://www.rfc-editor.org/rfc/rfc8725.html#name-use-explicit-typing>:
When explicit typing is employed for a JWT, it is RECOMMENDED that a media type name of the format "application/example+jwt" be used, where "example" is replaced by the identifier for the specific kind of JWT.

SD-JWT is doing the same thing, recommending the use of the media type suffix “+sd-jwt”.

This enables more fine-grained explicit typing.  For instance, when doing explicit typing for an SD-JWT in the Example use case, the “typ” value would be “example+sd-jwt”.  This can then be distinguished from an SD-JWT for the Other use case, which would use the “typ” value “other+sd-jwt” – meeting the goal of explicit typing.

                                                                -- Mike

From: Dick Hardt <[e-mail]>
Sent: Saturday, September 21, 2024 9:16 AM
To: Daniel Fett <[e-mail]>
Cc: [e-mail]; [e-mail]
Subject: [OAUTH-WG] Re: SD-JWT architecture feedback

…

Explicit Typing
Why leave the typing in the header to be determined by the application (10.11), and not just be 'sd-jwt' and be REQUIRED?

We had extensive discussions around typing, please refer to the following issues:

- https://github.com/oauth-wg/oauth-selective-disclosure-jwt/issues/267

- https://github.com/oauth-wg/oauth-selective-disclosure-jwt/issues/327

- https://github.com/oauth-wg/oauth-selective-disclosure-jwt/issues/345

Those issues don't really address the point.

Per RFC 8725: JSON Web Token Best Current Practices (rfc-editor.org)<https://www.rfc-editor.org/rfc/rfc8725.html#name-use-explicit-typing> -- the best practice would be to have a single type that would allow a library to know it is an SD-JWT. If additional context is needed, perhaps that should be a different header property?