[jose] preventing confusion of one kind of JWT for another in JWT BCP

[Brian Campbell <[e-mail]>]

During the first WG meeting last week
<https://datatracker.ietf.org/doc/minutes-99-oauth/> I asked if use of the
JOSE "crit" (Critical) Header Parameter had been considered as a
recommendation for preventing confusion of one kind of JWT for another.
Time was running short in the meeting so there wasn't much discussion and
it was requested that I take the question to the list. And so here on the
list is that.

Section 3.9
<https://tools.ietf.org/html/draft-sheffer-oauth-jwt-bcp-01#section-3.9> of
the JWT BCP draft now recommends explicit typing using the "typ" JWS/JWE
header parameter but does concede that 'the use of explicit typing may not
achieve disambiguation from existing kinds of JWTs, as the validation rules
for existing kinds JWTs often do not use the "typ" header parameter
value.'  And the recommendations for how to use the Type Header Parameter
in JWT <https://tools.ietf.org/html/rfc7519#section-5.1> strongly suggest
that it's not currently being used for any validation.

Alternatively using the JWS/JWE "crit" (Critical) Header Parameter
<https://tools.ietf.org/html/rfc7515#section-4.1.11> to signal the
type/intent/profile/application of a JWT could achieve disambiguation even
in validation of existing kinds of JWTs. The critical header lists other
headers which must be understood and processed by the receiver and that the
JWS/JWE is invalid if those listed aren't understood. So a new type/profile
of JWT that uses the "crit" header would produce JWTs that would be
rejected even by existing applications of JWT validation (that actually
implement "crit" properly anyway).

The JWT BCP could suggest the use of "crit" in conjunction with a
profile/application/type specific header. Or it could provide a bit more of
a framework like defining a registering a new JOSE header "p" (strawman of
p as a very short name for profile) and create a registry for its values. A
JWT header using that approach might look like the following where the
value 1 is registered as some cool new JWT profile/application. The
consumer of such a JWT would have to understand and process the "p" header,
which would mean checking that it had the value expected.

     {
      "alg":"ES256",
      "crit":["p"],
      "p":1
     }

A JOSE compliant JWT validator would reject such a JWT even for an OAuth
access token or OIDC id_token because the "p" header isn't known or
understood but is marked as critical.

To me, that seems like an approach to preventing confusion that has more
teeth than the "typ" header. Which is why I asked about it last week and am
now bringing it to the list.

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