[OAUTH-WG] DPoP and MTLS - friends or foes?

[Dmitry Telegin <[e-mail]>]

As an implementer of one binding mechanism (DPoP) for the AS (Keycloak)
that already features another (MTLS), I'm running into the question whether
we should allow those two to be used simultaneously (which could be of
course extrapolated to other hypothetical mechanisms). By "simultaneously"
I mean binding a single token using both methods given that the material
for both has been provided with the request.

I guess currently mutual exclusivity is implied. Though in theory the "cnf"
section of the AT could contain both "jkt" and "x5t#S256", the mechanisms
are using different values for "token_type" and authentication scheme
("DPoP" for DPoP, "Bearer" for MTLS, though the latter might change to
"MTLS" in the future) and we define no mechanism to combine them (could be
"Bearer+DPoP" or "DPoP+MTLS" for example, which would be valid as per RFCs
7230 and 7235).

I apologize if the question has been asked before; didn't find anything
relevant in the ML. The implementer of MTLS for Keycloak also voted for
mutually exclusive behavior.

- Dmitry
Backbase / Keycloak