[OAUTH-WG] some implementation feedback with the PKI method of OAuth MTLS client authentication

[Brian Campbell <[e-mail]>]

Some feedback was received recently off-list that pointed out difficulties
with implementation around the "tls_client_auth_root_dn" constraint in the
PKI method of OAuth MTLS client authentication from
draft-ietf-oauth-mtls-03. Basically the feedback was that popular web
servers such as Nginx and Apache don't expose sufficient information
(easily or in some cases at all) from the client certificate chain to the
application to enable proper checking of "tls_client_auth_root_dn".

Following from that and some additional reasoning below, I'm proposing that
"tls_client_auth_root_dn" be dropped from the draft-ietf-oauth-mtls draft.

The original idea behind the "tls_client_auth_root_dn" client metadata
parameter came from an MTLS client authentication feature we added to our
AS product years ago. The feature provided a way to allow the AS to more
tightly constrain trust in a particular context (from an otherwise global
list of trust anchors). It was fine as a specific product feature but
should have stayed at that. When I added metadata to the OAuth MTLS draft,
I added the "tls_client_auth_root_dn" parameter with that AS product
feature in mind as something an AS *might* want to be able to do (without
thinking thorough it all sufficiently). But having it as a client metadata
parameter has wider implications including shifting trust control to the
client and requiring ASs to support it. So, after thinking about it some
more and also seeing the potential implementation difficulties, I don't
believe it's appropriate to have in the specification. The AS should be at
liberty to do chain validation with the PKI method however is most
appropriate for it. And not be required to support one specific way of
doing things implied by "tls_client_auth_root_dn" (which is even infeasible
to implement in some environments).

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