Re: [OAUTH-WG] Omit "jwk" (or use "kid" instead) in DPoP Proof?

Hi Takahiro,

Thanks for pointing out. I forgot that OAuth2 client registration is basically
for a client application, not for a client instance. As you said, if we omit
"jwk" altogether and rely on client authentication to identify the signing key,
all instances of the client have to use the same key. That would be bad.

From: Takahiko Kawasaki <taka@authlete.com>
Sent: Tuesday, September 8, 2020 11:23 PM
To: ito toshio(伊藤 俊夫 ○ＲＤＣ□ＩＴ研○ＣＮＬ) <toshio9.ito@toshiba.co.jp>
Cc: oauth <oauth@ietf.org>
Subject: Re: [OAUTH-WG] Omit "jwk" (or use "kid" instead) in DPoP Proof?

To enable each "instance" of a client application to use a key pair which is dedicated to the instance, the public key needs to be included in the DPoP proof. On the other hand, in the scenario you described, all instances of the client application have to share one key pair. If client application instances don't have to share one key pair, it's better.

Illustrated DPoP (OAuth Access Token Security Enhancement)
https://medium.com/@darutk/illustrated-dpop-oauth-access-token-security-enhancement-801680d761ff

Best Regards,
Takahiko Kawasaki

On Tue, Sep 8, 2020 at 6:29 PM <toshio9.ito@toshiba.co.jp<mailto:toshio9.ito@toshiba.co.jp>> wrote:
Hi all,

In section 4.1 of draft-ietf-oauth-dpop-01, the "jwk" header parameter is
REQUIRED. However, there are some cases where "jwk" is not necessary in theory.

For example, consider a case where the client is registered with the
Authorization Server, and its one and only public key is also registered with
the AS. In that case, when the AS receives a request on Token endpoint, it can
just use the public key registered for the client to verify the DPoP Proof.
There is no need to send the public key in DPoP Proof.

The same goes for requests to the Resource Server, if the AS and RS share the
storage for clients' public keys. Things are a little difficult if the AS and RS
are separate. Probably the Access Token or its introspection result have to
include the public key (instead of its thumbprint as described in section 7).

If the client registers multiple keys with the AS, it needs to specify which key
it uses to sign the DPoP Proof. However, there is still no absolute need to send
the whole key in DPoP Proof. Instead, the client could use "kid" header
parameter to specify the key.

Daniel Fett once mentioned the above case in the GitHub issue #26 [*1], but I'm
not sure what happened to the discussion. There was also a comment on the latest
draft about the "jwk" header parameter [*2]. I agree with using the same DPoP
Proof structure for requests to AS and RS, but I think there are some cases
where we can omit "jwk" in BOTH requests. Making "jwk" OPTIONAL would allow
those cases to reduce some messaging overhead.

I'd like to hear your opinions about it.

[*1]: https://github.com/danielfett/draft-dpop/issues/26#issuecomment-480701746
[*2]: https://mailarchive.ietf.org/arch/msg/oauth/smwsONA6c4H2UICcZMzb8Yv2QRc/

Best regards,
Toshio Ito

-------------
Toshio Ito
Research and Development Center
Toshiba Corporation

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