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> 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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