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 <[email protected]> Sent: Tuesday, September 8, 2020 11:23 PM To: ito toshio(伊藤 俊夫 ○ＲＤＣ□ＩＴ研○ＣＮＬ) <[email protected]> Cc: oauth <[email protected]> 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 <[email protected]<mailto:[email protected]>> 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 _______________________________________________ OAuth mailing list [email protected]<mailto:[email protected]> https://www.ietf.org/mailman/listinfo/oauth
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