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

Filip Skokan <panva.ip@gmail.com> Wed, 09 September 2020 06:53 UTC

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From: Filip Skokan <panva.ip@gmail.com>
Date: Wed, 9 Sep 2020 08:53:03 +0200
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To: Brian Campbell <bcampbell=40pingidentity.com@dmarc.ietf.org>
Cc: toshio9.ito@toshiba.co.jp, oauth <oauth@ietf.org>
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Subject: Re: [OAUTH-WG] Omit "jwk" (or use "kid" instead) in DPoP Proof?
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+1 for keeping it simple. The design itself promotes unique-per-instance
keys, not pre-registration.

S pozdravem,
*Filip Skokan*


On Wed, 9 Sep 2020 at 00:46, Brian Campbell <bcampbell=
40pingidentity.com@dmarc.ietf.org> wrote:

> Indeed there are cases, as you point out, where the key might be knowable
> to the server via some other means, which makes the "jwk" header in the
> DPoP proof not strictly necessary. And while omitting the key in such cases
> would reduce the size of some messages (the DPoP proof anyway), such
> optionality would add complexity to implementations and deployments (and
> that kind of complexity can and often does degrade interoperability and
> even security). How, for example, would a client know if the access token
> includes the public key and thus whether or not to include the key with the
> proof? Sure the access token could always include the key (rather than
> thumbprint) but then there's the question of how to get the key to the AS.
> As well as the stated desire to utilize the same DPoP Proof structure for
> requests to both the AS and RS. There will be some clients that have public
> key(s) registered and some that won't (maybe a lot that won't as a driving
> use case for many for this is key binding access and refresh tokens for
> public clients). The protocol defined by the draft needs to account for
> both.
>
> Ultimately there are a number of different ways the necessary data could
> flow through the various protocol elements. And there are some tradeoffs
> with different approaches and/or trying to accommodate variations under one
> approach. The approach the draft has taken thus far is to prioritize
> consistency and simplicity as much as is possible. And that ethos has led
> to how it's currently defined, which is to always include the key in the
> proof and bind to a hash of the key in the access token.
>
>
> On Tue, Sep 8, 2020 at 3:29 AM <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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>> https://www.ietf.org/mailman/listinfo/oauth
>>
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