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

Dick Hardt <> Tue, 15 September 2020 22:51 UTC

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From: Dick Hardt <>
Date: Tue, 15 Sep 2020 15:51:06 -0700
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To: Brian Campbell <>
Cc: Neil Madden <>, oauth <>
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Subject: Re: [OAUTH-WG] Omit "jwk" (or use "kid" instead) in DPoP Proof?
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That was a lot of words Brian. What is the "tl;dr:"? (I read it, but I'm
not sure where you landed given your last sentence)

On Tue, Sep 15, 2020 at 3:16 PM Brian Campbell <bcampbell=> wrote:

> Sorry for the slow reply to this one, Neil. It's gotten me thinking a bit
> and it took some time to gather my thoughts and attempt to articulate them.
> It is true that, as defined in the current -01 draft, there is the
> additional step for the RS to check that the proof key matches the hash in
> the AT. But I think that equating it to "exactly" that kind of
> CVE-2018-0114 mistake or Moxie’s Cryptographic Doom Principle is a
> significant mischaracterization. The DPoP proof message and its signature
> are used to establish that the sender holds the corresponding private key.
> Then binding tokens to that key or checking such bindings can be done.
> That's quite different from checking the signature/integrity of a message
> against a trusted key before relying on the content of the message, which
> is what CVE-2018-0114 and erroneously trusting the key is about. I'm
> admittedly a little touchy about this because I tried to speak up about the
> potential problems with blindly trusting the jwk header back in the days of
> developing the JOSE specs. And have even joked more recently that the DPoP
> proof finally uncovered a legitimate use for it. So with all that said, I
> don't think it rises to the level of doom. And the step of checking the
> binding of the token to the proof key is pretty fundamental. It's
> conceptually the same as how the AT binding is done with MTLS / RFC 8705
> and the expired/defunct draft-ietf-oauth-token-binding. That doesn't make
> it right per se but there's some precedent for the general approach.
> Following that conceptual precedent was part of the motivation for the
> current approach - not so much as simplicity for the client but simplicity
> in the form of consistency in the protocol definition itself. But maybe
> that motivation was too much so as the hash based cnf binding is kinda
> necessary in those that are doing the key proof at or closer to the
> transport layer but DPoP has all the stuff at the same layer so could do it
> differently. Putting the jwk in the cnf of the AT would eliminate the
> possibility of resource servers forgetting to check the binding. Of course
> there are still mistakes that could be made but removing even the
> possibility of that particular one is compelling.
> The efficiency gains you mention are dependent on circumstance and not
> always applicable, as you've kinda alluded to. The many variations of
> deployment options (e.g., reference vs self-contained tokens, introspection
> vs. local validation and either with various caching strategies, location
> of entities and communication path between them) make quantifying it rather
> difficult and circumstantially dependent. However, moving the jwk from the
> DPoP proof to the AT (by value or reference) would reduce the overall
> amount of data that is conveyed across the two items from a jwk and its
> hash to just the jwk. That's not a huge gain but it is more than nothing.
> It would also eliminate the need to codify a hash function for the jwk
> thumbprint, which admittedly would be rather nice.
> I guess that's a lot of words to try and kinda defend the current approach
> while also admitting that I'm coming around to seeing the light in your
> suggestion.
> On Wed, Sep 9, 2020 at 12:43 AM Neil Madden <>
> wrote:
>> I disagree with this rationale, I’m afraid. I don’t think the simplicity
>> gain for the client is really very great and I think we will regret this
>> decision.
>> Most importantly, including the JWK directly in the proof token increases
>> the likelihood that somebody will just validate the signature using that
>> key and fail to check that it matches the hash in the AT. We’ve already
>> seen at least one vendor make exactly this kind of mistake in the past [1].
>> Letting the proof tell you the key to use to verify the proof is another
>> example of Moxie’s Cryptographic Doom Principle [2] - trusting the message
>> before you’ve authenticated it.
>> Moving the jwk from the DPoP proof to the AT/introspection response
>> reduces the chances of these kind of mistakes.
>> As a bonus it also has efficiency gains because the size of the DPoP
>> proof is reduced - potentially by a lot for RSA keys. Although the JWK
>> still has to be communicated to the RS, this can be more efficient because:
>> - For token introspection, in many deployments the introspection request
>> will be within a datacenter and so over a high-bandwidth low-latency
>> network. This is usually not the case for the DPoP proof, which may be over
>> a poor mobile connection.
>> - For embedded directly in a JWT access token, the JWK will be in the
>> claims rather than the header and so can at least be compressed. (Albeit
>> not by much, because public keys don’t compress well, but the rest of the
>> AT will and that can compensate a little for the extra bulk).
>> [1]:
>> [2]:
>> — Neil
>> On 8 Sep 2020, at 23:46, Brian Campbell <bcampbell=
>>> 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 <> 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]:
>>> [*2]:
>>> Best regards,
>>> Toshio Ito
>>> -------------
>>> Toshio Ito
>>> Research and Development Center
>>> Toshiba Corporation
>>> _______________________________________________
>>> OAuth mailing list
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