Re: [OAUTH-WG] Reminder - Interim Meeting to discuss DPoP

Brian Campbell <bcampbell@pingidentity.com> Tue, 01 December 2020 23:13 UTC

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From: Brian Campbell <bcampbell@pingidentity.com>
Date: Tue, 1 Dec 2020 16:12:32 -0700
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To: Dick Hardt <dick.hardt@gmail.com>
Cc: Daniel Fett <fett@danielfett.de>, oauth <oauth@ietf.org>
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Subject: Re: [OAUTH-WG] Reminder - Interim Meeting to discuss DPoP
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Thanks Dick.

On Tue, Dec 1, 2020 at 1:43 PM Dick Hardt <dick.hardt@gmail.com> wrote:

> I have 2 suggestions for the draft that I beleive address the issues Denis
> is bringing up:
>
> 1) call out that a DPoP proof can only be used once, and a new DPoP proof
> is needed for every API call. Apologies if that is in the text -- but I
> could not find it doing a skim over the document.
>

I think it is very strongly implied but not called out explicitly as such.
We can add that.



> 2) Provide additional guidance on what "a relatively brief period" is
> including what factors an implementation should consider when determining
> the length of time. I'm assuming it is a small number of seconds to
> accommodate for the delay between the client creating the DPoP proof and
> the server receiving it.
>

On the order of seconds is my assumption too. But yes we can add some more
guidance here with a bit more specificity.



> ᐧ
>
> On Tue, Dec 1, 2020 at 5:24 AM Daniel Fett <fett@danielfett.de> wrote:
>
>> So what you are proposing is that the time window in which an RS accepts
>> the DPoP proof is defined by the expiration time of the access token?
>>
>> DPoP proofs are intended to be generally be short-lived and fresh for
>> each request in order to provide some level of replay protection. There is
>> no point in making the time window as long as the (typically longer) time
>> window in which an AT would be accepted. A DPoP proof that is valid for 12
>> hours would not provide much replay protection.
>>
>> The time window is left unspecified because it is only meant to account
>> for clock differences and network latency. Its precise value can depend on
>> deployment considerations. It is not intended to give the client an option
>> to re-use proofs, which is prevented together with the jti.
>>
>> Also this would introduce new, unwanted and potentially surprising
>> dependencies between token lifetimes and the DPoP usage.
>>
>> And finally, as discussed before, not all access tokens are JWTs and we
>> are not going to mandate JWT access tokens in this spec.
>>
>> -Daniel
>>
>>
>> Am 01.12.20 um 09:54 schrieb Denis:
>>
>> Hi  Brian,
>>
>> Hi Denis,
>>
>> The choice to use "iat" vs. "exp" was made in the summer of last year.
>> You can see some of the discussion from then in
>> https://github.com/danielfett/draft-dpop/issues/38.
>> I believe it pretty well has consensus at this point and thus unlikely to
>> be changed.
>>
>> I fear that you misread my email or read it too fast. My point had
>> nothing to do whether using *either *of "iat" *o**r* "exp" in the DPoP
>> proof JWT sent by the client.
>>
>> The first sentence of my email was: "One comment on slide 5 about the *time
>> window*". So the topic was all about how the RS SHALL handle the "jti"
>> claim included
>> in the DPoP proof JWT when using a time window.
>>
>> While I do believe there are reasonable arguments that can be made on
>> both sides of using either of "iat" or "exp", it's difficult (and honestly
>> time consuming and very frustrating) to try and have such discussions or
>> even respond in a coherent way when fundamental aspects of the draft are
>> misrepresented or misunderstood. For example, the DPoP proof JWT is created
>> by the client not the AS so the advantages you put forward are
>> nonsensical in the context of the actual workings of the draft.
>>
>> Section 8.1 addresses the topic of the *time window*, but this topic
>> should not *only *be addressed in the "Security Considerations" section
>> but in the main body of the document, since some checks MUST be done by
>> the RS. "Security Considerations"are intended to provide
>> explanations but are not intended to be normative.
>>
>> Section 8.1 states:
>>
>>    " If an adversary is able to get hold of a DPoP proof JWT, the
>> adversary could replay that token at the same endpoint (the HTTP
>>    endpoint and method are enforced via the respective claims in the
>> JWTs).  To prevent this, servers MUST only accept DPoP proofs
>>    for a limited time window after their "iat" time, preferably only for
>> a relatively brief period.
>>
>>    Servers SHOULD store, in the context of the request URI, the "jti"
>> value of each DPoP proof for the time window in which the respective
>>    DPoP proof JWT would be accepted and decline HTTP requests to the same
>> URI for which the "jti" value has been seen before.  In order
>>    to guard against memory exhaustion attacks a server SHOULD reject DPoP
>> proof JWTs with unnecessarily large "jti" values or store only
>>    a hash thereof.
>>
>>    (...) ".
>>
>> The previous text makes the assumption that RSs MUST only accept DPoP
>> proofs for a relatively brief period after their "iat" time included
>> in the DPoP proof JWT. This assumption is rather restrictive. A client
>> might get an access token and associate it with DPoP proof JWT that
>> could be used during, e.g., 12 hours. A DPoP proof JWT/ access token JWT
>> pair could thus be used by a client during, e.g., one day for
>> several sessions with a RS.
>>
>> The *time window* is currently left at the discretion of each RS and is
>> supposed to be short (without stating explicitly what "short" may mean)..
>>
>> It would be possible to mandate in the JWT the inclusion of the exp
>> (Expiration Time) Claim. (I am *not* advocating the inclusion of the
>> "exp"
>> claim in the DPoP proof JWT).
>>
>> In this way, for a RS, the *time window *would be defined using the
>> "iat" claim defined in the DPoP proof JWT and the "exp" claim defined in
>> the JWT.
>>
>> Such a description should not be done in section 8, but in a section
>> earlier in the main body of the document.
>>
>> This would have the following advantages:
>>
>>    - The RS would be able to better manage the "jti" claim values,
>>    because it would be able to discard "jti" claim values as soon as they are
>>    outside the time window as defined above.
>>
>>
>>    - The client would know whether a DPoP proof JWT/ access token JWT
>>    pair is still usable, in particular using the "expires_in" status code
>>    returned in case of a successful response from the AS and is thus
>>    unlikely to get a rejection of both of them because of an unknown time
>>    window used by a RS.
>>
>> Denis
>>
>>
>> On Mon, Nov 30, 2020 at 8:45 AM Denis <denis.ietf@free.fr> wrote:
>>
>>> One comment on slide 5 about the *time window*.
>>>
>>> At the bottom, on the left, it is written: "Only valid for a limited *time
>>> window* relative to creation time".
>>>
>>> While the creation time is defined by "iat", the *time window* is
>>> currently left at the discretion of each RS.
>>>
>>> It would be preferable to mandate the inclusion in the JWT of the exp
>>> (Expiration Time) Claim.
>>> In this way, the *time window *would be defined by the AS using both
>>> the "iat" and the "exp" claims.
>>>
>>> This would have the following advantages:
>>>
>>>    - The client will know whether a token is still usable and is
>>>    unlikely to get a rejection of the token
>>>    because of an unknown time window defined by a RS.
>>>
>>>
>>>    - The RS is able to manage better the "jti" claim values, because it
>>>    will be able to discard "jti" claim values
>>>    as soon as they are outside the time window defined by the AS in a
>>>    JWT.
>>>
>>> Denis
>>>
>>> All,
>>>
>>> This is a reminder that we have an Interim meeting this Monday, Nov 30th
>>> @ 12:00pm ET, to discuss the latest with the *DPoP *document:
>>> https://datatracker.ietf.org/doc/draft-ietf-oauth-dpop/
>>>
>>> You can find the details of the meeting and the slides here:
>>> https://datatracker.ietf.org/meeting/interim-2020-oauth-16/session/oauth
>>>
>>> Regards,
>>>  Rifaat & Hannes
>>>
>>>
>>> _______________________________________________
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>>>
>>>
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>>>
>>
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