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

Daniel Fett <fett@danielfett.de> Tue, 01 December 2020 13:24 UTC

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From: Daniel Fett <fett@danielfett.de>
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Date: Tue, 1 Dec 2020 14:24:01 +0100
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Subject: Re: [OAUTH-WG] Reminder - Interim Meeting to discuss DPoP
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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
>> <mailto: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
>>>
>>>
>>>     _______________________________________________
>>>     OAuth mailing list
>>>     OAuth@ietf.org <mailto:OAuth@ietf.org>
>>>     https://www.ietf.org/mailman/listinfo/oauth
>>
>>
>>     _______________________________________________
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>>     OAuth@ietf.org <mailto:OAuth@ietf.org>
>>     https://www.ietf.org/mailman/listinfo/oauth
>>
>>
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