[OAUTH-WG] draft-ietf-oauth-dpop-02: The size of the "jti" is currently mandated to 96 bits minimum

Denis <denis.ietf@free.fr> Wed, 02 December 2020 16:11 UTC

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To: Brian Campbell <bcampbell@pingidentity.com>
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From: Denis <denis.ietf@free.fr>
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Subject: [OAUTH-WG] draft-ietf-oauth-dpop-02: The size of the "jti" is currently mandated to 96 bits minimum
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Hi Brian,

I changed the title of this thread from "Reminder - Interim Meeting to 
discuss DPoP" to "draft-ietf-oauth-dpop-02: The size of the "jti" is 
currently mandated to 96 bits minimum".

Thank you for the link. I read it but I am still not convinced that 
using a minimum of 96 bits is necessary.

Using both the "iat" claim and the "jti" claim, it is very unlikely that 
the same 32 bits jti will be picked up
at the very same "iat" time by two or more clients. Should such a 
condition happen, then another attempt
with a new DPoP proof JWT would very likely succeed for the second client.

In case of a collision, it would also be possible to return a specific 
error code saying something like "duplicate iat/jti pair".
So the client would be informed that it should perform another attempt 
using a new DPoP proof JWT.

Denis

> The conversation at 
> https://github.com/danielfett/draft-dpop/pull/51#discussion_r332377311 
> <https://github.com/danielfett/draft-dpop/pull/51#discussion_r332377311> 
> has a bit more of the rational behind the choice of 96 bit minimum.
>
> On Wed, Dec 2, 2020 at 7:07 AM Denis <denis.ietf@free.fr 
> <mailto:denis.ietf@free.fr>> wrote:
>
>     Hi Daniel,
>
>     All your arguments make sense. I agree.
>
>     A minor point however. The size of the jti" is currently mandated
>     to 96 bits minimum. This is unnecessarily long for a time window
>     of a few minutes.
>     The jti" does not need to be a unique identifier valid for ever.
>     It can simply be an identifier used during the time window which
>     complements the "iat" claim.
>
>     Using both the "iat" claim and a 32 bits pseudo-random number will
>     be quite sufficient.  It is also has the advantage of using less
>     memory and
>     it is easier to flush the entries looking at the 32 first bits only.
>
>     Denis
>
>>     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
>>>>     <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/
>>>>>         <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
>>>>>         <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  <https://www.ietf.org/mailman/listinfo/oauth>
>>>>
>>>>
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>>>>
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>>>
>>>
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>>
>>
>>     -- 
>>     https://danielfett.de  <https://danielfett.de>
>>
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