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

Denis <denis.ietf@free.fr> Mon, 07 December 2020 08:32 UTC

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To: Brian Campbell <bcampbell@pingidentity.com>, Neil Madden <neil.madden@forgerock.com>
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From: Denis <denis.ietf@free.fr>
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Subject: Re: [OAUTH-WG] Reminder - Interim Meeting to discuss DPoP
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Hi Brian,

> The client is not necessarily identified in requests to the RS (it 
> could be via the access token but that's an implementation detail that 
> can't be counted on in spec) so maintaining a per client list isn't 
> viable.
>
> That as well as some other considerations/approaches were talked about 
> in https://github.com/danielfett/draft-dpop/issues/47 
> <https://github.com/danielfett/draft-dpop/issues/47> with what's in 
> the spec now maybe not being perfect but good enough.

The message sent by b---c on September 23, 2019 states:

    I think there are a number of options. The ones that come to mind are:

     1. servers track only the jti value and require that jti be
        globally unique (a UUID or >= ~128 bit random)
     2. servers track the public key (or hash of it) + a jti that is
        incrementing or otherwise unique within that instance of the
        client (like random >= ~32 bits but I'm not sure and it really
        depends on how many requests are being made in the allowed
        timeframe)
     3. servers track the whole dpop jwt (or probably a hash of it) and
        jti is not required at all (or could be very very small). I
        think the iat and jwk and signature in the dpop jwt would
        provide more than sufficient entropy for this to work. Although
        I guess multiple requests to the same URI & same method during
        the same second (iat) when using a deterministic signature
        scheme would need to be differentiated somehow.

    I think 1 is the most straightforward to describe in a document. I'm
    drawn to 3 because it would allow for smaller dpops and move some of
    the complexity burden from the client to the server. 2 kinda does
    also but feels to me like not a great compromise that's maybe harder
    for both parties to implement.

    Thoughts? Preferences? Alternatives?

Method 1 is not "good enough" as you write: it is over-engineering. A 
method for flushing the cache of "whatever data is being stored" has not 
be considered in the discussion.
There are alternatives methods that have not been proposed, nor discussed.

In an earlier email, I proposed a pseudo random number with 2 ^ 32 
values associated with the"iat" time. For more details, see item 9 
(Section 4.2. DPoP Proof JWT Syntax)
from: 
https://mailarchive.ietf.org/arch/browse/oauth/?q=Proposed%20changes%20to%20draft-ietf-oauth-dpop-02

The text is as follows:

            *"jti": JWT identifier that complements the "iat" claim to 
handle replay protection for a given server (REQUIRED).

       The value MUST be assigned such that there is a negligible 
probability that the same value will be assigned to any other DPoP proof 
received by the server within the same second during a short time window 
of validity (i.e. a few minutes only). This MUST be accomplished by 
encoding 32 bits of pseudorandom data. I would add the following 
sentence: In case of a replay detection, an "replay_dpop_proof" error 
code MUST be returned

This will allow both to recover from an error by sending a new DPoP 
proof and to test that replay detection is indeed working.
Note that 64 bits of pseudorandom data would also make it, but 96 bits 
(or more) is oversized.

The following guidance in section 8.1 will then become unnecessary:

    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.

Denis


>
> On Thu, Dec 3, 2020 at 5:09 AM Neil Madden <neil.madden@forgerock.com 
> <mailto:neil.madden@forgerock.com>> wrote:
>
>     I think perhaps an assumption in the DPoP draft (and in the
>     description of “jti” in RFC 7519) is that the server will maintain
>     a single global list of recently used jti values to prevent
>     replay, rather than maintaining a separate list per client. That
>     could perhaps be spelled out more clearly in the draft, as I think
>     the entropy discussions only really make sense in that context. If
>     the RS instead maintains a separate list per client then a simple
>     counter is sufficient.
>
>     — Neil
>
>>     On 2 Dec 2020, at 15:17, Brian Campbell
>>     <bcampbell=40pingidentity.com@dmarc.ietf.org
>>     <mailto:bcampbell=40pingidentity.com@dmarc.ietf.org>> wrote:
>>
>>     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>
>>>>>
>>>>>
>>>>>             _______________________________________________
>>>>>             OAuth mailing list
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>>>>>             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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>>>
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