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

[Brian Campbell <[e-mail]>]

While there are certainly more than a few different ways of approaching it,
I am still not convinced of any significant advantage to tracking iat + a
slightly smaller jti value vs. what is currently in the draft. And as we
are trying to reflect WG consensus here rather than one person's opinion, I
do not plan to engage in discussing this item any further unless others
from the WG indicate their interest in making a change.

On Mon, Dec 7, 2020 at 1:31 AM Denis <[e-mail]> wrote:

> 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 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 <[e-mail]>
> 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=[e-mail]> wrote:
>>
>> The conversation at
>> 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 <[e-mail]> 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.
>>> 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 <[e-mail]> 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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