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.i...@free.fr
<mailto:denis.i...@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
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