Re: [OAUTH-WG] JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens

[Benjamin Kaduk <[e-mail]>]

Hi Denis,

On Tue, Jun 02, 2020 at 10:20:36AM +0200, Denis wrote:
> Hi Benjamin,
> 
> Responses are between the lines.
> 
> > On Fri, May 22, 2020 at 11:37:28AM +0200, Denis wrote:
> >> Hi Benjamin,
> >>> On Thu, May 14, 2020 at 04:29:43PM +0200, Denis wrote:
> >>>> Since then, I questioned myself how a client would be able to 
> >>>> request an access token that would be *strictly compliant with this 
> >>>> Profile*.
> >>> I don't understand why this is an interesting question to ask. The 
> >>> access token and interpretation thereof is (AIUI) generally seen as 
> >>> an internal matter between AS and RS, with the client having no need 
> >>> to care about the specifics.
> >> This document is*_a_* Profile for OAuth 2.0 Access Tokens. It is not 
> >> _*the*_ Profile for *_all_ *OAuth 2.0 Access Tokens.
> > Sure. But (in my understanding), in common usage, the contents and 
> > interpretation of the access token is set by common agreement between 
> > AS and RS, with the client serving only as a "dumb" channel for 
> > transporting the token. That is, we're providing a token format that 
> > an RS and AS can agree to use, most likely for all tokens issued by 
> > the AS for that RS. I don't know of any existing mechanisms, or desire 
> > for such mechanisms by deployments, for using a different token format 
> > for different tokens issued by a given AS for a given RS.
> 
> Since this document is *_a_* Profile for OAuth 2.0 Access Tokens, it 
> means that potentially other Profiles could be defined in the future.
> In the request, there is no parameter for a client to indicate that it 
> wants a JWT conformant to _this_ profile and no parameter either
> in the response to indicate to the client that it got a JWT that is 
> conformant to _this_ profile.

I don't think my point came through clearly.  Right now, the AS and RS have
to negotiate by some out-of-band means what token format and details to use
for tokens issued by AS with RS in audience, and there is not a
"well-known" description of how to use JWT to do so.  The goal of this
document is to simplify the out-of-band negotiation between AS and RS, so
they can just say "use RFC NNNN" instead of having to specify a lot of
details individually.  Nowhere does the client come into the current
negotiation of what token format to use, and the act of specifying a
profile of JWT for this usage does not create a requirement for the client
to be included in the negotiation.

Now, whether or not to include the client in this negotiation (and whether
or not to make the choice of token format finer-grained) may be a topic
worth discussion in its own right, but that discussion is untethered from
specifying a profile of JWT to simplify AS/RS negotiations.

> The processing mandated in the document of a request made by a client to 
> an AS only applies for a request conformant to this profile
> which may or may not include a scope parameter and which may or may not 
> include a "resource" parameter (and, if it does not, shall
> include an "aud" claim). Currently, it is not possible to make a 
> difference between a JWT request or response conformant to this profile
> and other JWT requests or responses.

I don't see where this document places constraints on a "conformant
request" made by a client; could you point that out to me?

(FWIW, the section heading for section 3 seems needlessly confusing, as the
contents of the section do not seem to say anything about specifically
requesting a JWT token.  I can see how this heading might cause some
confusion.)

> > Attempting to have the client provide input that would affect such a 
> > mechanism seems like complexity with no market demand; a "solution in 
> > search of a problem" as it were. Is there some pent-up demand among 
> > OAuth deployments that I'm not aware of? I freely admit to not being 
> > very on top of the broad spectrum of what's deployed...
> >> 1) A client may wish to obtain an Access Token that corresponds to 
> >> this Profile because, for example, this document mandates the "sub" 
> >> claim to be present". Hence, the content of the Access Token is not 
> >> totally "/an internal matter between AS and RS/". However, I have not 
> >> understood how a client could request an Access Token that 
> >> corresponds to *_this_***Profile, since there is no mandatory 
> >> parameter in the request (both the "scope" parameter and the 
> >> "resource" parameter are optional). In the future, we could define 
> >> _*another*_**Profile. Hence, there is the same question:  How could a 
> >> client request an Access Token that corresponds to *_that 
> >> other_***Profile ? 2) When getting a JWT,  how can the client make 
> >> sure that the access token it got is compliant with this Profile ? 
> >> RFC 8725 states in section 3.11 : 3.11. Use Explicit Typing 
> >> Sometimes, one kind of JWT can be confused for another. If a 
> >> particular kind of JWT is subject to such confusion, that JWT can 
> >> include an explicit JWT type value, and the validation rules can 
> >> specify checking the type (...). Explicit JWT typing is accomplished 
> >> by using the "typ" Header Parameter. Wouldn't be wise to include an 
> >> explicit JWT type value for JWTs conformant to this Profile ?
> > In the model where the client is a "dumb" communications channel, this 
> > question does not seem interesting. But the related question of "how 
> > can the RS make sure that the access token it got was generated 
> > according to this profile?" does seem interesting, and seems like it 
> > would benefit from the same proposed solution.
> 
> An explicit JWT type value added both in the JWT request and in the JWT 
> response would solve this problem.

An explicit JWT type is a solution to the problme I'm interested in, yes.
Though I don't know if that's what you mean by "this problem".

> >> This relates to an email posted by Dominick Baier under the topic 
> >> "JAR: JWT typ" on May 19 : This has been brought up before - but no 
> >> response. Either I can’t find it - or it is missing. But is the 
> >> setting the JWT typ explicitly mentioned somewhere?
> > It is fairly likely that I will remember to ask about explicit "typ" 
> > usage if I'm still on the IESG when this document gets there: I think 
> > I've been making a habit of doing so.
> 
> Once again, an explicit "typ" sould be considered for both the JWT 
> request and the JWT response. This implies that the client "MUST" be 
> able to inspect the content of the access token.

As above, I do not see how the client currently has input into the token
type/format agreed upon by the AS/RS out-of-band, so I dispute both the
premise and the supposed conclusion.

-Ben