Re: [Txauth] Use Case: Directed Tokens

[Denis <[e-mail]>]

Justin, I fear we are still far away from an agreement about what this 
BoF should do.

Tom Jones is saying " I am getting tired of the whack-a-mole approach ..."

I don't agree with you when you write: "I think it’s going to be 
overwhelmingly common that a given RS is going to trust exactly one AS
that it knows about ahead of time". Such an architecture would have 
exactly the same limitations as OAuth 2.0. and would not be scalable.

Before we start, we should have a clear view of the whole picture rather 
than starting from one scenario and expanding it in many different 
directions.
For building an architecture, a pre-requirement is to define ALL the 
trust relationships. I don't believe that we have an agreement at this 
time on what
these trust relationships are.

You are also using the following wording: "methods for the AS and RS to 
communicate what the token’s good for".
With such a paradigm, it would be impossible to protect the user's privacy.

A key question is : Will GNAP take care of the user's privacy or will 
GNAP *prevent *to take care of the user's privacy ?

About "the ability for the client to get several access tokens to get 
different resources from one request" is indeed a complex case.

No one (including ASs) is able to predict in advance which access tokens 
will be needed, since they depend upon the kind of operation(s)
the client will be willing to perform. For privacy reasons, ASs should 
be kept ignorant of all the operations that a client is going to perform
on one or more resource servers. I believe that every effort should be 
made to avoid the AS to be in a position to be able to trace the operations
performed by its clients on various RSs.

To handle the complex case you envision, the full workflow of operations 
needs to be considered with a detailed focus on the time
at which the user's *consent and choice* happens (/consent and choice/ 
is the first privacy principle from ISO 29100).

First of all, an access token could be targeted to a service rather than 
to a server. This would already solve many cases.

When a RS needs to call another RS (which does not support the same 
service) then the client should be informed by the first RS.
His "consent and choice" should then be obtained by the first RS and, 
when the user agrees, the client should then ask an access token
to one of the ASs trusted by the second RS in order to perform the 
subsequent operation.

Denis

PS.  In an email sent on June 23 you wrote: " I think an on-device AS is 
an important use case".  I am sorry, but I don't understand.
        However, I do understand what "a server-based AS" is.


> Denis, thanks for the great comments. I think that your trust model is 
> pretty different from what I’d laid out initially, and while it’s an 
> interesting and important one, it is not what I meant by “multiple 
> access tokens” in my discussion below. I think that might be the cause 
> of some disconnect here, but that doesn’t mean it’s out of reach for 
> what the WG is after.
>
> When I refer to multiple access tokens, and what the charter calls out 
> as multiple access tokens, is the ability for the client to get 
> several access tokens to get different resources from one request. I 
> personally see this as an optimization of a specific set of use cases, 
> some of which I discussed in my original email. There are others, I’m 
> sure, but all the ones I’ve seen are around the client needing to get 
> several different kinds of access through an AS.
>
> I think it’s going to be overwhelmingly common that a given RS is 
> going to trust exactly one AS that it knows about ahead of time. But 
> for RS’s that can trust multiple AS’s, then we should have a model 
> that can accommodate that as well. That’s why the charter calls out 
> methods for the AS and RS to communicate what the token’s good for. I 
> think the basis of those methods is going to start with a common token 
> model, and likely incorporate into things like token formats and 
> introspection-style token APIs.
>
>  — Justin
>
>> On Jun 22, 2020, at 3:55 AM, Denis <[e-mail] 
>> <mailto:[e-mail]>> wrote:
>>
>> Hello Justin,
>>
>> A few comments between the lines.
>>
>>> One of the most important aspects to GNAP is going to be an ability 
>>> to address things that OAuth 2 can’t, or at least can’t do without 
>>> significant gymnastics. So with that, I wanted to bring back a use 
>>> case discussion that originally came up while we were talking about 
>>> the possibility of multiple access tokens, a few months back. I 
>>> don’t know if this concept has a regular term, so I’m going to call 
>>> it “directed access tokens” until we come up with something better — 
>>> assuming we decide this is worthwhile.
>>
>> I don't understand what may mean "directed access tokens” but I 
>> understand what means "multiple access tokens".
>> When speaking of "multiple access tokens", these access tokens may 
>> come from one or more ASs.
>>
>>> In OAuth, the client’s supposed to always know where to send its 
>>> access token, and that knowledge is completely outside the protocol. 
>>> This makes a lot of sense for many (if not most) deployments, as 
>>> OAuth is really there to enable the API access that the client 
>>> already knows about.
>>>
>>> But we’re now in a world where a client could be talking to a 
>>> generic API that could be deployed at a number of different places, 
>>> or a cloud deployment that the AS wants to be able to dispatch the 
>>> client to.
>>
>> As soon the AS is placed (again) at the centre of the model, the AS 
>> will have the ability to act as "Big Brother".
>> OAuth 2.0 has not taken care of privacy issues. On the contrary, GNAP 
>> should take care of them.
>>
>>> In order to do this, the AS needs to be able to communicate to the 
>>> client not only the token information (and any related key and 
>>> presentation information), but also a set of/directions/ about what 
>>> that specific token is good for. This needs to be information 
>>> outside the token itself, since I believe we want to keep OAuth 2’s 
>>> feature of having the token be opaque to the client. Note: while we 
>>> could map all of these to different resource requests (in XYZ 
>>> parlance) or scopes (in OAuth 2 legacy parlance) on the request 
>>> side, this isn’t enough to tell the client what to do with the 
>>> token/if it doesn’t know already/.
>>>
>>> I know of two use cases already in the wild, where people are 
>>> addressing things using a mix of existing standards and some 
>>> proprietary extensions to address things within their silos. I’ll 
>>> try to summarize here, but I know the folks I’ve been talking to 
>>> about this are also on the list so hopefully they can chime in with 
>>> more detail or any corrections for something I’ve missed.
>>>
>>> (1) The client knows what resource it’s calling, but it doesn’t know 
>>> where it’s hosted. Everything is in a single security domain 
>>> controlled by the AS,
>>
>> Speaking of "multiple access tokens" is in contradiction with single 
>> security domain "controlled" by an AS.
>> A user may need to demonstrate some of his identity attributes 
>> granted e.g. by his bank but may also
>> need to demonstrate one or more diplomas granted by one or more 
>> universities. The bank cannot be
>> the "primary issuer" of a university diploma. It should not be either 
>> a "secondary issuer", because
>> the bank does not "/need to know"/what are the diplomas of the user.
>>
>>> but the AS needs to decide at runtime which specific instance of the 
>>> API to direct the client to. Since things are closely tied together, 
>>> the client just needs to effectively known an identifier for the RS, 
>>> and this is currently implemented as a URI. Once the client has that 
>>> identifier, it knows how to dispatch that token to that instance of 
>>> the RS.
>>
>> There is no good reason why the AS should be involved in that step.
>> OAuth 2.0 is/was implicitly mandating a strong relationship between 
>> ASs and RSs which makes it non scalable.
>> GNAP should be based on a simple trust relationship : a given RS 
>> trusts some ASs for access tokens that contains some types of attributes.
>> An AS should not need to know in advance (or even at the time of an 
>> access token request) the RSs that are trusting it.
>>
>> A client could first talk to a "global service provider" which has 
>> the knowledge of the different RSs affiliated to it.
>>
>>> (2) The client knows what kind of thing it’s looking for, but 
>>> doesn’t know who to ask it from.
>>
>> Once again, the client could first talk to a "global service 
>> provider" which has the knowledge of the different RSs affiliated to it.
>>
>>> There’s a cross-domain trust that’s bridged by the AS, and the AS 
>>> needs to dispatch to different resources depending on which user 
>>> logged in (and possibly what the user consented to). To make things 
>>> more concrete, the client needs to get driver’s license information, 
>>> but doesn’t know ahead of time which of the many state/provincial 
>>> bureaus to call to get that information because it doesn’t know yet 
>>> who the user is.
>>
>> When a user has a driving license, he knows its issuer. The user can 
>> then provide some hint to the client.
>>
>>> The AS will know who the user is once they log in and approve 
>>> things, and so it can direct the client to call the correct RS. 
>>> Since this is a relatively simple API with a pre-negotiated 
>>> cross-domain trust, the AS returns a URL that the client presents 
>>> the token at.
>>
>>  A single AS should not be aware of all the attributes a user has.
>>
>>>
>>> As far as I know, in both of these cases, the token is only good for 
>>> that API and not others — but more on that later.
>>>
>>> A simple thing to do is just give back a URL with the access token, 
>>> which tells the client where to go.
>>>
>>> {
>>> “access_token”: {
>>> “value”: “87yui843yfer”,
>>> “resource_uri”: “https://example/foo"
>>> }
>>> }
>>>
>>> This is good for some kinds of APIs, but it’s limiting because not 
>>> all APIs dispatch based on the URL. An AS might want to divvy up 
>>> access tokens to an API that’s keyed on headers, or verbs, or any 
>>> number of things. And it doesn’t tell us immediately what to do 
>>> about non-exact URL matches. Can the client add query parameters and 
>>> still use the token? What about path segments? I like that this 
>>> simple approach addresses some common deployments that we already 
>>> see today (see above), it’s not universal. Do we want or need a 
>>> universal description language for directing where tokens go?
>>>
>>> This also opens up a whole new set of security questions. If the AS 
>>> can now direct the client where to use the token, could a rogue AS 
>>> convince a legit client to use a stolen token at the wrong RS? And 
>>> what if the client ignores the directions from the AS entirely? 
>>> Could this open up new avenues of attack?
>>>
>>> This is just the start, too. Things get even more complex if the 
>>> client can ask for multiple different kinds of resources at once. 
>>> What if the AS decides that the client needs a hyper-focused 
>>> directed token for one part of the API, but can use a general token 
>>> for other stuff? Can it signal that to the client? And if it can, 
>>> does that mean that all clients need to be prepared for that kind of 
>>> thing?
>>>
>>> I firmly believe that whatever we build in GNAP, we need to optimize 
>>> for the overwhelmingly common use case of a client getting a single 
>>> access token to call APIs that it already knows about. Anything we 
>>> add on top of that really can’t get in the way of this, because if 
>>> it does, there’s very small chance that people will try to use this 
>>> for everyday things. Keep the simple things simple, and the complex 
>>> things possible, after all.
>>>
>>> I’m really looking forward to hearing what the community thinks 
>>> about these use cases, and hopefully the people I’ve chatted with 
>>> offline about this can join the conversation and provide more light 
>>> than I was able to.
>>
>> The two use cases which are considered above are:
>>
>>     (1) The client knows what resource it’s calling, but it doesn’t
>>     know where it’s hosted.
>>     (2) The client knows what kind of thing it’s looking for, but
>>     doesn’t know who to ask it from.
>>
>> That does not mean in any way that these two use cases should be 
>> solved by placing the AS at the centre of the solution.
>>
>> Denis
>>
>>>
>>>  — Justin
>>>
>>
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