Re: [OAUTH-WG] Followup on draft-ietf-oauth-token-exchange-12.txt

Denis <> Fri, 20 April 2018 17:38 UTC

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From: Denis <>
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Date: Fri, 20 Apr 2018 19:38:03 +0200
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Subject: Re: [OAUTH-WG] Followup on draft-ietf-oauth-token-exchange-12.txt
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Eric said: "what is the RP supposed to do when they encounter it? This 
seems kind of under specified".

After reading your explanations below, it looks like the RP can do 
anything he wants with the "actor".
It is a "claimed actor" and, if we keep the concept, it should be called 
as such. Such a claim cannot be verified.
A RP could copy and paste that claim in an audit log. No standard action 
related to the content of such a claim
can be specified in the spec. If the content of a "claimed actor" is 
used by the RP, it should be only used as an hint
and thus be subject to other verifications which are not specified in 
this specification.


> Eric, I realize you weren't particularly impressed by my prior 
> statements about the actor claim but, for lack of knowing what else to 
> say, I'm going to kind of repeat what I said about it over in the 
> Phabricator tool 
> <> and add 
> a little color.
> The actor claim is intended as a way to express that delegation has 
> happened and identify the entities involved. Access control or other 
> decisions based on it are at the discretion of the consumer of the 
> token based on whatever policy might be in place.
> There are JWT claims that have concise processing rules with respect 
> to whether or not the JWT can be accepted as valid. Some examples are 
> "aud" (Audience), "exp" (Expiration Time), and "nbf" (Not Before) from 
> RFC 7519. E.g. if the token is expired or was intended for someone or 
> something else, reject it.
> And there are JWT claims that appropriately don't specify such 
> processing rules and are solely statements of fact or circumstance. 
> Also from RFC 7519, the "sub" (Subject) and "iat" (Issued At) claims 
> are good examples of such. There might be application or policy 
> specific rules applied to the content of those kinds of claims (e.g. 
> only subjects from a particular organization are able to access tenant 
> specific data or, less realistic but still possible, disallow access 
> for tokens issued outside of regular business hours) but that's all 
> outside the scope of a specification's definition of the claim.
> The actor claim falls into the latter category. It's a way for the 
> issuer of the token to tell the consumer of the token what is going 
> on. But any action to take (or not) based on that information is at 
> the discretion of the token consumer. I honestly don't know it could 
> be anything more. And don't think it should be.
> There are two main expected uses of the actor claim (that I'm aware of 
> anyway) that describing here might help. Maybe. One is a human to 
> human delegation case like a customer service rep doing something on 
> behalf of an end user. The subject would be that user and the actor 
> would be the customer service rep. And there wouldn't be any chaining 
> or nesting of the actor. The other case is so called service chaining 
> where a system might exchange a token it receives for a new token that 
> it can use to call a downstream service. And that service in turn 
> might do another exchange to get a new token suitable to call yet 
> another downstream service. And again and so on and turtles all the 
> way. I'm not necessarily endorsing that level of granularity in 
> chaining but it's bound to happen somewhere/sometime. The nested actor 
> claim is able to express that all that has happened with the top level 
> or outermost one being the system currently using the token and prior 
> systems being nested.. What actually gets done with that information 
> is up to the respective systems involved. There might be policy about 
> what system is allowed to call what other system that is enforced. Or 
> maybe the info is just written to an audit log somewhere. Or something 
> else. I don't know. But whatever it is application/deployment/policy 
> dependent and not specifiable by a spec.
> On Fri, Apr 13, 2018 at 6:38 PM, Eric Rescorla < 
> <>> wrote:
>     Hi folks,
>     I've gone over draft-ietf-oauth-token-exchange-12 and things seem
>     generally OK. I do still have one remaining concern, which is about
>     the actor claim. Specifically, what is the RP supposed to do when they
>     encounter it? This seems kind of underspecified.
>     In particular:
>     1. What facts am I supposed to know here? Merely that everyone in
>        the chain signed off on the next person in the chain acting as
>     them?
>     2. Am I just supposed to pretend that the person presenting the token
>        is the identity at the top of the chain? Say I have the
>        delegation A -> B -> C, and there is some resource which
>        B can access but A and C cannot, should I give access?
>     I think the first question definitely needs an answer. The second
>     question I guess we could make not answer, but it's pretty hard
>     to know how to make a system with this left open..
>     -Ekr
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