This is incredibly useful.

I have a question regarding

“ROPolicy doesn’t let the scope go through, with one exception: if it’s in PrevRPT”

I am wondering whether this should always hold: RPTResult = subset(ROPolicy)

If the ROPolicy changed from when the prevRPT was granted, shouldn’t AS revoke the previous RPT?

Thanks,

--Cigdem

From: <wg-uma-bounces@kantarainitiative.org> on behalf of Justin Richer <jricher@mit.edu>
Date: Friday, 6 January 2017 at 22:56
To: "wg-uma@kantarainitiative.org UMA" <wg-uma@kantarainitiative.org>
Subject: [WG-UMA] Set Math Discussion

I’ve been working on the set math problem and I’ve been trying to lay out scenarios using this spreadsheet to help me sort my thoughts:

I’ve left a bit of space in the spreadsheet for additional combinations and use cases I might have missed, so please chime in and we can figure out an algorithm that we can all agree on. I can send out the .xlsx file if people care to play with it at home.

So we’ve got six sets of scopes to deal with here, and they’re somewhat independent of each other. 

ClientReg - client registers for these scopes at the RS

RSTicket - RS requests a ticket with these scopes

ROPolicy - RO sets a policy with these scopes (and it’s fulfilled by the RqP for our purposes)

ClientReq - client requests these scopes at the token endpoint

RSReg - RS registers these scopes at RS setup time

PrevRPT - client presents a previously-held RPT that’s got some other scopes on it and it’s trying to augment that with new scopes

And finally what the token includes:

RPTResult - the sets that the resulting token includes on output

The one direct relationship seems to be that:

ROPolicy = subset(RSReg)

Because otherwise the RO could set scopes on a resource that the resource didn’t register, which doesn’t make sense to me. That’s the error in column “F” above. 

It’s clear to me that “ROPolicy” is a limiting set, in that if a scope is NOT in that set, then it is not in the result. In other words:

RPTResult = subset(ROPolicy)

That’s column D, and nearly any other combination without ROPolicy doesn’t let the scope go through, with one exception: if it’s in PrevRPT, it gets carried through no matter what. But in particular, that, I believe, is an AS decision on whether it wants to honor previous RPTs at all. That’s why I’ve coded those K, L in green.

This also means that RSReg is also a limiting set, due to transitive subset operations:

RPTResult = subset(RSReg)

We have both the client and RS request scopes for the token at runtime in their parts, and it makes a lot of sense to combine them. So we get a working set of:

Requested = union(ClientReq, RSTicket)

We have an open question with what to do with column I: if a client hasn’t requested a scope, and a ticket didn’t request a scope, but the client registered for a scope, do we include it or not? We could choose to either ignore it and leave it out entirely; or add it in, and if it passes ROPolicy then we’ll pass it through to the token. These were the options we were discussing on the call on Thursday. Ignoring it here would effectively ignore the client’s registered scopes entirely, which is valid. The alternative would be coded something like:

Requested = union(ClientReg, ClientReq, RSTicket)

This could also be done conditionally, like the case where the client didn’t specifically request anything:

if (empty(ClientReq)) {

Requested = union(ClientReg, RSTicket)

}

Or if the client or ticket didn’t request anything:

if (empty(ClientReq) && empty(RSTicket)) {

Requested = ClientReg

}

So my proposed implementation would be something like this horrible pseudocode:

if (ROPolicy is not subset(RSReg)) {

throw error and fail <<sanity check>>

}

<< the requested set is made up of the client’s request to the token endpoint and the RS’s request during the ticket issuance step>>

Requested = union(ClientReq, RSTicket)

<< if nothing’s been requested, maybe the client’s registered for something we can default to >>

if (empty(Requested)) {

Requested = ClientReg

}

<< finally, we take the requested scopes and filter out anything not in a matched policy, and add in anything from a previous RPT if it’s there >>

RPTResult = union(

intersection(Requested, ROPolicy),

PrevRPT)

I’ve tested a little bit of this with the following python truth table code:

from truths import Truths

print Truths(['ClientReg', 'RSTicket', 'ROPolicy', 'ClientReq', 'RSReg', 'PrevRPT'], ['(((ClientReq or RSTicket) if not (ClientReq or RSTicket) else ClientReg) and ROPolicy) or PrevRPT'])

This seems to match up to expectations on a quick inspection, but I’m potentially missing something. 

Hopefully this makes sense, and this reflects my current thinking on this topic. I have almost certainly missed some use cases and might have some cases that don’t reflect reality. 

 — Justin