W3C home > Mailing lists > Public > public-credentials@w3.org > December 2020

Re: VCs - zCaps / OCap a Discussion

From: David Chadwick <D.W.Chadwick@kent.ac.uk>
Date: Mon, 7 Dec 2020 17:12:57 +0000
To: public-credentials@w3.org
Message-ID: <840a84d9-0e59-1c5c-b7cc-4ac16517e159@kent.ac.uk>
Hi Alan

we are using VCs for one thing only: authorisation.

Now it depends upon how fine grained you want to go. Do you want to use 
the same construct for all authorisations, or different constructs for 
each individual type and instance of authorisation?

Kind regards

David

On 06/12/2020 02:15, Alan Karp wrote:
> I want to start with a confession.  I suffer from a disease called 
> "overloading," using the same thing for two or more different 
> purposes.  The problem is I keep doing it even though I know I'll come 
> to regret it.  The worst of it is that the problems don't show up 
> until I've locked in a lot of things.  Using VCs for ocaps smells like 
> overloading.
>
> I have no doubt that VCs with chained credentials can be used for 
> ocaps.  I worry that some incompatibilities will arise later.  Will a 
> change to the chained credentials needed for VCs interfere with some 
> functionality of ocaps?  Will a new feature needed for ocaps turn out 
> to be incompatible with other uses of chained credentials?  Even if 
> these problems can be solved, how much time and work will it take to 
> resolve them?
>
> Don't get me wrong.  If VCs are the only standard for ocaps, I'll 
> gladly use them.  I'll complain about having to plow through all the 
> stuff that's not relevant to me, but I'll do it.
>
> --------------
> Alan Karp
>
>
> On Sat, Dec 5, 2020 at 5:40 PM Christopher Lemmer Webber 
> <cwebber@dustycloud.org <mailto:cwebber@dustycloud.org>> wrote:
>
>     I wrote my mega-response before reading Dave's; we raise some
>     different
>     points, but I think Dave frames things nicely as well.
>
>     Dave Longley writes:
>
>     > TL; DR: My current view is that the main confusion here may be
>     over the
>     > difference between VCs and LD Proofs, not VCs and ZCAPs. VCs are
>     not a
>     > generalized container for attaching a cryptographic proof to a
>     document.
>     > That's what LD proofs (or JOSE style proofs) are for. VCs *use* LD
>     > proofs (or JOSE style proofs) to attach an assertion proof to a
>     document
>     > that specifically models statements made by an issuer about some
>     > subject, which is therefore inherently about the identity of
>     that subject.
>     >
>     > Even if we were to say "let's combine ZCAPs and VCs" -- we'd
>     > *still* need to agree on a data model for expressing OCAPs
>     inside VCs.
>     > If we want that to be standard (which is what standardizing ZCAPs is
>     > about), then that work doesn't "go away" just because you put a
>     ZCAP in
>     > a VC as a "credentialSubject".
>     >
>     > Daniel,
>     >
>     >> ... snip ..
>     >>
>     >> Second, I have yet to see ANY justification for the hammer/screw
>     >> analogy. I gave an example of using a VC to express an ocap. It was
>     >> simple and elegant, and 100% compatible with the VC spec.
>     >
>     > I think your example has a flaw, however, because the data
>     itself does
>     > not express what you said it expresses. Rather, a plain reading
>     of it
>     > using the VC semantic model says something else.
>     >
>     > ```
>     > {
>     >   "@context": [ "https://www.w3.org/2018/credentials/v1
>     <https://www.w3.org/2018/credentials/v1>",
>     > "https://roboticlab.cam.ac.uk/grants
>     <https://roboticlab.cam.ac.uk/grants>" ],
>     >   "type": ["VerifiableCredential", "RobotPrivilege"],
>     >   "issuer": "https://facebook.com/Amy <https://facebook.com/Amy>",
>     >   "issuanceDate": "2021-01-01T19:73:24Z",
>     >   "credentialSubject": { "id":
>     > "https://roboticlab.cam.ac.uk/~amyt/capstone-project
>     <https://roboticlab.cam.ac.uk/~amyt/capstone-project>", "allow":
>     "operate"},
>     >   "proof": {...}
>     > }
>     > ```
>     >
>     > The above does not say "It's an assertion that the bearer is
>     > authorized." as you stated in your earlier email. It says:
>     >
>     > The subject identified by
>     > "https://roboticlab.cam.ac.uk/~amyt/capstone-project
>     <https://roboticlab.cam.ac.uk/~amyt/capstone-project>" has the
>     property
>     > "allow" with the value "operate".
>     >
>     > This has nothing to do with any bearer of the VC. In fact, if
>     one were
>     > to *guess* more about the meaning of this VC, it would be that Amy's
>     > "capstone project" is allowed to operate. Again, that's just
>     guessing
>     > from the English used in the terms and the identifier. This is
>     not what
>     > a machine would think without adding special-cased code that
>     bends the
>     > meaning to fit a use case. A machine would simply consider the
>     > statements to be about whatever this subject is. This highlights
>     that
>     > VCs are just about making statements of identity about a
>     particular subject.
>     >
>     > Note that the triples here are:
>     >
>     > <https://roboticlab.cam.ac.uk/~amyt/capstone-project
>     <https://roboticlab.cam.ac.uk/~amyt/capstone-project>> <allow>
>     <operate>
>     >
>     > (For some unambiguous mapping of "allow" and "operate" to full
>     URLs).
>     >
>     > These statements presumably say nothing about the bearer (unless the
>     > bearer is identified by that "/capstone-project" URL). Even if
>     so, what
>     > would they be allowed to operate on?
>     >
>     > So the semantics that you're layering on top through language in
>     your
>     > email are not present in the data itself. Another attempt will
>     have to
>     > be made to see what a VC would look like if it were to properly
>     model an
>     > OCAP.
>     >
>     > My guess is that the VC would have to contain a fully modeled OCAP
>     > as the `credentialSubject`. If true, it would become more
>     obvious (in my
>     > view) that the VC data model is merely being (mis-)used as a
>     container
>     > for an OCAP. Instead, applying a proof to the OCAP directly
>     would have
>     > sufficed and it could more accurately carry important semantics
>     such as
>     > that the purpose of the proof was for the delegation of
>     authority rather
>     > than merely asserting such authority exists (a claim of identity).
>     >
>     > Certainly, you could express a claim that someone possesses an OCAP.
>     > Note that this is still different from expressing a claim, for
>     > example, that someone possess a driving license. Possession of a
>     driving
>     > license does not authorize you to drive *my* car. It isn't bound
>     to a
>     > particular object and what you can do with it extends even
>     beyond just
>     > driving. You could potentially present your driving license and
>     get an
>     > authorization to drive *my* car in return. But even then you
>     don't get
>     > to drive my car simply because you are saying you have an
>     authorization
>     > to drive *my* car. That may cause you to drive my car when your
>     intent
>     > was different (merely to announce that you could if you wanted).
>     >
>     > I think this highlights some other problems with trying to fit OCAPs
>     > into the VC/VP model. There are some odd semantics to overcome if
>     > someone wants to "present" an OCAP using Verifiable Presentation
>     > machinery. As they are presenting the fact that someone said they
>     > possess an OCAP rather than creating a proof that is explicitly
>     invoking
>     > it. You don't invoke a Verifiable Presentation -- you invoke the
>     OCAP,
>     > so that's where the proofs should be attached. It seems to me that a
>     > number of rather semantically-confusing hacks would have to be
>     > implemented in order to cause OCAPs to travel inside of VCs.
>     >
>     > IMO, this suggests to me that trying to fit OCAPs into the existing
>     > VC/VP standard may be moving backwards toward a world where the
>     > semantics for VCs are in people's heads, not in the data itself.
>     A major
>     > reason for much of the VC work was to make data more
>     self-describing and
>     > capable of being used across contexts without needing to go
>     through a
>     > centralized authority for extensibility.
>     >
>     > Existing work (JWT claims, etc.) could not do this and were very
>     limited
>     > in what they could convey across contexts. This is because they
>     really
>     > were more useful as authorizations for a specific party, not merely
>     > statements of identity. The existing work emphasized the
>     authorization
>     > use cases first rather than the identity ones and then said
>     "This could
>     > work in the identity cases too" without a focus there. The
>     result was a
>     > half-measure that never really solved the bulk of the identity use
>     > cases; that's why VCs sprung into existence. I suspect we'd be
>     making
>     > the same mistake in the opposite direction to try and bend VCs back
>     > toward authorizations ... just because maybe we could squint and
>     make it
>     > work.
>     >
>     > OCAPs are an improvement over other authorization schemes for a
>     variety
>     > of reasons, and flavors such as ZCAP-LD do include some level of
>     > self-describing information as well, but it is less of a concern
>     because
>     > they are intentionally not used across contexts. There is only one
>     > "verifier" or one "type" of object for a given capability. What
>     you can
>     > "do" with an OCAP is not open-ended -- it is strictly defined by the
>     > verifier (which is the same party as the original "issuer"). VCs
>     are the
>     > opposite and intentionally split the issuer and verifier parties
>     apart
>     > -- even if you can have one party play both roles for some use
>     cases.
>     >
>     > This means OCAPs and VCs have very different economics as well.
>     VCs are
>     > designed for an ecosystem where issuers can create valuable VCs
>     to be
>     > broadly used across many verifiers precisely because they are fully
>     > self-describing and issuers are confined to only making statements
>     > that express properties of identity about subjects. Issuers
>     needn't be
>     > concerned about what authority is conferred because they do not
>     grant
>     > any authority. Only verifiers do that. Issuers do not *need* to
>     know or
>     > care about how these properties may be used -- and interesting
>     new ways
>     > to use them may crop up long after issuance.
>     >
>     > OCAPs are quite the opposite; their use is strictly defined and
>     limited
>     > by the original granter of authority. This also means that, as a
>     granter
>     > of a capability, you don't need to gather as much community
>     consensus
>     > around what the meaning of a particular authorization is in
>     order for
>     > what you're granting to be useful across contexts ... because
>     that would
>     > be an abuse of the OCAP. You are explicitly not making a
>     statement of
>     > authority that is intended to be used *anywhere* except on the
>     target
>     > object.
>     >
>     > Of course, since VCs are based on an open world model, you can
>     try and
>     > put anything you want into them. That doesn't mean it won't get
>     > cumbersome or that everything *should* be put into them.
>     Furthermore, a
>     > VC is always: "An issuer says these things are true about X"..
>     This is
>     > perhaps more restrictive than some people are thinking.
>     >
>     > Sure, you can substitute anything into "these things" or "X",
>     but you
>     > can't change the rest of that sentence. A VC cannot say you have
>     > authority, it can say an issuer says you have authority. It is
>     not an
>     > authorization itself. It could be used by some decision engine to
>     > *decide* to grant authorization, but that "granted authorization" is
>     > what an OCAP is. The OCAP *is* the authorization: delegatable and
>     > invocable at a particular target object -- and not to be
>     confused with a
>     > statement of identity. Similarly, as mentioned above, a VP does
>     not say
>     > you're invoking an OCAP -- if you were to present one "as a VC".
>     Rather,
>     > it would say that you're *announcing* that an issuer *says* you
>     have an
>     > OCAP. I think these differences are important and confusing them
>     leads
>     > to subtle vulnerabilities.
>     >
>     > My current view is that the main confusion here may be over the
>     > difference between VCs and LD Proofs, not VCs and ZCAPs. VCs are
>     not a
>     > generalized container for attaching a cryptographic proof to a
>     document.
>     > That's what LD proofs (or JOSE style proofs) are for. VCs *use* LD
>     > proofs (or JOSE style proofs) to attach an assertion proof to a
>     document
>     > that specifically models statements made by an issuer about some
>     > subject, which is therefore inherently about the identity of
>     that subject.
>     >
>     > Lastly, even if we were to say "let's combine ZCAPs and VCs" -- we'd
>     > *still* need to agree on a data model for expressing OCAPs
>     inside VCs.
>     > If we want that to be standard (which is what standardizing ZCAPs is
>     > about), then that work doesn't "go away" just because you put a
>     ZCAP in
>     > a VC as a "credentialSubject".
>
>
Received on Monday, 7 December 2020 17:13:15 UTC

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