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: Sun, 6 Dec 2020 10:31:31 +0000
To: public-credentials@w3.org
Message-ID: <1ee45598-5931-7bdc-2f8b-fc2a92b4968a@kent.ac.uk>
Conceptionally Verifiable Credentials are not new. They were first 
standardised in the 1960s by ECMA as PACs (Privilege Attribute 
Certificates).

"Authentication and Privilege Attribute Security Application with 
related key distribution
functions" Standard ECMA-219, 2nd edition - March 1996

They have always been authorisation tokens, whether directly or 
indirectly. Using them for identity is simply indirect authorisation via 
ABAC and RBAC.

Every plastic card I have in my physical wallet today is an 
authorisation token, whether directly or indirectly. If it did not 
confer some privilege on me, I would not make space for it.

I agree with Daniel, schemas will need to be defined to store different 
types of authz credentials in VCs, But that is not new either. Schemas 
are needed for all the properties that are assigned to VC subjects.

Kind regards

David

On 06/12/2020 01:37, Christopher Lemmer Webber 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://roboticlab.cam.ac.uk/grants" ],
>>    "type": ["VerifiableCredential", "RobotPrivilege"],
>>    "issuer": "https://facebook.com/Amy",
>>    "issuanceDate": "2021-01-01T19:73:24Z",
>>    "credentialSubject": { "id":
>> "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" 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> <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 Sunday, 6 December 2020 10:31:50 UTC

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