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Re: Understanding @contexts and credentialSchemas

From: Kim Hamilton <kimdhamilton@gmail.com>
Date: Fri, 11 Jun 2021 11:08:13 -0700
Message-ID: <CAFmmOzfkFTnBjLktmFxVJ5dji_E2AM1meqE5B6ccf7vFh3E9pQ@mail.gmail.com>
To: Kerri Lemoie <klemoie@concentricsky.com>
Cc: Orie Steele <orie@transmute.industries>, W3C Credentials CG <public-credentials@w3.org>
FYI, Orie’s latest response should be elevated to a CCG FAQ entry, if we
had such a thing. Having this handy would save a lot of time for folks
(having seen this be a major point of confusion over several years).

On Fri, Jun 11, 2021 at 11:03 AM Kerri Lemoie <klemoie@concentricsky.com>
wrote:

> Thank you, Orie!
>
>
> On Jun 11, 2021, at 9:43 AM, Orie Steele <orie@transmute.industries>
> wrote:
>
> a context describes semantics, a schema describes data shape... There is a
> right tool for each job, but sometimes you need both.
>
> `@context` can only help you define JSON-LD semantics.
>
> `credentialSchema` is much more flexible and can help you define JSON
> Schema, SHACL, CustomProprietarySchema, or any other schema technology you
> want to use.
>
> I would not recommend using `@context` for schemas related to data shape
> or `credentialSchema` for semantics... but JSON-LD is a lot of rope and it
> does let you define some data-shape like behaviors.... My experience with
> it is that there is always a better solution than using JSON-LD "data
> shape" features like @container...
>
> A concrete example, "@type": "@json" <- this is JSON-LD for the JSON
> Schema  "type": "oneOf <number, null, object, string, array, etc...>
>
> This can be very useful when your JSON data has no well defined semantics,
> or where its semantics are rapidly changing, or externally owned...
>
> For example: https://spec.smarthealth.cards/credential-modeling/
>
> "@context": [ "https://www.w3.org/2018/credentials/v1", { "@vocab": "
> https://smarthealth.cards# <https://smarthealth.cards/#>", "fhirBundle":
> { "@id": "https://smarthealth.cards#fhirBundle
> <https://smarthealth.cards/#fhirBundle>", "@type": "@json" } } ]
>
> In this case, all we know about "fhirBundle" is that it's JSON... this
> documentation is incomplete... because what version of FHIR is it?
>
> fhirVersion needs to be defined, and used to answer that, but together
> they can allow you to "define only the semantics you need"... in this case,
> FHIR.
>
> It's worth noting that there are lots of valid FHIR I could put in
> "fhirBundle" that could cause explosions... that's where a schema language
> like JSON Schema would be valuable.
>
> Working together, they could help both producers and consumers agree to be
> as specific as is helpful for the VC type, and not "overly pedantic /
> specific" in ways that are beyond the point of diminishing returns.
>
> One version of being as specific as is necessary is to say: "this payload
> is json"... I personally think that is harmful laziness, but you could also
> see it as a reflection of a lack of certainty regarding what is actually
> getting signed... and a need to allow anything... regardless of what it
> is... to be signed.
>
> When answering the question of "what am I signing or verifying" JSON-LD
> and JSON Schema (for example) give you different additional confidence...
> Omitting both is the equivalent of saying you are either highly unconfident
> in the content you are or will be signing in the future, or you feel that
> the security benefits of typing and input validation are not relevant to
> your credential... I would call credentials that lack semantic
> disambiguation and schema / data shape fundamentally LESS SECURE.
>
> Consider:
>
> https://owasp.org/www-project-top-ten/
>
> Of the top 10, the following are mitigated with stricter semantics and
> data shape constraints:
>
> - Injection
> - Sensitive Data Exposure
> - Insecure Deserialization
>
> A credential that lacks semantics and datashape is like a food product
> with no labeling or a mobile phone built with parts that might have been
> developed with poor labor practices... the point is that you don't really
> know, because the issuer chose not to meet those bars, when they decided to
> sign.
>
> OS
>
>
>
>
>
> On Thu, Jun 10, 2021 at 2:28 PM Kerri Lemoie <klemoie@concentricsky.com>
> wrote:
>
>> This is helpful, Orie.
>>
>> While VC has this credentialSchema property, I assume it would still be
>> acceptable for a context to reference schema(s) directly?
>>
>> Thanks!
>>
>> K
>>
>>
>>
>>
>> On Jun 10, 2021, at 10:09 AM, Orie Steele <orie@transmute.industries>
>> wrote:
>>
>> This won't be a complete answer, but at the time of publication I
>> believe that field was used in 2 ways.
>>
>> 1. with json schema, see this for example -
>> https://w3c-ccg.github.io/vc-json-schemas/
>> 2. with hyperledger indy zkp-cl signature vc's
>>
>> In both cases, "credentialSchemas" was more about the VC data shape and
>> type, whereas contexts and JSON-LD are best used only for semantics.
>>
>> There are other tools like SHACL that can help do linked data shape
>> constraints, perhaps someone might use them with credentialSchemas in the
>> future.
>>
>> but AFAIK, "credentialSchemas" is focused on the credential data shape.
>> and "@context" is focused on the semantics and term definitions used in the
>> credential.
>>
>> OS
>>
>> On Wed, Jun 9, 2021 at 5:15 PM Kerri Lemoie <klemoie@concentricsky.com>
>> wrote:
>>
>>> Hello all,
>>>
>>> I’m reviewing this: https://www.w3.org/TR/vc-data-model/#data-schemas
>>>
>>> Could folks please explain to me the uses of credentialSchemas in
>>> comparison to @context files in JSON-LD? Is it that @context files name the
>>> attributes and credentialSchemas provide the information about how to
>>> validate the data/semantics?
>>>
>>> Can you provide some real-world examples? Bonus points for human
>>> centered examples such as identity,  education, &  workforce. :)
>>>
>>> Thanks!
>>>
>>> Kerri
>>>
>>>
>>>
>>> --------
>>> Kerri Lemoie, PhD
>>> Director, Digital Credentials Research & Innovation
>>> badgr.com <https://info.badgr.com/> | concentricsky.com
>>> she/her/hers
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>
>> --
>> *ORIE STEELE*
>> Chief Technical Officer
>> www.transmute.industries
>>
>> <https://www.transmute.industries/>
>>
>>
>>
>
> --
> *ORIE STEELE*
> Chief Technical Officer
> www.transmute.industries
>
> <https://www.transmute.industries/>
>
>
>
Received on Friday, 11 June 2021 18:08:58 UTC

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