# Composing Credentials via Links and Cryptographic Binding

[](https://hackmd.io/r8CsaGShSIu2z1K6iTHi6A)

## Co-Authors

* Phillip D. Long - @longpd
* Dmitri Zagidulin - @dmitrizagidulin
* Golda Velez - @gvelez17
* Oliver Klingefjord - @Klingefjord

## Abstract

The Verifiable Credential ecosystem has encountered several use cases that require a third-party assertion, or an endorsement of an existing object (another VC, a PDF, a webpage, etc). Whether it is product reviews, endorsements of self-created credentials, provenance of academic paper reviews, or some other general purpose third party assertion, these use cases have several requirements in common. Each use case may also require a domain-specific set of fields.

We propose a minimal format for connecting credentials that will allow each use case of 3rd party assertions to be represented as a set of LinkedClaims.

Further, we propose to demonstrate the ability to compose several LinkedClaims into a single domain-specific credential, specifically a Verifiable Endorsement, that will satisfy the domain requirements of the likely users.

This approach will enable rich shared datasets to inform trust decisions, while satisfying the requirements of domain-specific end users. If time permits a sample score can be built over the linked claim dataset.

## Introduction

As the Verifiable Credential community gathers implementation experience based on the VC Data Model v1.0, and as it starts working on the next version of the spec, we see the VC landscape adopt two different (but interoperable) architectures.

The first approach favors large **compound credentials** that contain many claims, signed by the issuer as a set. In the education space, the CLRv2 (Comprehensive Learner Record v2) spec is one such canonical example, intended to be, well, comprehensive, containing a person's entire educational history associated with enrollment overseen by a single issuer, the institution awarding the degree. Another example, both in the physical world of plastic cards, as well as in the world of digital credentials, is the Drivers License, a collection of unrelated claims (name, address, age, picture, driving ability), grouped together, for historical reasons of convenience.

The second approach, which is isomorphic/functionally equivalent to the first, uses linked micro-credentials with cryptographic binding.

The compound credential provides the recipient with a rich set of claims of potentially multiple types, along with alignments and other metadata needed to sematically express complex relationships among them. This is useful, but can lead to large payloads and introduce complexities on the part of systems consuming them. It also means any change to any part of the complex payload that needs updating requires the revoation of the compound VC, and any of the compents within it. After corrections are made those edited parts need re-signing/reissusing, as well as the the overal compound payload package.

Micro-credentials allow for easier data minimization, editing and reissuance of only those microcredentials whose data elements need updating, facilitates partial disclosure, and allows for secure re-composition down the line.

### A Brief Primer on Verifiable Presentations

VPs are a general purpose wrapper/envelope for one or more VCs. When an Issuer sends VCs to a wallet, it sends them wrapped inside a VP envelope. When a wallet holder presents VCs to a requesting party, the wallet also wraps them inside a VP envelope.

Aside from serving as a lightweight container for one or more VCs, VPs allow the presenter to perform DID-Authentication, to prove cryptographic control over a particular DID, and in this case conveying the sender of the VC is reponsible for which is useful to some use cases.
- VPs don't have to be signed. Performing DID-Authentication is optional. It's reserved for use cases where it's important for the receiver to know who the presentation is coming from.
- Also, by (optionally) signing a VP, you're adding a tamper-proof binding on the bundle itself (in addition to the fact that any given VC inside a VP is already tamper-proof by itself).
- Little known fact: the VCs inside a presentation don't have to be about the presenter. The holder may have in their possession credentials issued by third parties. For example, when applying to a job, an individual can obtain (and pass along) their school's accreditation credential. They can also obtain and pass along a VC of the job description they're applying for.
Neither of those VCs (the school accreditation, and the job description) mention the person at all (the job applicant is not the subject of either of those). As part of the application process, the job applicant submits a bundle of credentials (the Verifiable Presentation), some of which may be about the applicant (such as their awards and achievements), and others that are not directly related, but that provide contextual value to the recipient (such as the job description VC).

## Requirements for Linked Claims and Endorsements

1. They must use the W3C Verifiable Credentials (v2) envelope data model.
2. They must be able to refer to (make a statement about) an external object, such as:
* Another VC
* A URL (web page, PDF, image, etc)
* A subsection of another VC (for example, an individual achievement or course in a transcript VC that contains multiple courses, or a one among a set of competences in VC).
3. They must provide an (optional) mechanism for cryptographic binding between VCs or between a VC and another URL. (Hashlink mechanism.)
4. They should define properties that describe the endorsement or assertion:
* a property describing that is an extension of the VC issuer, describing who is making the endorsement, what their expertise/bonafides is, what their relationship to the subject, etc.
* a property containing evidence of the assertion (images, URLs, published papers, Github repos, etc.)
* human-readable description of the assertion
* an optional rating (such as for ecommerce product reviews)

Nice to haves include:

1. Ability to create "local names" from the perspective of the issuer to generate URIs for things that do not have authoritative URIs
2. Ability to compose one or more minimal endorsement VCs into a composite for a specific use case

## Existing Standards Overview

* Coalition for Content Provenance and Authenticity (C2PA)
https://c2pa.org/specifications/specifications/1.0/specs/C2PA_Specification.html

## Use Cases

### Endorsements of self-asserted skill credentials
Alice has worked in warehouse distribution centers since she dropped out of high school to earn money to support her mother, and siblings, after her mother was forced to recuperate from injuries incurred in a vehicle accident.

Alice started as a custodian but over time and through observation and conversation with her co-workers she learned she could get trained to drive a warehouse forklift and significantly increase her earning potential. She enrolled in class 4 OSHA certified forklift driver training course while continuing her night custodian job. After receiving her certification she applied for a forklift driver opening at the company where she was employed but sought corroboration of her skills both from her instructors at ISETA (Bootcamp 1.0) and the forklift specialization course, along with an endorsement from her current supervisor attesting to her reliability, teamwork, and attention to detail she has expressed in her current job.

Alice sends the self-asserted credential describing her skills relevant to the forklift driver position to Bob, her instructor ay ISETA of the OSHA forklift training course by email and points out the attributes she thinks he can attest to amongst her skill claims.

Alice does the same thing sending her self-assertion credential to Juanita,her current supervisor, pointing out those attributes she suggests Juanita has direct knowledge about her performance.

Both Bob and Juanita construct a linked-claims Verifiable Credential endorsing Alice for those attributes they have expertise in and can address with professional credibility based on their own bona fides included in their relevance arrestations describing their own training and background.

Juanita and Bob send their self-asserted skill endorsements to Alice after binding their attributions to those skills and competencies based on their knowledge of Alice's capabilities. Alice selects these credentials for inclusion in the set she prepares as a verifiable presentation to submit to the job application site.

### An endorsement of an institution/busniess/gov issued credential to an individual by a third party.
Claudio recently complete a certificate program in cybersecurity. The technical college issued the certificate in both paper and VC formats. Through conversations with some of his instructors he's become aware of an opening in a local cybersecurity sourcing company who provides short (1 to 6 month consulting project contracts) and medium term placements for contract employees). Claudio asks one of his instructors, Laura,for a reference. Laura agrees to serve as a reference and suggests that she provide a verifable endorsement credential to Claudio's digital credential wallet.

Laura asks Claudio for (a copy of) his VC conferring his cybersecurity certifiate awarded to him for successfully completing his course of study that awarded him a Certificate in Cyber Threat Analytics and Prevention (CTAP).Laura composes a self asserted verifiable endorsement VC that establishes her credentials or bona fides as a credible judge of Claudios knowldge, skills and abilities in this domain. She provides verifiable documentation of her work in the field through links to papers she has published, reports to which she contributed on cyberthreat assessment, and grants she has received for her workmin this domain.

She describes the the work she has seen done by Claudio and her view of the level of performance he demonstrated. Laura also has offeres additional evidence of Claudios knowledge in this discipline from the help he provided as a contracted worker under a cyberassessment contract she oversaw for a local regional bank. These included statistical analyses of penetration tests, attack simulations and summary reports all in his Github account, all hashlinked into the evidence section of her endorsement VC for Claudio.

Laura signs her verifiable endorsement VC and sends it as a VP to Claidios credential wallet. He accepts the presentation which puts the endorsement VC in his wallet and generates a VP including his CTAP certificate VC, the endorsement VC and several others that document related credentials pertinent to the job requirements. This VP is then sent to the cybersecurity sourcing company as part of his Verifiable Resume (TM).

### A review of a paper submitted to an academic conference
Layla Soliman registered to attend her disciplinary conference in molecular genetics. The conference organizers asked her to review a paper submitted for presentation at the meeting to which she agreed. The organizers send Layla a link to the paper in a Google docs directory.

Layla follows the link to the paper, reads the reviewer guidelines and sets out reading and taking notes and preparing her review comments.On completion of the reading the submitted paper manuscript she prepares her final comments and creates a self-assertion credential of type equals "review". Essentially this is a narrative credential that is cryptographically signed and linked to hashlinked to the Google doc location of the manuscript.

Layla creates a simple Verifiable Presentation to send her review to the conference organizer's digital wallet where reviews are collated. The linked data endorsement is of type: review and contains the narrative of her review comments

### An ecommerce product review (5-star type)

### Assertion that an image/video was taken by a camera crew has not been altered from the time the images were captured.
Ansel Abrams is a professional photographer who publishes his images in magazines, newpapers/services and on his own photo gallery on the web. Recently he has been embroiled in several controversies over the authenticity of what were purported to be 'his' images but which were in fact subtly edited representations of his work. One of these resukted in expensive legal costs in defending his claims of manipulation without his consent.

Ansel has now introduced into his workflow a requirement that any publication of any image he creates must be cryptogrphically hashlinked and signed by him. This both makes it simple for him to prove tampering has occurred to one of his photos. It also imposes a requirement that any purchuser of his work of does have a legitimate reason to edit his image contact him for permission to do so and either provide him the edited version that he can cryptographically add this derivative work to his repository or request the publisher to state their provenance for the derivative image.

### A person was harmed by an entity (may be known or unknown)

### An endorsement that attests to the provenance of an article posted in a published news service

Island News, the daily morning paper for the Signal Islands, has a reputation for in depth investigative reporting. Recently, controversies have emerged when a report on inappropriate use of provincial revenue collected from bond financing was published stating that government ministers involved in the oversight of the fund accounts associated with a new economic development, anchored by Wind Casinos, redirected substantial amounts of Bitcoin to private ministerial accounts.

Representatives of these ministers countered that these accusations were fabricated and published purported notes from the reporting journaistis corroborating their claims.The journalists contested these representations of their work, with the back and forth of accusations and countering claims devolving into a confusing and ultimately unresolvable incident that diverted public attention tomthe serious concerns initially raised.

Island News decided to hashlink future investigative reporting and associate them with a verifiable credential signed by the reporters involved. This was further substantiated by a verifiable endorsement credential issued by The Internation Consortium of Investigative Journalism (the TICIJ), corroborating by a respected third party the claims of authenticity derived bybtheor review of the investigative journalism process and specufic transcripts of notes, photographic evidence and recordings the Island News submitted to them and they independently analyzed.

Future investigative journalism reports contain the links to these credentials, publicly accessible, and verifiable for all who wish to see them.

### Social impact - philanthropic grant helped attesting individual (or witnessed)

When philanthropic organizations grant funds to help a target population, currently the grantee submits an impact report about the impact. The individuals intended to benefit are more or less passive objects. In the proposed model, the individuals intended to benefit from a grant could be asked to individually attest to the benefit received, as well as witnesses. This way bottom-up accountability can be achieved and the impact of philanthropy can be measured and receive feedback from a broad set of recipients and witnesses.

Linked claims will enable the many signed attestations to be credible as the signatories may have additional credentials linked to them. It also enables auditability as individuals who are surveyed can verify their signatures cryptographically.

### Hashlink Prior Art
A hashlink cryptographically binds a VC to another VC or object by including a digest hash of the liked object itself. For further information see these links.

- [Cryptographic Hyperlinks IETF Draft](https://datatracker.ietf.org/doc/html/draft-sporny-hashlink-07)
- [ACDC - Authentic Chained Data Containers Draft](https://www.ietf.org/archive/id/draft-ssmith-acdc-02.html)
- Adobe/Intel/etc [Content Authenticity Initiative](https://contentauthenticity.org/)

### Proposal
This paper introduces two new related properties: `anchoredResource` and `digestMultibase`.

#### Anchored Resource
An anchored resource points to one or more linked or bound resources. It can appear in the `credentialSubject` section (which implies that the resource is linked to the subject), or in the top level credential (same level as issuer, issuanceDate, etc), which implies that the resource is linked to the credential itself.

#### digestMultibase
A `digestMultibase` property provides a way to lock down a link using a hash of its content, by specifying the hash algorithm, and (optionally) the canonicalization mechanism to perform before hashing.

### Compound vs Single-assertion (Micro-) Credentials

explain, add picture -- the VC model in general, and the hash linking mechanism, enables a set of claims to be expressed either as standalone credentials, or as a single composed credential.

## Data Model

Format in development see [composable claims examples](https://github.com/WebOfTrustInfo/rwot11-the-hague/tree/master/draft-documents/vc-le/composable-sample1/)

## Mental Model

### Firsthand Observation

English: I witnessed an accident.
Subject: The accident
Claim: happened
Object: <something about the accident. For example, when & where it happened> -- this is a set of claims
Evidence: <I witnessed it firsthand>

{
  issuer: "Me", // person who witnessed the accident
  credentialSubject: {
    "id": "https://accidents.example.com/reports/accident-1",
    "statement": "I saw a collision on Elm St Sunday Oct 23rd, 2022.",
    "dateObserved": "Sunday",
    "location": "corner of Elm St"
  },
  evidence: [{
    "source": "firsthand witness"  
  }]
}

### Secondhand Observation

English: I heard Charles say that he witnessed the accident
Subject: The Accident
Claim: happened
Object: "(same as previous example - the accident happened)"
Evidence: <I heard Charles said that he witnessed accident>

{
  issuer: "Golda", // person who heard Charles say he witnessed the accident
  credentialSubject: {
    "id": "https://accidents.example.com/reports/accident-1",
    "statement": "Some stuff happened at Elm St on Sunday.",
    "dateObserved": "Sunday",
    "location": "corner of Elm St"
  },
  evidence: [
    {
      "type": "secondhand",  // evidence type
      "witness": {
        "id": "<charless homepage>"
        "name": "Charles"
      },
      "statement": "I heard Charles say this"
    },
    {
      "type": "firsthand"
      "witness": "Golda",
      "statement": "I heard a noise at such & such time."
    }
  ]
}

## Examples

### Standalone Claim - Review

Example of a linked claim representing a product review.

- References an external product (no hashlink)
- No claimant expertise
- No evidence section

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
  ],
  "type": [ "VerifiableCredential", "LinkedClaim" ],
  "issuer": {
    "id": "spider",
    "name": "spider"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "effectiveDate": "1970-01-01",  // core vc 2.0 see original
  "credentialSubject": {
    "id": "https://example.com/products/coffee-mug",

    "linkedClaim": {
      "type": "Review",
      "statement": "This is a great coffee mug, would buy again.",
      "review": {
        // "5-star" type review
        rating: 5,
        ratingMax: 5,
        ratingMin: 1
      } 
    }
  },
  "proof": { ... }
}

### Standalone Claim - Witnessing an Event
Example of a linked claim representing a witnessing of an event.

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
  ],
  "type": [ "VerifiableCredential", "LinkedClaim" ],
  "issuer": {
    "id": "did:web:bob.example.com",
    "name": "Bob"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "effectiveDate": "1970-01-01",  // core vc 2.0 see original
  "credentialSubject": {
    // The subject id here is the id of the **report**
    "id": "https://reports.example.com/accidents/123",
    "linkedClaim": {
      "type": "EventWitnessClaim",
      "statement": "I have witnessed a bicycle accident on the corner of Elm and East 3rd St.",
      "source": "firsthand"
    }
  },
  "proof": { ... }
}

### Example - A Verifiable Endorsement

This example is composed of two components -- an initial standalone VC, and then an _endorsement_ of that VC, with a one-way cryptographic binding to it.

**Initial (Self-issued) VC:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/openbadges/v3"
  ],
  "type": [
    "VerifiableCredential",
    "OpenBadgeCredential"
  ],
  "issuer": {
    "id": "did:key:z6MkrHKzgsahxBLyNAbLQyB1pcWNYC9GmywiWPgkrvntAZcj",
    "name": "Alice Jones"
  },
  "issuanceDate": "2022-05-01T00:00:00Z",
  "credentialSubject": {
    "type": "AchievementSubject",
    // Note that the subject of the VC is the issuer, hence self-issued
    "id": "did:key:z6MkrHKzgsahxBLyNAbLQyB1pcWNYC9GmywiWPgkrvntAZcj",
    "achievement": {
      "id": "urn:uuid:e8096060-ce7c-47b3-a682-57098685d48d",
      "type": "Achievement",
      "name": "UAV Control System for Drone Navigation",
      "description": "<description goes here>",
      "criteria": {
         "type": "Criteria",
         "narrative": "<narrative>"
       }
    }
  },
  "proof": {
    // Signature goes here
  }
}

**Endorsement of above VC:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
    "https://w3id.org/security/suites/ed25519-2020/v1"
  ],
  "type": [ "VerifiableCredential", "VerifiableEndorsement" ],
  "issuer": {
    "id": "did:web:bob.example.com",
    "name": "Bob"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "credentialSubject": {
    // Note that the credentialSubject.id is the id of an individual Achievement in the target VC
    // It could just as readily be the id of the VC, but the authors wanted to highlight that a section of the VC could be targeted
    "id": "urn:uuid:e8096060-ce7c-47b3-a682-57098685d48d",
    "digestMultibase": "zb1B1M6Bve5JEaNqeJSmuE", // digest of the Achievement being endorsed
    "endorsement": {
       "type": "Endorsement",
       "statement": "This is an endorsement regarding Alice's 'UAV Control System for Drone Navigation' achievement. Alice has an exceptional skill set as an UAV guidance control engineer. See also the attached evidence.",
       "endorser": {
          "id": "did:web:bob.example.com",  // MUST be same as issuer
          // the endorsing entity's bona fides, tailored to the specific endorsement area or claims on a per-use basis
          "relevance": [
            // Generic expertise claims (such as CV / resume / degrees)
            {
             "id": "https://SmartResume.com",
             "type": "SmartResumeProfile"
            },
            {
             "id": "https://linkedin.com/Bob",
             "type": "LinkedInProfile"
            },
            { 
             // link to a credential I received saying I have a degree to this subject
             "id": "https://example.edu/degrees/class-of-2021/bob",
             "name": "University Degree Credential"
            },
            {
             "id": "https://sigspatial.acm.org/members/12345",
             "description": 
                      "https://www.acm.org/special-interest-groups/sigs/sigspatial",
             "name": "SigSpatial Membership Credential"
            },
            // Specific expertise item tailored to the endorsement
            {
             "id": "https://example-journal.com/my-article.pdf",
              // optional hashlink (note that 'multibase' is a part of the in-progress 
              // IETF spec https://datatracker.ietf.org/doc/html/draft-multiformats-multibase
             "digestMultibase":"zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n",
             "name": "Control Systems in Unmanned Flight",
             "citation": "...",
             "description": "I have published an article in a peer-reviewed journal." 
            },
            // TODO: Add a CID / Ceramic link as an example.
         ]
      }
    }
  },
  "evidence": [
    {
      "id": "https://github.com/example-org/control-test-suite",
      "type": ["EndorsementEvidence"],
      "name": "Control System Test Suite",
      "description": "The code used to control a UAV delivering packages to an address.",
      "digestMultibase": "..."
    },
    {
       "id": "https://control-systems-journal.example.com/12345.pdf",
       "digestMultibase": "zQmdfRKkx7Uf8Rpr079Uh",
        "name": "Geopositioning in Control Systems",
        "citation": "...",
        "description": "A particularly insightful implementation of geopositioning with precision; I was very impressed with Alice's approach."
    }
  ],
  "proof": {
    // Signature goes here
  }
}

### Verifiable Endorsement - De-structured Into Linked Standalone VCs

This example is functionally identical to the previous (an initial standalone self-issued VC combined an _endorsement_ of that VC), except that it de-composes the parts of the endorsement into separate standalone VCs (that are still linked, cryptographically). These would typically be presented as a bundle, in a Verifiable Presentation:

1. A self-issued VC (a skill achievement badge) by Alice. Subject: Alice
2. An endorsement of the skill VC, issued by Bob. Subject: the ID of the VC #1.
3. A VC listing Bob's qualifications/bonafides (explaining why Bob is qualified to make the recommendation). Subject: The ID of the endorsement (#2)
4. A VC listing the _evidence_ behind Bob's endorsement. Subject: The ID of the endorsement (#2)

**#1 - Initial (Self-issued) VC:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3id.org/openbadges/v3"
  ],
  "type": [
    "VerifiableCredential",
    "OpenBadgeCredential"
  ],
  "issuer": {
    "id": "did:key:z6MkrHKzgsahxBLyNAbLQyB1pcWNYC9GmywiWPgkrvntAZcj",
    "name": "Alice Jones"
  },
  "id": "urn:uuid:f5a8b09a-424f-4aa1",
  "issuanceDate": "2022-05-01T00:00:00Z",
  "credentialSubject": {
    "type": "AchievementSubject",
    // Note that the subject of the VC is the issuer, hence self-issued
    "id": "did:key:z6MkrHKzgsahxBLyNAbLQyB1pcWNYC9GmywiWPgkrvntAZcj",
    "achievement": {
     "type": "Achievement",
     "name": "UAV Control System for Drone Navigation",
     "description": "<description goes here>",
     "criteria": {
        "type": "Criteria",
        "narrative": "<narrative>"
      }
    }
  },
  "proof": {
    // Signature goes here
  }
}

**#2 - Endorsement of VC #1:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
    "https://w3id.org/security/suites/ed25519-2020/v1"
  ],
  "id": "urn:uuid:4b4d-8d0f-0ad47cf4e64c",
  "type": [ "VerifiableCredential", "VerifiableEndorsement" ],
  "issuer": {
    "id": "did:web:bob.example.com",
    "name": "Bob"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "credentialSubject": {
    // Subject: the ID of VC #1
    "id": "urn:uuid:f5a8b09a-424f-4aa1",
    "digestMultibase": "zb1B1M6Bve5JEaNqeJSmuE", // digest of the VC being endorsed
    "linkedClaim": {
      "statement": "This is an endorsement regarding Alice's 'UAV Control System for Drone Navigation' achievement. Alice has an exceptional skill set as an UAV guidance control engineer. See also the attached evidence.",
    }
  },
  "proof": {
    // Signature goes here
  }
}

**#3 - Endorser Qualifications/Bonafides for Endorsement VC #2:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
    "https://w3id.org/security/suites/ed25519-2020/v1"
  ],
  "type": [ "VerifiableCredential", "VerifiableEndorsement" ],
  "issuer": {
    "id": "did:web:bob.example.com",
    "name": "Bob"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "credentialSubject": {
    // Subject: the ID of Endorsement VC #2
    "id": "urn:uuid:4b4d-8d0f-0ad47cf4e64c",
    "digestMultibase": "zb1B1M6Bve5JEaNqeJSmuE", // digest of the VC being referenced
    "endorsement": {
       "endorser": {
          "id": "did:web:bob.example.com",  // MUST be same as issuer
          // the endorsing entity's bona fides, tailored to the specific endorsement area or claims on a per-use basis
          "relevance": [
            // Generic expertise claims (such as CV / resume / degrees)
            {
             "id": "https://SmartResume.com",
             "type": "SmartResumeProfile"
            },
            {
             "id": "https://linkedin.com/Bob",
             "type": "LinkedInProfile"
            },
            { 
             // link to a credential I received saying I have a degree to this subject
             "id": "https://example.edu/degrees/class-of-2021/bob",
             "name": "University Degree Credential"
            },
            {
             "id": "https://sigspatial.acm.org/members/12345",
             "description": 
                      "https://www.acm.org/special-interest-groups/sigs/sigspatial",
             "name": "SigSpatial Membership Credential"
            },
            // Specific expertise item tailored to the endorsement
            {
             "id": "https://example-journal.com/my-article.pdf",
              // optional hashlink (note that 'multibase' is a part of the in-progress 
              // IETF spec https://datatracker.ietf.org/doc/html/draft-multiformats-multibase
             "digestMultibase":"zQmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n",
             "name": "Control Systems in Unmanned Flight",
             "citation": "...",
             "description": "I have published an article in a peer-reviewed journal." 
            },
            // TODO: Add a CID / Ceramic link as an example.
         ]
      }
    }
  },
  "proof": {
    // Signature goes here
  }
}

**#4 - Evidence for Endorsement #2:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://w3.id/vc/linked-claims" // <- TODO: decide on url
    "https://w3id.org/security/suites/ed25519-2020/v1"
  ],
  "id": "urn:uuid:4b4d-8d0f-0ad47cf4e64c",
  "type": [ "VerifiableCredential", "VerifiableEndorsement" ],
  "issuer": {
    "id": "did:web:bob.example.com",
    "name": "Bob"
  },
  "issuanceDate": "2010-01-01T00:00:00Z",
  "expirationDate": "2020-01-01T00:00:00Z",
  "credentialSubject": {
    // Subject: the ID of the endorsement #2
    "id": "urn:uuid:f5a8b09a-424f-4aa1",
    "digestMultibase": "zb1B1M6Bve5JEaNqeJSmuE", // digest of the VC being endorsed
        
    // OPTIONAL: Also hashlink to the original VC
    "anchoredResource": {
       "id": "<id of the original VC #1>",
       "digestMultibase": "<hash of the VC #1>"
    }
  },
  "evidence": [
    {
      "id": "https://github.com/example-org/control-test-suite",
      "type": ["EndorsementEvidence"],
      "name": "Control System Test Suite",
      "description": "The code used to control a UAV delivering packages to an address.",
      "digestMultibase": "..."
    },
    {
       "id": "https://control-systems-journal.example.com/12345.pdf",
       "digestMultibase": "zQmdfRKkx7Uf8Rpr079Uh",
        "name": "Geopositioning in Control Systems",
        "citation": "...",
        "description": "A particularly insightful implementation of geopositioning with precision; I was very impressed with Alice's approach."
    }
  ],
  "proof": {
    // Signature goes here
  }
}

### Verifying Linked VCs

To make this meaningful a verifier for a VP must validate that the composed documents are truly composed the way the business logic asserts; AND that linked documents are linked (by issuer/source) the way business logic asserts

A new 'verifyAnchoredResources' callback is needed for VC libraries.

## References

Replabs DAO: https://replabs.web.app/dao/tec
Replabs Twitter: https://replabs.web.app/twitter/1548015533710123010

### Relates to Advanced Readings

https://github.com/WebOfTrustInfo/rwot11-the-hague/blob/master/advance-readings/endorsements.md

https://github.com/WebOfTrustInfo/rwot11-the-hague/blob/master/advance-readings/a-minimal-approach-to-linked-trust-with-uncertainty.md

https://github.com/WebOfTrustInfo/rwot11-the-hague/blob/master/advance-readings/Multi-dimensional%20reputation%20systems%20using%20webs-of-trust.md

### Standards

https://www.w3.org/TR/vc-data-model-2.0/#dfn-verifiable-credentials

Note: effectiveDate is in a proposed modification to the standard

## TODO

- Decide on top-level credential type
- Decide on context url
- best way to express controlled/suggested vocabs?
- check with Oliver if he wants to flesh out the "### Reputation systems for social media platforms" usecase

- Verifier for correctly composed and correctly linked credentials

---
Syncs to -> https://github.com/WebOfTrustInfo/rwot11-the-hague/blob/master/draft-documents/verifiable-endorsements-from-linked-claims.md

Original draft at https://docs.google.com/document/d/1mgIKC3U3OaYtq2koht9aQwAgJIrAEFCHq3XJvUOie5o/edit#

Working Addenda

Contact @BRiIYS-0T9GKZAz7VgyEQg / @gvelez17 if you want to push a change from HackMD