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Presentation Definition

If you accept verifiable credentials, you'll want to define what sort of information is necessary to meet the requirements. This specification of expected claims is a Presentation Definition (PD).

For example, assume an Employer accepts verifiable credentials as part of a job application. The presentation definition would specify exactly what the Employer will look for in those credentials. This could include things like proof of a degree, past employment history, and maybe a specific certification.

They're essentially saying, "To consider your application, we need to see these specific pieces of information."

Designing a Presentation Definition

An employer decides that an applicant must provide proof of:

• employment history
• degree in a field related to Engineering, Computer Science, or Security
• Certified Ethical Hacker certification issued by a specific trusted organization

Example Presentation Definition

To represent these requirements, the employer creates the PD.

Presentation Definition

{
  "id": "PD_JobApplication_123456",
  "name": "Credentials Verification for Ethical Hacker Job Application",
  "purpose": "To verify the applicant's employment history, and either their academic degree or Certified Ethical Hacker certification",
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
      "constraints": {
        "fields": [
          {
            "path": ["$.type[*]"],
            "filter": {
              "type": "string",
              "pattern": "Employment"
            }
          }
        ]
      }
    },
    {
      "id": "degreeVerification",
      "name": "Degree",
      "purpose": "Confirm the applicant's academic qualification",
      "constraints": {
        "fields": [
          {
            "path": ["$.vc.credentialSubject.degree.type"],
            "filter": {
              "type": "string",
              "pattern": "(Engineering|Computer|Cyber|Security)"
            }
          }
        ]
      }
    },
    {
      "id": "CEH_CertificationVerification",
      "name": "Certified Ethical Hacker Certification",
      "purpose": "Confirm the applicant holds a Certified Ethical Hacker certification",
      "constraints": {
        "fields": [
          {
            "path": ["$.vc.credentialSubject.certifications[*].name"],
            "filter": {
              "type": "string",
              "pattern": "Certified Ethical Hacker"
            }
          },
          {
            "path": ["$.issuer"],
            "filter": {
              "type": "string",
              "const": "did:example:123456789abcdefghi"
            }
          }
        ]
      }
    }
  ]
}

Properties

The following are the top-level properties of a PD:

• id: A required unique identifier that distinguishes this PD from others
• name: An optional, human-friendly name for easier identification of the PD
• purpose: An optional description outlining why the information requested by the PD is needed
• format: The optional technical format requirements for the data to be submitted. Example: { "jwt_vc": {"alg": ["ES256K", "ES384"]} }
• input_descriptors: This is the most important property of the PD. It details the exact data requirements

info

For more in-depth information on the schema and options, refer to the Presentation Definition spec.

Breakdown of PD

Let's break down the creation of each part:

id

To keep the IDs unique, the Employer uses a format within their system: 'PD_JobApplication' followed by the ID of the job req. This allows them to easily match PDs with their open roles.

{
  "id": "PD_JobApplication_123456",
}

name and purpose

The name and purpose are clear and human-readable, as they may be presented to the applicant when they need to decide if to share their credentials with the employer.

{
  "id": "PD_JobApplication_123456",
  "name": "Credentials Verification for Ethical Hacker Job Application",
  "purpose": "To verify the applicant's employment history, and either their academic degree or Certified Ethical Hacker certification",
}

input_selectors

The input_selectors property is an array of required claims and specifications on exactly how they will be evaluated.

{
  "input_descriptors": [

  ]
}

In this first input descriptor, the Employer provides an id for this descriptor.

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
    }
  ]
}

The name and purpose should be human-readable as they may be shown to the user.

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
    }
  ]
}

constraints

The constraints object contains the exact test that the VC will need to pass in order to meet the requirement.

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
      "constraints": {

      }
    },
  ]
}

fields

Under constraints is an array of fields objects, each object representing a specific piece of data that the Employer is interested in. Each field can have its own constraints.

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
      "constraints": {
        "fields": [

        ]
      }
    },
  ]
}

path

The path key specifies the location within the VC where the data is expected to be found. Here, $.type[*] indicates that the Employer will verify the type attribute of the submitted VCs to see if any of its values match the filter.

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
      "constraints": {
        "fields": [
          {
            "path": ["$.type[*]"],
            "filter": {

            }
          }
        ]
      }
    },
  ]
}

filter

The filter provides the specific conditions that the data in the specified path must satisfy. Here, it's saying that the expected data type of the field is a string, and the pattern property indicates that the value should match the provided regular expression.

In this case, the Employer expects the applicant to submit a VC whose type includes the word "Employment".

{
  "input_descriptors": [
    {
      "id": "employmentHistoryVerification",
      "name": "Employment History",
      "purpose": "Verify the applicant's previous employment experiences",
      "constraints": {
        "fields": [
          {
            "path": ["$.type[*]"],
            "filter": {
              "type": "string",
              "pattern": "Employment"
            }
          }
        ]
      }
    },
  ]
}

info

The filter property offers a lot of flexibility. Refer to the Presentation Definition spec for more examples.

The next input descriptor describes that if a degree VC is provided, it must be a degree with one of the following terms in the title: Engineering, Computer, Cyber, or Security.

If someone has a degree in Biology, for example, that won't satisfy this requirement.

    {
      "id": "degreeVerification",
      "name": "Degree",
      "purpose": "Confirm the applicant's academic qualification",
      "constraints": {
        "fields": [
          {
            "path": ["$.credentialSubject.degree.type"],
            "filter": {
              "type": "string",
              "pattern": "(Engineering|Computer|Cyber|Security)"
            }
          }
        ]
      }
    },

This Employer also requires a Ethical Hacking certification from a specific reputable institution.

For this credential, there's two fields that the Employer wants to add constraints for: the name of the certification, and the organization that issued the credential.

Notice the filter for the certification's name uses pattern so that it checks the name with a regular expression. This gives flexibility in case the certification is for "Advanced Certified Ethical Hacker" or "Certified Ethical Hacker II", etc.

But the filter for the issuer uses const to indicate that the issuer's DID must match this value exactly.

    {
      "id": "CEH_CertificationVerification",
      "name": "Certified Ethical Hacker Certification",
      "purpose": "Confirm the applicant holds a Certified Ethical Hacker certification",
      "constraints": {
        "fields": [
          {
            "path": ["$.credentialSubject.certifications[*].name"],
            "filter": {
              "type": "string",
              "pattern": "Certified Ethical Hacker"
            }
          },
          {
            "path": ["$.issuer"],
            "filter": {
              "type": "string",
              "const": "did:example:123456789abcdefghi"
            }
          }
        ]
      }
    }

For the issuer, the Employer only accepts VCs from the top trusted source who issues these certifications, so they enter the organization's DID as a constraint. If an applicant provides a Ethical Hacker Certification that was issued by anyone other than this organization, that VC will not meet the requirements.

    {
      "path": ["$.issuer"],
      "filter": {
        "type": "string",
        "const": "did:example:123456789abcdefghi"
      }
    }

Validate the Presentation Definition

Once the PD is created, you can validate it to ensure there are no errors in its design.

1. import classes needed for a Presentation Exchange

JavaScript

Kotlin

Swift

import { PresentationExchange } from '@web5/credentials';

2. call validateDefinition()

This assumes the presentation definition is assigned to variable pd

JavaScript

Kotlin

Swift

If there are errors with the PD, an error will be thrown:

This is not a valid PresentationDefinition

Example Credentials

For your reference, below are example VCs that could be used to satisfy the Employment Application PD.

Employment at Tech Inc

{
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1"
    ],
    "type": [
      "VerifiableCredential",
      "EmploymentCredential"
    ],
    "id": "urn:uuid:d377379b-3699-44b7-b15c-0831a849ff7c",
    "issuer": "did:example:techinc",
    "issuanceDate": "2024-01-05T17:43:45Z",
    "credentialSubject": {
      "id": "did:example:alice",
      "employmentHistory": {
        "type": "Employment",
        "position": "Software Engineer",
        "company": "Tech Inc.",
        "startDate": "2019-01-01",
        "endDate": "2021-12-31"
      }
    }
  },
  "iss": "did:example:techinc",
  "sub": "did:example:alice"
}

Employment at Innovate LLC

{
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1"
    ],
    "type": [
      "VerifiableCredential",
      "EmploymentCredential"
    ],
    "id": "urn:uuid:2d80375b-834c-4817-a79b-33f1fe3cd80a",
    "issuer": "did:example:innovatellc",
    "issuanceDate": "2024-01-05T17:43:45Z",
    "credentialSubject": {
      "id": "did:example:alice",
      "employmentHistory": {
        "type": "Employment",
        "position": "Project Manager",
        "company": "Innovate LLC",
        "startDate": "2022-01-03",
      }
    }
  },
  "iss": "did:example:innovatellc",
  "sub": "did:example:alice"
}

Computer Science Degree

{
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1"
    ],
    "type": [
      "VerifiableCredential",
      "EducationalCredential"
    ],
    "id": "urn:uuid:a09b7d2d-c7ac-42de-9230-05e7fd894692",
    "issuer": "did:example:stateuniversity",
    "issuanceDate": "2024-01-05T17:43:45Z",
    "credentialSubject": {
      "id": "did:example:alice",
      "degree": {
        "type": "Bachelor of Computer Science",
        "university": "State University",
        "graduationYear": "2018"
      }
    }
  },
  "iss": "did:example:stateuniversity",
  "sub": "did:example:alice"
}

Certified Ethical Hacker

{
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1"
    ],
    "type": [
      "VerifiableCredential",
      "CertificationCredential"
    ],
    "id": "urn:uuid:7fba00eb-2f47-4113-aea6-4c5b5799f95f",
    "issuer": "did:example:123456789abcdefghi",
    "issuanceDate": "2023-05-01T16:06:00Z",
    "credentialSubject": {
      "id": "did:example:alice",
      "certifications": [
        {
          "name": "Certified Ethical Hacker",
          "date": "2023-05-01"
        }
      ]
    }
  },
  "iss": "did:example:123456789abcdefghi",
  "sub": "did:example:alice"
}