Abstract

Among other things, the [[VC-DATA-MODEL-2]] specifies the models used for Verifiable Credentials, Verifiable Presentations, and explains the relationships between three parties: issuers, holders, and verifiers. Verifiability, extensibility, and semantic interoperability are critical pieces of functionality referenced throughout the [VC-DATA-MODEL-2]. This specification provides a mechanism to make use of a Credential Schema in Verifiable Credential, leveraging the existing Data Schemas concept.

Status of This Document

This document is experimental and is undergoing heavy development. It is inadvisable to implement the specification in its current form. An experimental implementation is available.

Introduction

This specification provides a mechanism for the use of JSON Schemas with Verifiable Credentials. A significant part of the integrity of a Verifiable Credential comes from the ability to structure its contents so that all three parties — issuer, holder, verifier — may have a consistent mechanism of trust in interpreting the data that they are provided with. We introducing a new data model for an object to facilitate backing Credentials with JSON Schemas that we call a Credential Schema.

This specification provides a standardized way of creating Credential Schemas to be used in credentialing systems. Credential Schemas may apply to any portion of a Verifiable Credential. Multiple JSON Schemas may back a single Verifiable Credential, e.g. a schema for the `credentialSubject` and another for other credential properties.

Terminology

Data Model

The following sections outline the data models for this document, of which there are two: JsonSchema2023 for usage of a [[JSON-Schema]] directly in a credentialSchema property, and CredentialSchema2023 for usage of a [[JSON-Schema]] represented as a verifiable credential.

JsonSchema2023

This term definition is https://www.w3.org/ns/credentials#JsonSchema2023.

JsonSchema2023 is for the validation of W3C Verifiable Credentials, based on JSON Schema. The version of [[JSON-Schema]] can be any version noted in the section on JSON Schema Specifications.

Property Description
id The constraints on the id property are listed in the Verifiable Credentials Data Model specification [[VC-DATA-MODEL-2]]. The value MUST be a URL that identifies the schema associated with the verifiable credential.
type The type property MUST be JsonSchema2023.

An example of utilizing the VC Data Model's credentialSchema is provided below:
               {
                 "@context": [
                   "https://www.w3.org/ns/credentials/v2",
                   "https://www.w3.org/ns/credentials/examples/v2"
                 ],
                 "id": "https://example.com/credentials/3732",
                 "type": ["VerifiableCredential", "EmailCredential"],
                 "issuer": "https://example.com/issuers/14",
                 "issuanceDate": "2010-01-01T19:23:24Z",
                 "credentialSubject": {
                   "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
                   "emailAddress": "subject@example.com"
                 },
                 "credentialSchema": {
                   "id": "https://example.com/schemas/email.json",
                   "type": "JsonSchema2023"
                 }
               }
              

              {
                "$id": "https://example.com/schemas/email.json",
                "$schema": "https://json-schema.org/draft/2020-12/schema",
                "name": "EmailCredential",
                "description": "EmailCredential using JsonSchema2023",
                "type": "object",
                "properties": {
                  "credentialSubject": {
                    "type": "object",
                    "properties": {
                      "emailAddress": {
                        "type": "string",
                        "format": "email"
                      }
                    },
                    "required": [
                      "emailAddress"
                    ]
                  }
                }
              }
              

CredentialSchema2023

This term definition is https://www.w3.org/ns/credentials#CredentialSchema2023.

CredentialSchema2023 is used for the validation of W3C Verifiable Credentials, based on representing JSON Schema in a verifiable credential. The version of [[JSON-Schema]] can be any version noted in the section on JSON Schema Specifications.

Property Description
id The constraints on the id property are listed in the Verifiable Credentials Data Model specification [[VC-DATA-MODEL-2]]. The value MUST be a URL that identifies the verifiable credential which contains a credential schema.
type The type property MUST be CredentialSchema2023

An example of utilizing the VC Data Model's credentialSchema is provided below:
               {
                 "@context": [
                   "https://www.w3.org/ns/credentials/v2",
                   "https://www.w3.org/ns/credentials/examples/v2"
                 ],
                 "id": "https://example.com/credentials/3733",
                 "type": ["VerifiableCredential", "EmailCredential"],
                 "issuer": "https://example.com/issuers/14",
                 "issuanceDate": "2010-01-01T19:23:24Z",
                 "credentialSubject": {
                   "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
                   "emailAddress": "subject@example.com"
                 },
                 "credentialSchema": {
                   "id": "https://example.com/credentials/3734",
                   "type": "CredentialSchema2023"
                 }
               }
              

              {
                "@context": [
                    "https://www.w3.org/ns/credentials/v2",
                    "https://www.w3.org/ns/credentials/examples/v2"
                ],
                "id": "https://example.com/credentials/3734",
                "type": ["VerifiableCredential", "CredentialSchema2023"],
                "issuer": "https://example.com/issuers/14",
                "issuanceDate": "2010-01-01T19:23:24Z",
                "credentialSubject": {
                  "$id": "https://example.com/schemas/email-credential-schema.json",
                  "$schema": "https://json-schema.org/draft/2020-12/schema",
                  "name": "EmailCredential",
                  "description": "EmailCredential using CredentialSchema2023",
                  "type": "object",
                  "properties": {
                    "credentialSubject": {
                      "type": "object",
                      "properties": {
                        "emailAddress": {
                          "type": "string",
                          "format": "email"
                        }
                      },
                      "required": ["emailAddress"]
                    }
                  }
                }
              }
              

Add language about CredentialSchema2023 credentials having a credentialSchema property and the risks of nesting.

JSON Schema Specifications

The following section describes the allowed specifications for using a [[JSON-Schema]] with a credential schema.

JSON Schema Specification Date of Publication References
draft-bhutton-json-schema-01 10 June 2022 JSON Schema: A Media Type for Describing JSON Documents
JSON Schema Validation: A Vocabulary for Structural Validation of JSON
Relative JSON Pointers
Draft 2020-12, draft-bhutton-json-schema-00 11 June 2021 JSON Schema: A Media Type for Describing JSON Documents
JSON Schema Validation: A Vocabulary for Structural Validation of JSON
Relative JSON Pointers
Draft 2019-09, Draft 8 19 March 2020 JSON Schema: A Media Type for Describing JSON Documents
JSON Schema Validation: A Vocabulary for Structural Validation of JSON
JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON

Write considerations for normatively referencing JSON Schema.

Write section on using different representations of JSON Schema, such as YAML.

Write section on implementation considerations, such as using name, description and reserved keywords such as $id and $schema.

Processing

This section details instructions on how to process Credential Schemas.

Integrity Validation

Credential Schemas MAY be packaged as verifiable credentials as defined by usage of the CredentialSchema2023 type. The credential containing a credential schema may include a proof, either embedded according to [[VC-DATA-INTEGRITY]] or packaged as a [[VC-JWT]].

Secured credentials representing credential schemas SHOULD first be validated according to the rules set out in the aforementioned securing specifications before proceeding with additional processing.

Provide examples for Data Integrity and VC-JWT Credential Schemas

Evaluation

Validation of a given Credential against its schema is to be performed according to its associated [[JSON-SCHEMA]] specification. If validation succeeds the credential is considered to be valid against its credential schema.

Implementation Considerations

This section details some issues implementers of the specification may consider.

Validation of Specific Properties in a Credential

Implementers may wish to validate certain sections of a verifiable credential. To do this, credential schemas can be constructed to specify application to subsets of a given credential.

One example of such a construction would be to validate the presence of certain top-level properties in a verifiable credential. The following example demonstrates a schema which enforces that a credential issued against it has an validUntil property and includes evidence.

            {
              "$id": "validuntil-and-evidence-schema",
              "$schema": "https://json-schema.org/draft/2020-12/schema",
              "description": "Schema requiring validUntil and evidence properties",
              "type": "object",
              "properties": {
                "validUntil": {
                  "type": "object"
                },
                "evidence": {
                  "type": "object"
                }
              },
              "required": ["validUntil", "evidence"]
            }
            

Additional Properties

In using [[JSON-Schema]] it is advised that implementers avoid setting the additionalProperties to false. Doing so could inadvertently exclude properties in a credential from passing validation.

As an example, consider a credential schema that is intended to validate the credentialSubject property of a credential. It is common for the credentialSubject property to include an id, denoting the identifier the subject. Not including this id property in a given schema would result in validation failure. The simple alternative is to avoid setting additionalProperties to false.

            {
              "$id": "name-schema",
              "$schema": "https://json-schema.org/draft/2020-12/schema",
              "description": "Name",
              "type": "object",
              "properties": {
                "credentialSubject": {
                  "type": "object",
                  "properties": {
                    "name": {
                      "type": "object",
                      "properties": {
                        "firstName": {
                          "type": "string"
                        },
                        "lastName": {
                          "type": "string"
                        },
                        "additionalProperties": false
                      },
                      "required": [
                        "firstName",
                        "lastName"
                      ]
                    }
                  }
                }
              }
            }
            

Versioning

Versioning is not provided as an explicit feature of this specification. It is advised to make backwards compatabile changes to schemas, should they be adjusted. Otherwise, it is advised that a new credential schema is created with a unique identifier.

Add language on enabling schema immutability/versioning, such as making use of hashlinks.

Storage

Credential schemas MAY be created and made available as immutable objects. They can be stored on any number of storage mediums such as a distributed ledger, traditional database, or decentralized file storage. The same schema could be replicated across multiple file stores with the same identifier.

Immutability is key to enabling a consistent source of truth for usage with verifiable credentials which are tamper-evident by design.

Resolution

Add section on schema resolution.

Multiple Schemas

A common use case is to include multiple schemas to validate against which a single verifiable Credential. One such use case is to utilize the JSON Schema defined by the [[VC-DATA-MODEL-2]] in addition to a schema to validate a specific property in the credential, such as the credentialSubject. Multiple schemas MAY be combined using native constructs from the [[JSON-SCHEMA]] specification, through utilizing properties such as oneOf, anyOf, or allOf.

An example of how to construct such a schema using the [[JSON-SCHEMA]] property allOf is provided below, combining schemas for a verifiable credential, name, and email address:

            {
              "allOf": [
                {
                  "$ref": "https://raw.githubusercontent.com/w3c/vc-data-model/main/schema/verifiable-credential/verifiable-credential-schema.json"
                },
                {
                  "$id": "name-schema",
                  "$schema": "https://json-schema.org/draft/2020-12/schema",
                  "description": "Name",
                  "type": "object",
                  "properties": {
                    "credentialSubject": {
                      "type": "object",
                      "properties": {
                        "name": {
                          "type": "object",
                          "properties": {
                            "firstName": {
                              "type": "string"
                            },
                            "lastName": {
                              "type": "string"
                            },
                            "additionalProperties": false
                          },
                          "required": [
                            "firstName",
                            "lastName"
                          ]
                        }
                      }
                    }
                  }
                },
                {
                  "$id": "email-schema-1.0",
                  "$schema": "https://json-schema.org/draft/2020-12/schema",
                  "description": "Email",
                  "type": "object",
                  "properties": {
                    "credentialSubject": {
                      "type": "object",
                      "properties": {
                        "email": {
                          "type": "object",
                          "properties": {
                            "emailAddress": {
                              "type": "string",
                              "format": "email"
                            }
                          },
                          "required": ["emailAddress"]
                        }
                      }
                    }
                  }
                }
              ]
            }
            

The example above is used to validate every property in the following verifiable credential:

            {
                "@context": ["https://www.w3.org/ns/credentials/v2"],
                "id": "4995c86c-851f-43a6-9dd2-03dc891091fd",
                "type": ["VerifiableCredential"],
                "issuer": "did:example:1234",
                "validFrom": "2023-01-01T05:05:05Z",
                "credentialSubject": {
                    "firstName": "Alice",
                    "lastName": "Bobertson",
                    "emailAddress": "alice@bobertson.com"
                },
                "credentialSchema": {
                    "id": "multiple-credential-schema-test",
                    "type": "CredentialSchema2023"
                },
                "proof": { ... }
            }
            

Add warning for improperly formed schemas and risks associated with using multiple schemas.

Validity of a Verifiable Credential

Validation against a [[JSON-SCHEMA]] may be confused with validation or verification of a Verifiable Credential. A valid credential according to a [[JSON-SCHEMA]] refers only to the structure of the claims comprising a Verifiable Credential. This idea of validity does not imply anything about the validity of the Verifiable Credential itself. It's possible for a Verifiable Credential to be considered valid by one verifier, while another verifier would not consider it valid.

Relationship to Verifiable Credential Type Property

It is common to define a credential schema that will be set for Verifiable Credentials whose type property contains a specific type. In this scenario, it is advised to use the value of the specific type in the id or in a name or description property. of a [[JSON-Schema]].

The example below illustrates this for EmailCredential:

              {
                "@context": [
                  "https://www.w3.org/ns/credentials/v2",
                  "https://www.w3.org/ns/credentials/examples/v2"
                ],
                "id": "https://example.com/credentials/email-credential",
                "type": ["VerifiableCredential", "EmailCredential"],
                "issuer": "https://example.com/issuers/14",
                "issuanceDate": "2010-01-01T19:23:24Z",
                "credentialSubject": {
                  "id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
                  "emailAddress": "tester@example.com"
                },
                "credentialSchema": {
                  "id": "https://example.org/examples/email.json",
                  "type": "JsonSchema2023"
                }
              }
            

              {
                "$id": "https://example.com/schemas/email.json",
                "$schema": "https://json-schema.org/draft/2020-12/schema",
                "name": "EmailCredential",
                "description": "Email Credential Schema for usage in JsonSchema2023",
                "type": "object",
                "properties": {
                  "credentialSubject": {
                    "type": "object",
                    "properties": {
                      "emailAddress": {
                        "type": "string",
                        "format": "email"
                      }
                    },
                    "required": [
                      "emailAddress"
                    ]
                  }
                }
              }
            

It is important to note that a credential schema enables issuers to communicate how to process the structure of data inside a verifiable credential, whereas the type property of a verifiable credential lets issuers communicate the semantics of the data. It is advised to associate all properties that have a semantic mapping with a property in a credential schema.

Privacy Considerations

This section details the general privacy considerations and specific privacy implications of deploying this specification into production environments.

Verifier Caching

Since schemas are immutable, they are highly cachable. It is possible for verifiers to increase the privacy of the holder whose verifiable credential is being checked by caching schemas that have been fetched from remote servers. By caching the content locally, less correlatable information can be inferred from verifier-based access patterns on the schema.

Content Distribution Networks

The use of content distribution networks by issuers can increase the privacy of holders by reducing or eliminating requests for the schemas lists from the issuer. Often, a request for a schema list will be served by an edge device and thus be faster and reduce the load on the server as well as cloaking verifiers and holders from issuers.

Security Considerations

There are a number of security considerations that implementers should be aware of when processing data described by this specification. Ignoring or not understanding the implications of this section can result in security vulnerabilities.

Issuer Impersonation

It is possible for a schema to become authoritative, such as schemas provided by a recognized industry group like a consoritum of financial companies. To avoid confusion as to the authorship of credential schemas it is advised that they are packaged as secured verifiable credentials.

Accessibility Considerations

There are a number of accessibility considerations implementers should be aware of when processing data described in this specification. As with any web standards or protocols implementation, ignoring accessibility issues makes this information unusable to a large subset of the population. It is important to follow accessibility guidelines and standards, such as [[WCAG21]], to ensure all people, regardless of ability, can make use of this data. This is especially important when establishing systems utilizing cryptography, which have historically created problems for assistive technologies.

This section details the general accessibility considerations to take into account when utilizing this data model.

Write accessibility considerations.

Internationalization Considerations

There are a number of internationalization considerations implementers should be aware of when publishing data described in this specification. As with any web standards or protocols implementation, ignoring internationalization makes it difficult for data to be produced and consumed across a disparate set of languages and societies, which would limit the applicability of the specification and significantly diminish its value as a standard.

This section outlines general internationalization considerations to take into account when utilizing this data model.

Write i18n considerations.