Prolog VC Tools

This library provides a set of tools for converting Verifiable Credentials (VCs) into Prolog facts and rules. This allows for expressive and powerful policy-based access control and other logic-based systems to be built on top of a verifiable data model.

Docs

How it Works

The library takes a VC as input, validates it against the corresponding schema, and then converts it into a Prolog fact or rule. These facts and rules can then be asserted into a Prolog engine to build a knowledge base.

Example: Asserting a Person

Here is the schema for a VC's credentialSubject for a person:

{
  "title": "Person",
  "type": "object",
  "properties": {
    "claimType": {
      "type": "string",
      "const": "person"
    },
    "id": { "type": "string", "description": "Person's ID" },
    "updateView": {
      "type": "string",
      "enum": ["assert", "retract", "assertz", "asserta"],
      "default": "assert"
    }
  },
  "required": ["claimType", "id", "updateView"]
}
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So following that schema we can construct this VC:

{
  "credentialSubject": {
    "claimType": "person",
    "id": "person1",
    "updateView": "assert"
  }
}
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And pass to the extractPrologStatement function which will return the following Prolog fact:

assert(person(person1)).
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Example: Asserting a Rule

Here is the schema for a VC's credentialSubject for a rule.

{
  "title": "Rule Definition (Generalized Boolean Logic)",
  "type": "object",
  "properties": {
    "claimType": {
      "type": "string",
      "const": "rule"
    },
    "name": { "type": "string" },
    "evaluate": { "$ref": "#/$defs/logicNode" },
    "variables": {
      "type": "array",
      "items": { "type": "string" }
    },
    "returns": { "enum": ["boolean"] },
    "updateView": {
      "type": "string",
      "enum": ["assert", "retract"],
      "default": "assert"
    }
  },
  "required": [
    "claimType",
    "name",
    "evaluate",
    "variables",
    "returns",
    "updateView"
  ],

  "$defs": {
    "logicNode": {
      "description": "A logical expression node (predicate or combinator)",
      "type": "object",
      "oneOf": [
        {
          "description": "A predicate (atomic condition)",
          "properties": {
            "predicate": { "type": "string" },
            "args": {
              "type": "array",
              "items": { "type": "string" }
            }
          },
          "required": ["predicate", "args"],
          "additionalProperties": false
        },
        {
          "description": "Logical AND combinator",
          "properties": {
            "and": {
              "type": "array",
              "items": { "$ref": "#/$defs/logicNode" },
              "minItems": 1
            }
          },
          "required": ["and"],
          "additionalProperties": false
        },
        {
          "description": "Logical OR combinator",
          "properties": {
            "or": {
              "type": "array",
              "items": { "$ref": "#/$defs/logicNode" },
              "minItems": 1
            }
          },
          "required": ["or"],
          "additionalProperties": false
        },
        {
          "description": "Logical NOT combinator",
          "properties": {
            "not": { "$ref": "#/$defs/logicNode" }
          },
          "required": ["not"],
          "additionalProperties": false
        }
      ]
    }
  }
}
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Which is a bit complicated but lets us chain and embed multiple rules with ANDs, ORs and NOTs.

So following that schema we can construct this VC:

{
  "credentialSubject": {
    "claimType": "rule",
    "name": "my_rule",
    "evaluate": {
      "and": [
        { "predicate": "p", "args": ["a"] },
        { "predicate": "q", "args": ["b"] }
      ]
    },
    "variables": ["X"],
    "returns": "boolean",
    "updateView": "assert"
  }
}
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And pass to the extractPrologStatement function which will return the following Prolog fact:

assert(my_rule(X) :- (p(a), q(b))).
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Core Entities

The library is built around a set of schemas that define the structure of the VCs. These schemas represent the core concepts of a system that could be built using this library. Below are the categories of statements that get constructed using the VCs that implement the corresponding schema.

Person

• Person: Represents a user in the system.

assert(person(personId)).
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• Person Custom Property: Allows for adding arbitrary properties to a person.

assert(person_custom_property(person1, age, 30)).
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Group

• Group: Represents a collection of people.

assert(group(exampleGroupId)).
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• Group Custom Property: Allows for adding arbitrary properties to a group.

assert(group_custom_property(group1, department, engineering)).
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Entity

• Entity Represents a more abstract entity.

assert(entity(entityId)).
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• Entity Custom Property: Allows for adding arbitrary properties to an entity.

assert(entity_custom_property(entityId, property, value)).
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Entity Group

• Entity Group: Reresents a collection of entities.

assert(entity_group(entityGroupId)).
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• Entity Group Custom Property: Allows for adding arbitrary properties to an entity group.

assert(entity_group_custom_property(entity_group_id, property, value)).
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Relations

These are pre-defined rules that provide relations between our existing entities.

• Person Belongs to Group: A relationship that links a person to a group.

assert(person_belongs_to_group(person1, group1))."
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• Resource Owned By Person: A relationship that indicates a person owns a resource.

assert(resource_owned_by_person(resource1, person1)).
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• Resource Shared With Person: A relationship that indicates a resource is shared with a person.

assert(resource_shared_with_person(sharer1, resource1, person1)).
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• Resource Shared With Group: A relationship that indicates a resource is shared with a group.

assert(resource_shared_with_group(sharer1, resource1, group1)).
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• Resource Contained In: A relationship that links a resource to a folder.

assert(resource_contained_in(resource1, folder1)).
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• Custom Relation: A relationship that links a resource to a folder.

assert(trust(person, resource)).
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• Entity Belongs to Entity Group

assert(entity_belongs_to_entity_group(entityID, entityGroupID)).
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Resource

• Resource: A generic entity that can be accessed.

assert(resource(resource1)).
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• File: A specific type of resource.

assert(file(resource1)).
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• Folder: A container for other resources.

assert(folder(resource1)).
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Rules

• Rule: Defines a custom rule using a JSON-based logic structure. Supports conjunction, disjunction and negation of other rules.

assert(my_rule(X) :- (p(a), q(b))).
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• Rule Custom: Defines a custom rule using raw Prolog.

assert(my_rule(X) :- a(X), b(X)).
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Query

• Query: Represents a query to the Prolog system.

parent(john, mary).
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• Query Custom: Represents a custom query using raw Prolog.

parent(john, X).
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Claim Types

The claimType property in the credential subject of a VC is used to determine which schema to validate against and how to generate the Prolog statement. The possible values for claimType are defined by the ClaimType enum:

Person

Claim Type	Description
person	Represents a user in the system.
person_custom_property	A custom property for a person.

Group

Claim Type	Description
group	Represents a group of people.
group_custom_property	A custom property for a group.

Entity

Claim Type	Description
entity	Represents a entity in the system.
entity_custom_property	A custom property for an entity.

Relations

Claim Type	Description
person_belongs_to_group	Links a person to a group.
resource_owned_by_person	Indicates a person owns a resource.
resource_shared_with_group	Indicates a resource is shared with a group.
resource_shared_with_person	Indicates a resource is shared with a person.
resource_contained_in	Links a resource to a folder.
entity_belongs_to_entity_group	Links an entity to an entity group.
relation_custom	A custom relationship.

Resource

Claim Type	Description
resource	A generic entity that can be accessed.
file	Labels a resource as a file.
folder	Labels a resource as a folder.

Rules

Claim Type	Description
rule	Defines a custom rule using a JSON-based logic structure.
rule_custom	Defines a custom rule using raw Prolog.

Query

Claim Type	Description
query	Represents a query to the Prolog system.
query_custom	A custom query using raw Prolog.

Installation

In a Node.js project, run npm i prolog-vc-tools.

Usage

The primary function of this library is extractPrologStatement, which takes a VC that conforms to a supported schema and returns a Prolog statement.

import { extractPrologStatement } from "prolog-vc-tools";

const vc = {
  credentialSubject: {
    claimType: "person",
    id: "person1",
    updateView: "assert",
  },
};

const { fact, type } = extractPrologStatement(vc);

console.log(fact); // assert(person(person1)).
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You can also pass an updateView argument which will override any updateView set in the credentialSubject.

import { extractPrologStatement, UpdateView } from "prolog-vc-tools";

const vc = {
  credentialSubject: {
    claimType: "person",
    id: "person1",
    updateView: "assert",
  },
};

const { fact, type } = extractPrologStatement(vc, UpdateView.Retract);

console.log(fact); // retract(person(person1)).
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The library also provides a function isStringValidTerm to check if a string is a valid Prolog term.

import { isStringValidTerm } from "prolog-vc-tools";

const isValid = await isStringValidTerm("person(person1).");

console.log(isValid); // true
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Extra Resources

Demo Applications

An application with multiple example implementations of systems using this library can be found here: policy-use-cases.

A demo application that implements this library can be found here: prolog-vc-tools-app.

Schemas

Full schemas can be found here: GitHub.

Example VCs

Example VCs implementing those schemas can be found here: Github.