---
tags: CWU
title: Notes on Linked Data Proofs
description: An overview of Linked Data Proofs
---

# Notes on Linked Data Proofs

## Table of Contents
[TOC]

## Introduction
Linked Data Proofs provide a mechanism to for ensuring the authenticity and integrity of Linked Data documents using mathematical proofs.

### Related Documents
- [Notes on JSON-LD](/QFzgME-VSSS1m6ySVXQ4Pg?edit)
- [Verifiable Credentials & Linked Data Proofs](/wEYKVHnbTo2UHfiWRqcWIA)

### Resources Used
- [Linked Data Proofs Specification](https://w3c-ccg.github.io/ld-proofs)
- [Security Vocabulary Specification](https://w3c-ccg.github.io/security-vocab)
- [Verifiable Credentials Implementation Guideline](https://w3c.github.io/vc-imp-guide)
- [Benefits of JSON-LD and LD-Proofs](https://w3c.github.io/vc-imp-guide/#benefits-of-json-ld-and-ld-proofs)

## The Purpose of Linked Data Proofs
Linked Data Proofs can be used to:
- Make statements without the loss of trust (e.g. VCs or social media posts)
- Authenticate an entity is identified by a certain identifier (e.g. DIDs)
- Delegate authorization for actions to remote environments (e.g. [ZCAP-LD](https://w3c-ccg.github.io/zcap-ld/))
- Agree to contracts (where agreement can be verified by other parties)
- Integrity protection (e.g. making document tamper-evident)

## Common Properties
This section describes the most common attributes of a Linked Data Proof.

### `type` (required)
Indicates the [proof type](https://w3c-ccg.github.io/ld-proofs/#dfn-proof-type) that is used. For example, an `Ed25519Signature2018` type indicates that the proof includes a digital signature produced by an `ed25519` cryptographic key.

### `proofPurpose` (required)
Indicates the purpose the proof was created for. It acts as a safeguard to prevent the proof from being misused for a purpose other than the one it was intended for. For example, a proof can be used for purposes of `authentication`, for asserting control of a Verifiable Credential (`assertionMethod`), and several others.

### `verificationMethod` (required)
Indicates when the proof was generated. Must be specified in the [ISO8601](https://www.iso.org/standard/40874.html) format.

### `domain` (optional)
A string value that specifies the operational domain of a digital proof. This could be an Internet domain name like example.com, an ad-hoc value such as mycorp-level3-access, or a very specific transaction value like 8zF6T$mqP.

### `proofValue` (required)
A valid representation of the proof value that can be used to verify the proof. This value is generated by the [proof algorithm](https://w3c-ccg.github.io/ld-proofs/#proof-algorithm). What representation is considered valid depends on the specified `verificationMethod`. More info can be found [here](https://w3c-ccg.github.io/security-vocab/#proofValue).

## Proof Sets
A proof set is simply an array of proof objects under the `proof` key. It's useful when the same data needs to be secured by multiple entities, but where the order of proods doesn't matter.

**For example:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "title": "Hello World!",
  "proof": [{
    "type": "Ed25519Signature2018",
    "proofPurpose": "assertionMethod",
    "created": "2019-08-23T20:21:34Z",
    "verificationMethod": "did:example:123456#key1",
    "challenge": "2bbgh3dgjg2302d-d2b3gi423d42",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  },
  {
    "type": "RsaSignature2018",
    "proofPurpose": "assertionMethod",
    "created": "2017-09-23T20:21:34Z",
    "verificationMethod": "https://example.com/i/pat/keys/5",
    "challenge": "2bbgh3dgjg2302d-d2b3gi423d42",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  }]
}

## Proof Chains
A proof chain is the same as a [proof set](#proof-sets), except that in a proof chain the order of proofs **does** matter. In this case the array is specified by the `proofChain` key (instead of `proof`).

**For example:**

{
  "@context": [
    "https://www.w3.org/2018/credentials/v1",
    "https://www.w3.org/2018/credentials/examples/v1"
  ],
  "title": "Hello World!",
  "proofChain": [{
    "type": "Ed25519Signature2018",
    "proofPurpose": "assertionMethod",
    "created": "2019-08-23T20:21:34Z",
    "verificationMethod": "did:example:123456#key1",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  },
  {
    "type": "RsaSignature2018",
    "proofPurpose": "assertionMethod",
    "created": "2017-09-23T20:21:34Z",
    "verificationMethod": "https://example.com/i/pat/keys/5",
    "domain": "example.org",
    "jws": "eyJ0eXAiOiJK...gFWFOEjXk"
  }]
}