title: "Ethr DID Resolver" index: 7 category: "ethr-did-resolver" type: "reference" source: "https://github.com/decentralized-identity/ethr-did-resolver/blob/develop/README.md"

ethr DID Resolver

This library is intended to use ethereum addresses as fully self managed Decentralized Identifiers and wrap them in a DID Document

It supports the proposed Decentralized Identifiers spec from the W3C Credentials Community Group.

It requires the did-resolver library, which is the primary interface for resolving DIDs.

The DID method relies on the ethr-did-registry.

DID method

To encode a DID for an Ethereum address, simply prepend did:ethr:

eg:

did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74

DID Document

The did resolver takes the ethereum address, checks for the current owner, looks at contract events and builds a simple DID document.

The minimal DID document for a an ethereum address 0xb9c5714089478a327f09197987f16f9e5d936e8a with no transactions to the registry looks like this:

{
  '@context': 'https://w3id.org/did/v1',
  id: 'did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a',
  publicKey: [{
       id: 'did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a#owner',
       type: 'Secp256k1VerificationKey2018',
       owner: 'did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a',
       ethereumAddress: '0xb9c5714089478a327f09197987f16f9e5d936e8a'}],
  authentication: [{
       type: 'Secp256k1SignatureAuthentication2018',
       publicKey: 'did:ethr:0xb9c5714089478a327f09197987f16f9e5d936e8a#owner'}]
}

Note this uses the Secp256k1VerificationKey2018 type and an ethereumAddress instead of a publicKeyHex.

Building a DID document

The DID document is built by using read only functions and contract events on the ethr-did-registry Ethereum smart contract.

Any value from the registry that returns an ethereum address will be added to the publicKey array of the DID document with type Secp256k1VerificationKey2018 and an ethereumAddress attribute containing the address.

Owner Address

Each identity always has an owner address. By default it's the same as the identity address, but check the read only contract function identityOwner(address identity) on the deployed version of the EthrDIDRegistry contract.

The Identity owner will always have a publicKey with the id set as the DID with the fragment #owner appended.

An entry is also added to the authentication array of the DID document with type Secp256k1SignatureAuthentication2018.

Enumerating contract events for an identity

The EthereumDIDRegistry contract publishes 3 types of events for each identity.

• DIDOwnerChanged
• DIDDelegateChanged
• DIDAttributeChanged

If a change has ever been made for an identity the block number is stored in the changed mapping.

The latest event can be efficiently looked up by checking for one of the 3 above events at that exact block.

Each event contains a previousChange value which contains the block number of the previous change (if any)

To see all changes in history for an identity use the following pseudo code:

1. call changed(address identity) contract
2. if result is null return
3. filter for events for all the above types with the contracts address on the specified block
4. if event has a previous change then go to 3

Delegate Keys

Delegate Keys are ethereum addresses that can either be general signing keys or optionally also perform authentication.

They are also verifiable from solidity (see ethr-did-registry for more info).

A DIDDelegateChanged event is published that is used to build a DID.

event DIDDelegateChanged(
    address indexed identity,
    bytes32 delegateType,
    address delegate,
    uint validTo,
    uint previousChange
  );

The only 2 delegateTypes that are currently published in the DID Document are:

• veriKey Which adds a Secp256k1VerificationKey2018 to the publicKey section of document
• sigAuth Which adds a Secp256k1SignatureAuthentication2018 to the publicKey section of document. An entry is also added to the authentication section of document.

Note The delegateType is a bytes32 type for Ethereum gas efficiency reasons and not a string. This restricts us to 32 bytes, which is why we use the short hand versions above.

Only events with a validTo in seconds greater or equal to current time should be included in the DID document.

Non Ethereum Attributes

Non ethereum keys, service elements etc can be added using attributes. Attributes only exist on the blockchain as contract events of type DIDAttributeChanged and can thus not be queried from within solidity code.

event DIDAttributeChanged(
    address indexed identity,
    bytes32 name,
    bytes value,
    uint validTo,
    uint previousChange
  );

Note The name is a bytes32 type for Ethereum gas efficiency reasons and not a string. This restricts us to 32 bytes, which is why we use the short hand attribute versions below.

While any attribute can be stored. For the DID document we currently support adding to each of these sections of the DID document:

• Public Keys
• Service Endpoints

Public Keys

The name of the attribute should follow this format:

did/pub/(Secp256k1|RSA|Ed25519)/(veriKey|sigAuth)/(hex|base64)

Hex encoded Secp256k1 Verification Key

A DIDAttributeChanged event for the identity 0xf3beac30c498d9e26865f34fcaa57dbb935b0d74 with the name did/pub/Secp256k1/veriKey/hex and the value of 0x02b97c30de767f084ce3080168ee293053ba33b235d7116a3263d29f1450936b71 generates a PublicKey entry like this:

{
  id: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74#delegate-1",
  type: "Secp256k1VerificationKey2018",
  owner: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74",
  publicKeyHex: '02b97c30de767f084ce3080168ee293053ba33b235d7116a3263d29f1450936b71'
}

Base64 encoded Ed25519 Verification Key

A DIDAttributeChanged event for the identity 0xf3beac30c498d9e26865f34fcaa57dbb935b0d74 with the name did/pub/Ed25519/veriKey/base64 and the value of 0xb97c30de767f084ce3080168ee293053ba33b235d7116a3263d29f1450936b71 generates a PublicKey entry like this:

{
  id: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74#delegate-1",
  type: "Ed25519VerificationKey2018",
  owner: "did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74",
  publicKeyBase64: "uXww3nZ/CEzjCAFo7ikwU7ozsjXXEWoyY9KfFFCTa3E="
}

We are looking for people to submit support for pem, base58 and jwk key formats as well.

Service Endpoints

The name of the attribute should follow this format:

did/svc/[ServiceName]

Hex encoded Secp256k1 Verification Key

A DIDAttributeChanged event for the identity 0xf3beac30c498d9e26865f34fcaa57dbb935b0d74 with the name did/svc/HubService and value of the url https://hubs.uport.me hex encoded as 0x68747470733a2f2f687562732e75706f72742e6d65 generates a Service entry like this:

{
  type: "HubService",
  serviceEndpoint: "https://hubs.uport.me"
}

Resolving a DID document

The resolver presents a simple resolver() function that returns a ES6 Promise returning the DID document.

import { Resolver } from 'did-resolver'
import getResolver from 'ethr-did-resolver'

// You can set a rpc endpoint to be used by the web3 provider
// You can also set an address for your own ethr-did-registry contract
const providerConfig = { rpcUrl: 'https://rinkeby.infura.io/ethr-did', registry: registry.address }

// getResolver will return an object with a key/value pair of { "ethr": resolver } where resolver is a function used by the generic did resolver. 
const ethrDidResolver = getResolver(providerConfig)
const didResolver = Resolver(ethrDidResolver)

didResolver.resolve('did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74').then(doc => console.log)

// You can also use ES7 async/await syntax
const doc = await didResolver.resolve('did:ethr:0xf3beac30c498d9e26865f34fcaa57dbb935b0d74')