did:key method driver (@digitalcredentials/did-method-key)

A DID (Decentralized Identifier) method driver for the did-io library and for standalone use

Table of Contents

• Background
  • Example DID Document
• Security
• Install
• Usage
• Contribute
• Commercial Support
• License

Background

(Forked from digitalbazaar/did-method-key v1.0.0 to provide TypeScript compatibility.)

See also (related specs):

• Decentralized Identifiers (DIDs)
• Linked Data Cryptographic Suite Registry
• Linked Data Proofs

A did:key method driver for the did-io client library and for standalone use.

The did:key method is used to express public keys in a way that doesn't require a DID Registry of any kind. Its general format is:

did:key:<multibase encoded, multicodec identified, public key>

So, for example, the following DID would be derived from a base-58 encoded ed25519 public key:

did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH

That DID would correspond to the following DID Document:

Example DID Document

{
  "@context": [
    "https://www.w3.org/ns/did/v1",
    "https://w3id.org/security/suites/ed25519-2020/v1",
    "https://w3id.org/security/suites/x25519-2020/v1"
  ],
  "id": "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK",
  "verificationMethod": [{
    "id": "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK",
    "type": "Ed25519VerificationKey2020",
    "controller": "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK",
    "publicKeyMultibase": "z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
  }],
  "authentication": [
    "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
  ],
  "assertionMethod": [
    "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
  ],
  "capabilityDelegation": [
    "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
  ],
  "capabilityInvocation": [
    "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK"
  ],
  "keyAgreement": [{
    "id": "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK#z6LSj72tK8brWgZja8NLRwPigth2T9QRiG1uH9oKZuKjdh9p",
    "type": "X25519KeyAgreementKey2020",
    "controller": "did:key:z6MkhaXgBZDvotDkL5257faiztiGiC2QtKLGpbnnEGta2doK",
    "publicKeyMultibase": "z6LSj72tK8brWgZja8NLRwPigth2T9QRiG1uH9oKZuKjdh9p"
  }]
}

Security

The keyAgreement key is a Curve25519 public key (suitable for Diffie-Hellman key exchange) that is deterministically derived from the source Ed25519 key, using ed2curve-js.

Note that this derived key is optional -- there's currently no proof that this is safe to do.

Install

Requires Node.js 12+

To install from npm:

npm install --save @digitalcredentials/did-method-key

To install locally (for development):

git clone https://github.com/digitalcredentials/did-method-key-js.git
cd did-method-key-js
npm install

Usage

generate()

To generate a new key and get its corresponding did:key method DID Document:

const didKeyDriver = require('@digitalcredentials/did-method-key').driver();

// generate did:key using Ed25519 key type by default
const {didDocument, keyPairs, methodFor} = await didKeyDriver.generate();

// print the DID Document above
console.log(JSON.stringify(didDocument, null, 2));

// keyPairs will be set like so ->
Map(2) {
  'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv#z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv' => Ed25519VerificationKey2020 {
    id: 'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv#z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv',
    controller: 'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv',
    type: 'Ed25519VerificationKey2020',
    publicKeyMultibase: 'z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv',
    privateKeyMultibase: 'z3zDo1wXuXGcFkJa9SPE7VYpdutmHq8gJsvFRMKJckTWMykoHsAjWNbHXqzrZ8qa7aWdDTjmJNJ1amYEG2mCvZZeY'
  },
  'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv#z6LSeRSE5Em5oJpwdk3NBaLVERBS332ULC7EQq5EtMsmXhsM' => X25519KeyAgreementKey2020 {
    id: 'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv#z6LSeRSE5Em5oJpwdk3NBaLVERBS332ULC7EQq5EtMsmXhsM',
    controller: 'did:key:z6MkuBLrjSGt1PPADAvuv6rmvj4FfSAfffJotC6K8ZEorYmv',
    type: 'X25519KeyAgreementKey2020',
    publicKeyMultibase: 'z6LSeRSE5Em5oJpwdk3NBaLVERBS332ULC7EQq5EtMsmXhsM',
    privateKeyMultibase: 'z3weeMD56C1T347EmB6kYNS7trpQwjvtQCpCYRpqGz6mcemT'
  }
}

methodFor is a convenience function that returns a public/private key pair instance for a given purpose. For example, a verification key (containing a signer() and verifier() functions) are frequently useful for jsonld-signatures or vc-js operations. After generating a new did:key DID, you can do:

// For signing Verifiable Credentials
const assertionKeyPair = methodFor({purpose: 'assertionMethod'});
// For Authorization Capabilities (zCaps)
const invocationKeyPair = methodFor({purpose: 'capabilityInvocation'});
// For Encryption using `@digitalbazaar/minimal-cipher`
const keyAgreementPair = methodFor({purpose: 'keyAgreement'});

Note that methodFor returns a key pair that contains both a public and private key pair (since it has access to the keyPairs map from generate()). This makes it useful for signing and encrypting operations (unlike the publicMethodFor that's returned by get(), below).

publicKeyToDidDoc()

If you already have an Ed25519VerificationKey2020 public key object (as an LDKeyPair instance, or a plain key description object), you can turn it into a DID Document:

const {didDocument} = await didKeyDriver.publicKeyToDidDoc({publicKeyDescription});

get()

Getting a full DID Document from a did:key DID

To get a DID Document for an existing did:key DID:

const did = 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T';
const didDocument = await didKeyDriver.get({did});

(Results in the example DID Doc above).

Getting just the key object by key id

You can also use a .get() to retrieve an individual key, if you know it's id already (this is useful for constructing documentLoaders for JSON-LD Signature libs, and the resulting key does include the appropriate @context).

const verificationKeyId = 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T';
await didKeyDriver.get({url: verificationKeyId});
// ->
{
  "@context": "https://w3id.org/security/suites/ed25519-2020/v1",
  "id": "did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T",
  "type": "Ed25519VerificationKey2020",
  "controller": "did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T",
  "publicKeyMultibase": "z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T"
}

const keyAgreementKeyId = 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW';
await didKeyDriver.get({url: keyAgreementKeyId});
// ->
{
  "@context": "https://w3id.org/security/suites/x25519-2020/v1",
  "id": "did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW",
  "type": "X25519KeyAgreementKey2020",
  "controller": "did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T",
  "publicKeyMultibase": "z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW"
}

publicMethodFor()

Often, you have just a did:key DID, and you need to get a key for a particular purpose from it, such as an assertionMethod key to verify a VC signature, or a keyAgreement key to encrypt a document for that DID's controller.

For that purpose, you can use a combination of get() and publicMethodFor:

// Start with the DID
const didDocument = await didKeyDriver.get({did});
// This lets you use `publicMethodFor()` to get a key for a specific purpose
const keyAgreementData = didKeyDriver.publicMethodFor({
  didDocument, purpose: 'keyAgreement'
});
const assertionMethodData = didKeyDriver.publicMethodFor({
  didDocument, purpose: 'assertionMethod'
});

// If you're using a `crypto-ld` driver harness, you can create key instances
// which allow you to get access to a `verify()` function.
const assertionMethodPublicKey = await cryptoLd.from(assertionMethodData);
const {verify} = assertionMethodPublicKey.verifier();

publicMethodFor will return undefined if no key is found for a given purpose.

Contribute

See the contribute file!

PRs accepted.

If editing the Readme, please conform to the standard-readme specification.

License

• MIT License - DCC - TypeScript compatibility.
• New BSD License (3-clause) © 2020-2021 Digital Bazaar - Initial implementation.