@digitalbazaar/did-method-key/README.md

A did:key method resolver.

# did:key method driver _(@digitalbazaar/did-method-key)_

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> A [DID](https://w3c.github.io/did-core) (Decentralized Identifier) method driver for the `did-io` library and for standalone use

## Table of Contents

- [Background](#background)
  * [Example DID Document](#example-did-document)
- [Security](#security)
- [Install](#install)
- [Usage](#usage)
- [Contribute](#contribute)
- [Commercial Support](#commercial-support)
- [License](#license)

## Background

See also (related specs):

* [Decentralized Identifiers (DIDs)](https://w3c.github.io/did-core)
* [Linked Data Cryptographic Suite Registry](https://w3c-ccg.github.io/ld-cryptosuite-registry/)
* [Linked Data Proofs](https://w3c-dvcg.github.io/ld-proofs/)

A `did:key` method driver for the [`did-io`](https://github.com/digitalbazaar/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

```json
{
  "@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:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH",
  "verificationMethod": [
    {
      "id": "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH",
      "type": "Ed25519VerificationKey2020",
      "controller": "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH",
      "publicKeyMultibase": "z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH"
    }
  ],
  "authentication": [
    "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH"
  ],
  "assertionMethod": [
    "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH"
  ],
  "capabilityDelegation": [
    "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH"
  ],
  "capabilityInvocation": [
    "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH"
  ],
  "keyAgreement": [
    {
      "id": "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH#z6LSbysY2xFMRpGMhb7tFTLMpeuPRaqaWM1yECx2AtzE3KCc",
      "type": "X25519KeyAgreementKey2020",
      "controller": "did:key:z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH",
      "publicKeyMultibase": "z6LSbysY2xFMRpGMhb7tFTLMpeuPRaqaWM1yECx2AtzE3KCc"
    }
  ]
}
```

## 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`](https://github.com/dchest/ed2curve-js).

Note that this derived key is optional -- there's at least
[one proof](https://eprint.iacr.org/2021/509) that this is safe to do.

## Install

Requires Node.js 16+

To install from `npm`:

```
npm install --save @digitalbazaar/did-method-key
```

To install locally (for development):

```
git clone https://github.com/digitalbazaar/did-method-key.git
cd did-method-key
npm install
```

## Usage

### `use()`

This method registers a multibase-multikey header and a multibase-multikey
deserializer and configures a driver to use a multibase-multikey deserializer
to handle data using that multibase-multikey header.

```js
import * as EcdsaMultikey from '@digitalbazaar/ecdsa-multikey';
import {driver} from '@digitalbazaar/did-method-key';

const didKeyDriverMultikey = driver();

didKeyDriverMultikey.use({
  multibaseMultikeyHeader: 'zDna',
  fromMultibase: EcdsaMultikey.from
});
```

### `createFromMultibase()`

This utility function can be used to adapt legacy verification suites such as
`Ed25519VerificationSuite2018` to work properly with `fromMultibase()`
calls in `DidKeyDriver`.

```js
import {driver} from '@digitalbazaar/did-method-key';
import {Ed25519VerificationKey2018} from
  '@digitalbazaar/ed25519-verification-key-2018';

const didKeyDriver2018 = driver();

didKeyDriver2018.use({
  multibaseMultikeyHeader: header,
  fromMultibase: createFromMultibase(Ed25519VerificationKey2018)
});
```

### `fromKeyPair()`

To generate a new key and get its corresponding `did:key` method DID Document
from a verification keypair.

```js
import {driver} from '@digitalbazaar/did-method-key';
import {Ed25519VerificationKey2020} from
  '@digitalbazaar/ed25519-verification-key-2020';

const didKeyDriver = driver();

didKeyDriver.use({
  multibaseMultikeyHeader: 'z6Mk',
  fromMultibase: Ed25519VerificationKey2020.from
});

const publicKeyMultibase = 'z6MkpTHR8VNsBxYAAWHut2Geadd9jSwuBV8xRoAnwWsdvktH';
const verificationKeyPair = await Ed25519VerificationKey2020.from({
  publicKeyMultibase
});
// or perhaps:
// const verificationKeyPair = await Ed25519VerificationKey2020.generate();

const {didDocument, keyPairs, methodFor} = await didKeyDriver.fromKeyPair({
  verificationKeyPair
});

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

// keyPairs will be set like so =>
Map(2) {
  'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T' => Ed25519VerificationKey2020 {
    id: 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T',
    controller: 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T',
    revoked: undefined,
    type: 'Ed25519VerificationKey2020',
    publicKeyMultibase: 'z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T',
    privateKeyMultibase: undefined
  },
  'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW' => X25519KeyAgreementKey2020 {
    id: 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW',
    controller: 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T',
    revoked: undefined,
    type: 'X25519KeyAgreementKey2020',
    publicKeyMultibase: 'z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW',
    privateKeyMultibase: undefined
  }
}
```

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

```js
// 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 a `publicKeyMultibase`.
This makes it useful for _verifying_ and _encrypting_ operations.

### `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:

```js
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:

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

(Results in the [example DID Doc](#example-did-document) above).

#### Getting the DID Document from key id

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

```js
const verificationKeyId = 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T';

const keyAgreementKeyId = 'did:key:z6MknCCLeeHBUaHu4aHSVLDCYQW9gjVJ7a63FpMvtuVMy53T#z6LSotGbgPCJD2Y6TSvvgxERLTfVZxCh9KSrez3WNrNp7vKW';
const didDocument = await didKeyDriver.get({url: verificationKeyId});
// OR
const didDocument = await didKeyDriver.get({url: keyAgreementKeyId});

// DID Document ->
console.log(JSON.stringify(didDocument, null, 2));
```

### `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`:

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

// If you have a known key type, for example, `Ed25519VerificationKey2020`,
// you can create key instances which allow you to get access to a
// `verify()` function.
const assertionMethodPublicKey = await Ed25519VerificationKey2020.from(
  assertionMethod);
const {verify} = assertionMethodPublicKey.verifier();
```

`publicMethodFor` will throw an error if no key is found for a given purpose.

## Contribute

See [the contribute file](https://github.com/digitalbazaar/bedrock/blob/master/CONTRIBUTING.md)!

PRs accepted.

If editing the Readme, please conform to the
[standard-readme](https://github.com/RichardLitt/standard-readme) specification.

## Commercial Support

Commercial support for this library is available upon request from
Digital Bazaar: support@digitalbazaar.com

## License

[New BSD License (3-clause)](LICENSE) © Digital Bazaar