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

[![NPM Version](https://img.shields.io/npm/v/@digitalcredentials/did-method-key)](https://www.npmjs.com/package/@digitalcredentials/did-method-key)

> 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

(Forked from [`digitalbazaar/did-method-key` v1.0.0](https://github.com/digitalbazaar/did-method-key-js)
to provide TypeScript compatibility.)

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

Note that this derived key is optional -- there's currently
[no proof](https://crypto.stackexchange.com/questions/3260/using-same-keypair-for-diffie-hellman-and-signing/3311#3311)
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:

```js
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`](https://github.com/digitalbazaar/jsonld-signatures) or
[`vc-js`](https://github.com/digitalcredentials/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 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:

```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 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 `documentLoader`s for JSON-LD Signature
libs, and the resulting key does include the appropriate `@context`).

```js
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`:

```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 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](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.

## License

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