deriveBits

It takes as its arguments the base key, the derivation algorithm to use, and the length of the bits to derive. It returns a Promise which will be fulfilled with an ArrayBuffer containing the derived bits. This array of bits can be used as a key for encryption or decryption as a shared secret.

Usage

deriveBits(algorithm, baseKey, length)

Parameters

Name	Type	Description
`algorithm`	EcdhKeyDeriveParams	An object defining a derivation algorithm to use.
`baseKey`	CryptoKey	Represent an input to derivation algorithm. Currently it could be only a private ECDH key.
`length`	`number`	Optional. A length of the bits to derive. Currently, only multiplies of 8 are supported.

Supported algorithms

ECDH	HKDF	PBKDF2
✅ EcdhKeyDeriveParams	❌	❌

Return Value

A Promise that resolves to a new ArrayBuffer containing the derived bits.

Example

export default async function () {
  // Generate a key pair for Alice
  const aliceKeyPair = await crypto.subtle.generateKey(
    {
      name: 'ECDH',
      namedCurve: 'P-256',
    },
    true,
    ['deriveKey', 'deriveBits']
  );

  // Generate a key pair for Bob
  const bobKeyPair = await crypto.subtle.generateKey(
    {
      name: 'ECDH',
      namedCurve: 'P-256',
    },
    true,
    ['deriveKey', 'deriveBits']
  );

  // Derive shared secret for Alice
  const aliceSharedSecret = await deriveSharedSecret(aliceKeyPair.privateKey, bobKeyPair.publicKey);

  // Derive shared secret for Bob
  const bobSharedSecret = await deriveSharedSecret(bobKeyPair.privateKey, aliceKeyPair.publicKey);

  console.log('alice shared secret: ' + printArrayBuffer(aliceSharedSecret));
  console.log('bob shared secret: ' + printArrayBuffer(bobSharedSecret));
}

async function deriveSharedSecret(privateKey, publicKey) {
  return crypto.subtle.deriveBits(
    {
      name: 'ECDH',
      public: publicKey,
    },
    privateKey,
    256
  );
}

const printArrayBuffer = (buffer) => {
  const view = new Uint8Array(buffer);
  return Array.from(view);
};