HKDF derive key web crypto subtle

example-hkdf.js

// We generated this above as an example IND key derivation key
let openSSLSecretKey = '0de81e851cd7995626ad4c3e160ae1c449af4e15c8ceabd44fb75be581adfbaa';

// Parse the hex string into a Uint8Array
let ikm = Uint8Array.from(openSSLSecretKey
  .match(/.{1,2}/g)
  .map((byte) => parseInt(byte, 16)));

// Import the raw key data
let kdk = await crypto.subtle.importKey(
  'raw',
  ikm,
  'HKDF',
  false, // KDF keys cannot be exported
  ['deriveKey', 'deriveBits']);

// We are going to create a signing key from the secret
// If we create other keys too, they should not have the
// same label!
let label = 'signing key';

// This function works with bytes.
// Therefore we must encode our label which is text to bytes.
let encoder = new TextEncoder();
let info = encoder.encode(label);

// A salt is a required property, even though it is empty.
let salt = new Uint8Array(); // Nothing inside!

// Derive a signing key from the key derivation key
let signingKey = await crypto.subtle.deriveKey(
  // Again, the salt is empty
  // The info will uniquely describe this key
  {name: 'HKDF', salt, info, hash: 'SHA-256'},
  // The input key material we decoded from hex above
  // and then wrapped in a CryptoKey
  kdk,
  // We're creating an HMAC-SHA-256 key
  {name: 'HMAC', hash: 'SHA-256'},
  // We do not need to export it,
  // since we can create it deterministically.
  false,
  // it needs to sign and verify
  ['sign', 'verify']);

// Prove that it works
// Let's sign "Hello world"
let message = 'Hello world';
let encodedMessage = encoder.encode(message);
let tag = await crypto.subtle.sign(
  {name: 'HMAC'},
  signingKey,
  encodedMessage);

console.log(`Message ${message} - tag: ${btoa(tag)}`);
// Message Hello world - tag: W29iamVjdCBBcnJheUJ1ZmZlcl0=

// And prove that it can match its own mac too.
let verified = await crypto.subtle.verify(
  {name: 'HMAC'},
  signingKey,
  tag,
  encodedMessage);

console.log(`Verify?: ${verified}`);
// Verify?: true