SubtleCrypto: deriveKey() メソッド

deriveKey(algorithm, baseKey, derivedKeyAlgorithm, extractable, keyUsages)

/*
Derive an AES key, given:
- our ECDH private key
- their ECDH public key
*/
function deriveSecretKey(privateKey, publicKey) {
  return window.crypto.subtle.deriveKey(
    {
      name: "ECDH",
      public: publicKey,
    },
    privateKey,
    {
      name: "AES-GCM",
      length: 256,
    },
    false,
    ["encrypt", "decrypt"],
  );
}

async function agreeSharedSecretKey() {
  // Generate 2 ECDH key pairs: one for Alice and one for Bob
  // In more normal usage, they would generate their key pairs
  // separately and exchange public keys securely
  let alicesKeyPair = await window.crypto.subtle.generateKey(
    {
      name: "ECDH",
      namedCurve: "P-384",
    },
    false,
    ["deriveKey"],
  );

  let bobsKeyPair = await window.crypto.subtle.generateKey(
    {
      name: "ECDH",
      namedCurve: "P-384",
    },
    false,
    ["deriveKey"],
  );

  // Alice then generates a secret key using her private key and Bob's public key.
  let alicesSecretKey = await deriveSecretKey(
    alicesKeyPair.privateKey,
    bobsKeyPair.publicKey,
  );

  // Bob generates the same secret key using his private key and Alice's public key.
  let bobsSecretKey = await deriveSecretKey(
    bobsKeyPair.privateKey,
    alicesKeyPair.publicKey,
  );

  // Alice can then use her copy of the secret key to encrypt a message to Bob.
  let encryptButton = document.querySelector(".ecdh .encrypt-button");
  encryptButton.addEventListener("click", () => {
    encrypt(alicesSecretKey);
  });

  // Bob can use his copy to decrypt the message.
  let decryptButton = document.querySelector(".ecdh .decrypt-button");
  decryptButton.addEventListener("click", () => {
    decrypt(bobsSecretKey);
  });
}

/*
deriveKeyメソッドの入力として使用する鍵素材を取得します。
鍵素材は、ユーザーから提供されたパスワードです。
*/
function getKeyMaterial() {
  const password = window.prompt("Enter your password");
  const enc = new TextEncoder();
  return window.crypto.subtle.importKey(
    "raw",
    enc.encode(password),
    "PBKDF2",
    false,
    ["deriveBits", "deriveKey"],
  );
}

async function encrypt(plaintext, salt, iv) {
  const keyMaterial = await getKeyMaterial();
  const key = await window.crypto.subtle.deriveKey(
    {
      name: "PBKDF2",
      salt,
      iterations: 100000,
      hash: "SHA-256",
    },
    keyMaterial,
    { name: "AES-GCM", length: 256 },
    true,
    ["encrypt", "decrypt"],
  );

  return window.crypto.subtle.encrypt({ name: "AES-GCM", iv }, key, plaintext);
}