SubtleCrypto: deriveKey() method

algorithm

An object defining the derivation algorithm to use.

• To use ECDH, pass an EcdhKeyDeriveParams object.
• To use HKDF, pass an HkdfParams object.
• To use PBKDF2, pass a Pbkdf2Params object.

baseKey

A CryptoKey representing the input to the derivation algorithm. If algorithm is ECDH, then this will be the ECDH private key. Otherwise it will be the initial key material for the derivation function: for example, for PBKDF2 it might be a password, imported as a CryptoKey using SubtleCrypto.importKey().

derivedKeyAlgorithm

An object defining the algorithm the derived key will be used for.

• For HMAC: pass an HmacKeyGenParams object.
• For AES-CTR, AES-CBC, AES-GCM, or AES-KW: pass an AesKeyGenParams object.

extractable

A boolean value indicating whether it will be possible to export the key using SubtleCrypto.exportKey() or SubtleCrypto.wrapKey().

keyUsages

An Array indicating what can be done with the derived key. Note that the key usages must be allowed by the algorithm set in derivedKeyAlgorithm. Possible values of the array are:

• encrypt: The key may be used to encrypt messages.
• decrypt: The key may be used to decrypt messages.
• sign: The key may be used to sign messages.
• verify: The key may be used to verify signatures.
• deriveKey: The key may be used in deriving a new key.
• deriveBits: The key may be used in deriving bits.
• wrapKey: The key may be used to wrap a key.
• unwrapKey: The key may be used to unwrap a key.

A Promise that fulfills with a CryptoKey.

The promise is rejected when one of the following exceptions are encountered:

InvalidAccessError DOMException

Raised when the master key is not a key for the requested derivation algorithm or if the keyUsages value of that key doesn't contain deriveKey.

NotSupported DOMException

Raised when trying to use an algorithm that is either unknown or isn't suitable for derivation, or if the algorithm requested for the derived key doesn't define a key length.

SyntaxError DOMException

Raised when keyUsages is empty but the unwrapped key is of type secret or private.

The three algorithms supported by deriveKey() have quite different characteristics and are appropriate in different situations.

ECDH (Elliptic Curve Diffie-Hellman) is a key-agreement algorithm. It enables two people who each have an ECDH public/private key pair to generate a shared secret: that is, a secret that they — and no one else — share. They can then use this shared secret as a symmetric key to secure their communication, or can use the secret as an input to derive such a key (for example, using the HKDF algorithm).

ECDH is specified in RFC 6090.

HKDF is a key derivation function. It's designed to derive key material from some high-entropy input, such as the output of an ECDH key agreement operation.

It's not designed to derive keys from relatively low-entropy inputs such as passwords. For that, use PBKDF2.

HKDF is specified in RFC 5869.

PBKDF2 is also a key derivation function. It's designed to derive key material from some relatively low-entropy input, such as a password. It derives key material by applying a function such as HMAC to the input password along with some salt, and repeating this process many times. The more times the process is repeated, the more computationally expensive key derivation is: this makes it harder for an attacker to use brute-force to discover the key using a dictionary attack.

PBKDF2 is specified in RFC 2898.

In this example Alice and Bob each generate an ECDH key pair, then exchange public keys. They then use deriveKey() to derive a shared AES key, that they could use to encrypt messages. See the complete code on GitHub.

/*
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);
  });
}

In this example we ask the user for a password, then use it to derive an AES key using PBKDF2, then use the AES key to encrypt a message. See the complete code on GitHub.

/*
Get some key material to use as input to the deriveKey method.
The key material is a password supplied by the user.
*/
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);
}

• HKDF specification.
• NIST guidelines for password-based key derivation.
• Password storage cheat sheet.
• Advice on choosing an iteration count for PBKDF2.