SubtleCrypto: deriveKey() method
================================

**Secure context:** This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

 The **`deriveKey()`** method of the `SubtleCrypto`
 interface can be used to derive a secret key from a master key.

 It takes as arguments some initial key material, the derivation algorithm to use, and
 the desired properties for the key to derive. It returns a `Promise`
 which will be fulfilled with a `CryptoKey` object representing the new key.

 It's worth noting that the three key derivation algorithms you can use have quite
 different characteristics and are appropriate in quite different situations. See Supported algorithms for some more detail on this.

Syntax
------


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

### Parameters

`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.
* For HKDF, pass an `HkdfParams` object.
* For PBKDF2, pass a `Pbkdf2Params` 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`.

### Return value

A `Promise` that fulfills with a `CryptoKey`.

### Exceptions

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`.


Supported algorithms
--------------------

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

### ECDH

 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

 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

 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.

Examples
--------

**Note:** You can try the working examples on GitHub.

### ECDH

 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.


```js
/\*
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);
  });
}
```

### PBKDF2

 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.


```js
/\*
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

In this example, we encrypt a message `plainText` given a shared secret `secret`, which might itself have been derived using an algorithm such as ECDH. Instead of using the shared secret directly, we use it as key material for the HKDF function, to derive an AES-GCM encryption key, which we then use to encrypt the message. See the complete code on GitHub.


```js
/\*
 Given some key material and some random salt,
 derive an AES-GCM key using HKDF.
 \*/
function getKey(keyMaterial, salt) {
  return window.crypto.subtle.deriveKey(
    {
      name: "HKDF",
      salt: salt,
      info: new Uint8Array("Encryption example"),
      hash: "SHA-256",
    },
    keyMaterial,
    { name: "AES-GCM", length: 256 },
    true,
    ["encrypt", "decrypt"],
  );
}

async function encrypt(secret, plainText) {
  const message = {
    salt: window.crypto.getRandomValues(new Uint8Array(16)),
    iv: window.crypto.getRandomValues(new Uint8Array(12)),
  };

  const key = await getKey(secret, message.salt);

  message.ciphertext = await window.crypto.subtle.encrypt(
    {
      name: "AES-GCM",
      iv: message.iv,
    },
    key,
    plainText,
  );

  return message;
}
```

Specifications
--------------

| Specification |
| --- |
| Web Cryptography API # SubtleCrypto-method-deriveKey |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

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