SubtleCrypto: deriveBits() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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

The deriveBits() method of the SubtleCrypto interface can be used to derive an array of bits from a base key.

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 method is very similar to SubtleCrypto.deriveKey(), except that deriveKey() returns a CryptoKey object rather than an ArrayBuffer. Essentially deriveKey() is composed of deriveBits() followed by importKey().

This function supports the same derivation algorithms as deriveKey(): ECDH, HKDF, PBKDF2, and X25519. See Supported algorithms for some more detail on these algorithms.

Syntax

deriveBits(algorithm, baseKey, length)

Parameters

algorithm

An object defining the derivation algorithm to use.

To use ECDH, pass an EcdhKeyDeriveParams object, specifying the string ECDH as the name property.
To use HKDF, pass an HkdfParams object.
To use PBKDF2, pass a Pbkdf2Params object.
To use X25519, pass an EcdhKeyDeriveParams object, specifying the string X25519 as the name property.

baseKey

A CryptoKey representing the input to the derivation algorithm. If algorithm is ECDH, 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().

length

A number representing the number of bits to derive. To be compatible with all browsers, the number should be a multiple of 8.

Return value

A Promise that fulfills with an ArrayBuffer containing the derived bits.

Exceptions

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

OperationError DOMException: Raised if the length parameter of the deriveBits() call is null, and also in some cases if the length parameter is not a multiple of 8.
InvalidAccessError DOMException: Raised when the base key is not a key for the requested derivation algorithm or if the CryptoKey.usages value of that key doesn't contain deriveBits.
NotSupported DOMException: Raised when trying to use an algorithm that is either unknown or isn't suitable for derivation.

Supported algorithms

See the Supported algorithms section of the deriveKey() documentation.

Examples

Note: You can try the working examples on GitHub.

ECDH

In this example Alice and Bob each generate an ECDH key pair.

We then use Alice's private key and Bob's public key to derive a shared secret. See the complete code on GitHub.

async function deriveSharedSecret(privateKey, publicKey) {
  const sharedSecret = await window.crypto.subtle.deriveBits(
    {
      name: "ECDH",
      namedCurve: "P-384",
      public: publicKey,
    },
    privateKey,
    128,
  );

  const buffer = new Uint8Array(sharedSecret, 0, 5);
  const sharedSecretValue = document.querySelector(".ecdh .derived-bits-value");
  sharedSecretValue.classList.add("fade-in");
  sharedSecretValue.addEventListener("animationend", () => {
    sharedSecretValue.classList.remove("fade-in");
  });
  sharedSecretValue.textContent = `${buffer}…[${sharedSecret.byteLength} bytes total]`;
}

// 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
const generateAlicesKeyPair = window.crypto.subtle.generateKey(
  {
    name: "ECDH",
    namedCurve: "P-384",
  },
  false,
  ["deriveBits"],
);

const generateBobsKeyPair = window.crypto.subtle.generateKey(
  {
    name: "ECDH",
    namedCurve: "P-384",
  },
  false,
  ["deriveBits"],
);

Promise.all([generateAlicesKeyPair, generateBobsKeyPair]).then((values) => {
  const alicesKeyPair = values[0];
  const bobsKeyPair = values[1];

  const deriveBitsButton = document.querySelector(".ecdh .derive-bits-button");
  deriveBitsButton.addEventListener("click", () => {
    // Alice then generates a secret using her private key and Bob's public key.
    // Bob could generate the same secret using his private key and Alice's public key.
    deriveSharedSecret(alicesKeyPair.privateKey, bobsKeyPair.publicKey);
  });
});

X25519

In this example Alice and Bob each generate an X25519 key pair. We then use Alice's private key and Bob's public key to derive a secret, and compare that with the secret generated using Bobs's private key and Alices's public key to show that they are shared/identical.

HTML

The HTML is defines two buttons. The "Change keys" button is pressed to generate new key pairs for Alice and Bob. The "Derive bits" button is pressed to derive a shared secret with the current set of key pairs.

html

<input id="buttonDeriveKeys" type="button" value="Derive bits" />
<input id="buttonChangeKeys" type="button" value="Change keys" />

<pre id="log"></pre>

#log {
  height: 150px;
  width: 90%;
  white-space: pre-wrap; /* wrap pre blocks */
  overflow-wrap: break-word; /* break on words */
  overflow-y: auto;
  padding: 0.5rem;
  border: 1px solid black;
}

JavaScript

const logElement = document.querySelector("#log");
function log(text) {
  logElement.innerText = `${logElement.innerText}${text}\n`;
  logElement.scrollTop = logElement.scrollHeight;
}

The function to generate a shared secret using the X25519 algorithm is shown below. This takes a private key from one party and the public key from another.

async function deriveSharedSecret(privateKey, publicKey) {
  return await window.crypto.subtle.deriveBits(
    {
      name: "X25519",
      public: publicKey,
    },
    privateKey,
    128,
  );
}

The code below adds a function to generate new keys for Alice and Bob. This is done the first time the JavaScript is loaded, and repeated whenever the "Change keys" button is pressed (this allows us to see the effect of changing the keys on the shared secret).

let alicesKeyPair;
let bobsKeyPair;

async function changeKeys() {
  try {
    alicesKeyPair = await window.crypto.subtle.generateKey(
      {
        name: "X25519",
      },
      false,
      ["deriveBits"],
    );

    bobsKeyPair = await window.crypto.subtle.generateKey(
      {
        name: "X25519",
      },
      false,
      ["deriveBits"],
    );

    log("Keys changed");
  } catch (e) {
    log(e);
  }
}

changeKeys();

const changeKeysButton = document.querySelector("#buttonChangeKeys");

// Generate 2 X25519 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
changeKeysButton.addEventListener("click", changeKeys);

The code below adds a handler function that is invoked every time the "Derive bits" button is pressed. The handler generates the shared secrets for Alice and Bob using the deriveSharedSecret() method defined above, and logs them for easy comparison.

const deriveBitsButton = document.querySelector("#buttonDeriveKeys");

deriveBitsButton.addEventListener("click", async () => {
  // Generate 2 X25519 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

  // Alice then generates a secret using her private key and Bob's public key.
  // Bob could generate the same secret using his private key and Alice's public key.

  const sharedSecretAlice = await deriveSharedSecret(
    alicesKeyPair.privateKey,
    bobsKeyPair.publicKey,
  );

  let buffer = new Uint8Array(sharedSecretAlice, 0, 10);
  log(`${buffer}…[${sharedSecretAlice.byteLength} bytes total] (Alice secret)`);

  const sharedSecretBob = await deriveSharedSecret(
    bobsKeyPair.privateKey,
    alicesKeyPair.publicKey,
  );

  buffer = new Uint8Array(sharedSecretBob, 0, 10);
  log(`${buffer}…[${sharedSecretAlice.byteLength} bytes total] (Bob secret)`);
});

Result

Press the "Derive bits" button to generate and log a shared secret from Bob and Alice's keys. Press the "Change keys" button to change the X25519 keys used by both parties.

PBKDF2

In this example we ask the user for a password, then use it to derive some bits using PBKDF2. See the complete code on GitHub.

let salt;

/*
Get some key material to use as input to the deriveBits 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),
    { name: "PBKDF2" },
    false,
    ["deriveBits", "deriveKey"],
  );
}

/*
Derive some bits from a password supplied by the user.
*/
async function getDerivedBits() {
  const keyMaterial = await getKeyMaterial();
  salt = window.crypto.getRandomValues(new Uint8Array(16));
  const derivedBits = await window.crypto.subtle.deriveBits(
    {
      name: "PBKDF2",
      salt,
      iterations: 100000,
      hash: "SHA-256",
    },
    keyMaterial,
    256,
  );

  const buffer = new Uint8Array(derivedBits, 0, 5);
  const derivedBitsValue = document.querySelector(
    ".pbkdf2 .derived-bits-value",
  );
  derivedBitsValue.classList.add("fade-in");
  derivedBitsValue.addEventListener("animationend", () => {
    derivedBitsValue.classList.remove("fade-in");
  });
  derivedBitsValue.textContent = `${buffer}…[${derivedBits.byteLength} bytes total]`;
}

const deriveBitsButton = document.querySelector(".pbkdf2 .derive-bits-button");
deriveBitsButton.addEventListener("click", () => {
  getDerivedBits();
});

Specifications

Specification
Web Cryptography API # SubtleCrypto-method-deriveBits

Browser compatibility

BCD tables only load in the browser