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.
Note: This feature is available in Web Workers.
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 stringECDH
as thename
property. - To use HKDF, pass an
HkdfParams
object. - To use PBKDF2, pass a
Pbkdf2Params
object. - To use X25519, pass an
EcdhKeyDeriveParams
object, specifying the stringX25519
as thename
property.
- To use ECDH, pass an
baseKey
-
A
CryptoKey
representing the input to the derivation algorithm. Ifalgorithm
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 aCryptoKey
usingSubtleCrypto.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 containderiveBits
. NotSupported
DOMException
-
Raised when trying to use an algorithm that is either unknown or isn't suitable for derivation.
Supported algorithms
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 generateAliceKeyPair = window.crypto.subtle.generateKey(
{
name: "ECDH",
namedCurve: "P-384",
},
false,
["deriveBits"],
);
const generateBobKeyPair = window.crypto.subtle.generateKey(
{
name: "ECDH",
namedCurve: "P-384",
},
false,
["deriveBits"],
);
Promise.all([generateAliceKeyPair, generateBobKeyPair]).then((values) => {
const aliceKeyPair = values[0];
const bobKeyPair = 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(aliceKeyPair.privateKey, bobKeyPair.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 Bob's private key and Alice'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.
<input id="buttonDeriveKeys" type="button" value="Derive bits" />
<input id="buttonChangeKeys" type="button" value="Change keys" />
JavaScript
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 aliceKeyPair;
let bobKeyPair;
async function changeKeys() {
try {
aliceKeyPair = await window.crypto.subtle.generateKey(
{
name: "X25519",
},
false,
["deriveBits"],
);
bobKeyPair = 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(
aliceKeyPair.privateKey,
bobKeyPair.publicKey,
);
let buffer = new Uint8Array(sharedSecretAlice, 0, 10);
log(`${buffer}…[${sharedSecretAlice.byteLength} bytes total] (Alice secret)`);
const sharedSecretBob = await deriveSharedSecret(
bobKeyPair.privateKey,
aliceKeyPair.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