SubtleCrypto.deriveBits()
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 bit string 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, and PBKDF2. See Supported
algorithms for some more detail on these algorithms.
Syntax
const result = crypto.subtle.deriveBits(
algorithm,
baseKey,
length
);
Parameters
algorithm
is 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.
-
To use ECDH, pass
an
-
baseKey
is aCryptoKey
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
is 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
-
result
is aPromise
that fulfills with anArrayBuffer
containing the derived bits.
Exceptions
The promise is rejected when one of the following exceptions are encountered:
OperationError
-
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
-
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
-
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.
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 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);
});
});
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: 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