SubtleCrypto.deriveKey()
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
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.

To use ECDH, pass an
baseKey

A
CryptoKey
representing the input to the derivation algorithm. Ifalgorithm
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 aCryptoKey
usingSubtleCrypto.importKey()
. derivedKeyAlgorithm

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

For HMAC: pass an
HmacKeyGenParams
object. 
For AESCTR, AESCBC,
AESGCM, or AESKW: pass an
AesKeyGenParams
object.

For HMAC: pass an
extractable

A boolean value indicating whether it will be possible to export the key using
SubtleCrypto.exportKey()
orSubtleCrypto.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 inderivedKeyAlgorithm
. Possible values of the array are:encrypt
: The key may be used toencrypt
messages.decrypt
: The key may be used todecrypt
messages.sign
: The key may be used tosign
messages.verify
: The key may be used toverify
signatures.deriveKey
: The key may be used inderiving a new key
.deriveBits
: The key may be used inderiving bits
.wrapKey
: The key may be used towrap a key
.unwrapKey
: The key may be used tounwrap a key
.
Return value
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 containderiveKey
. 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 typesecret
orprivate
.
Supported algorithms
The three algorithms supported by deriveKey()
have quite different
characteristics and are appropriate in different situations.
ECDH
ECDH (Elliptic Curve DiffieHellman) is a keyagreement 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 highentropy input, such as the output of an ECDH key agreement operation.
It's not designed to derive keys from relatively lowentropy 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 lowentropy 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 bruteforce 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.
/*
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: "AESGCM",
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: "P384"
},
false,
["deriveKey"]
);
let bobsKeyPair = await window.crypto.subtle.generateKey(
{
name: "ECDH",
namedCurve: "P384"
},
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 .encryptbutton");
encryptButton.addEventListener("click", () => {
encrypt(alicesSecretKey);
});
// Bob can use his copy to decrypt the message.
let decryptButton = document.querySelector(".ecdh .decryptbutton");
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.
/*
Get some key material to use as input to the deriveKey method.
The key material is a password supplied by the user.
*/
function getKeyMaterial() {
let password = window.prompt("Enter your password");
let enc = new TextEncoder();
return window.crypto.subtle.importKey(
"raw",
enc.encode(password),
"PBKDF2",
false,
["deriveBits", "deriveKey"]
);
}
async function encrypt(plaintext, salt, iv) {
let keyMaterial = await getKeyMaterial();
let key = await window.crypto.subtle.deriveKey(
{
"name": "PBKDF2",
salt: salt,
"iterations": 100000,
"hash": "SHA256"
},
keyMaterial,
{ "name": "AESGCM", "length": 256},
true,
[ "encrypt", "decrypt" ]
);
return window.crypto.subtle.encrypt(
{
name: "AESGCM",
iv: iv
},
key,
plaintext
);
}
Specifications
Specification 

Web Cryptography API # SubtleCryptomethodderiveKey 
Browser compatibility
BCD tables only load in the browser