hash_hkdf()

hash_hkdf() — Generate a HKDF key derivation of a supplied key input.

Built-in function of PHP 8.5. The manual lists it as available in PHP 7 >= 7.1.2, PHP 8. This page is the reference card: signature, parameters and return value, plus the changelog and the example the manual provides.

Signature

hash_hkdf(string $algo, string $key, int $length = 0, string $info = "", string $salt = ""): string

Parameters

Parameter	Type	What it takes
$algo	string	Name of selected hashing algorithm (e.g. "sha256"). For a list of supported algorithms see hash_hmac_algos(). Note: Non-cryptographic hash functions are not allowed.
$key	string	Input keying material (raw binary). Cannot be empty.
$length	int	Desired output length in bytes. Cannot be greater than 255 times the chosen hash function size.
$info	string	Application/context-specific info string.
$salt	string	Salt to use during derivation.

Return value

Returns a string containing a raw binary representation of the derived key (also known as output keying material - OKM).

Example

A runnable snippet:

// Generate a random key, and salt to strengthen it during derivation. $inputKey = random_bytes(32); $salt = random_bytes(16); // Derive a pair of separate keys, using the same input created above. $encryptionKey = hash_hkdf('sha256', $inputKey, 32, 'aes-256-encryption', $salt); $authenticationKey = hash_hkdf('sha256', $inputKey, 32, 'sha-256-authentication', $salt); var_dump($encryptionKey !== $authenticationKey); // bool(true)

Changelog

Version	Change
8.0.0	Now throws a ValueError exception on error. Previously, false was returned and an E_WARNING message was emitted.