PHP Manual: Generate a HKDF key derivation of a supplied key input

Description

string hash_hkdf(
    string $algo,
    #[\SensitiveParameter]string $key,
    int $length = 0,
    string $info = "",
    string $salt = ""
)

Parameters

algo

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

Input keying material (raw binary). Cannot be empty.

length

Desired output length in bytes. Cannot be greater than 255 times the chosen hash function size.

If length is 0, the output length will default to the chosen hash function size.

info

Application/context-specific info string.

salt

Salt to use during derivation.

While optional, adding random salt significantly improves the strength of HKDF.

Return Values

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

Errors/Exceptions

Throws a ValueError exception if key is empty, algo is unknown/non-cryptographic, length is less than 0 or too large (greater than 255 times the size of the hash function).

Changelog

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

Examples

Example #1 hash_hkdf example

<?php
// 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)
?>

The above example produces a pair of separate keys, suitable for creation of an encrypt-then-HMAC construct, using AES-256 and SHA-256 for encryption and authentication respectively.

hash_hkdf