(PHP 7 >= 7.1.2)
hash_hkdf — Generate a HKDF key derivation of a supplied key input
$algo
, string $ikm
[, int $length = 0
[, string $info = ''
[, string $salt = ''
]]] ) : string
algoName of selected hashing algorithm (i.e. "sha256", "sha512", "haval160,4", etc..) See hash_algos() for a list of supported algorithms.
Note:
Non-cryptographic hash functions are not allowed.
ikmInput keying material (raw binary). Cannot be empty.
lengthDesired 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.
infoApplication/context-specific info string.
saltSalt to use during derivation.
While optional, adding random salt significantly improves the strength of HKDF.
Returns a string containing a raw binary representation of the derived key
(also known as output keying material - OKM); or FALSE on failure.
An E_WARNING will be raised if ikm
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).
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.