/**
* Key derivation wth pbkdf2: http://en.wikipedia.org/wiki/PBKDF2
* @param string $key payload
* @param string $salt random string from generate_salt
* @param string $length result length
* @param string $count iterations
* @param string $algo hash algorithm to use
*/
public static function pbkdf2($key, $salt, $length, $count, $algo)
{
/* requires PHP >= 5.5 */
if (Hm_Functions::function_exists('openssl_pbkdf2')) {
return openssl_pbkdf2($key, $salt, $length, $count, $algo);
}
/* manual version */
$size = strlen(hash($algo, '', true));
$len = ceil($length / $size);
$result = '';
for ($i = 1; $i <= $len; $i++) {
$tmp = hash_hmac($algo, $salt . pack('N', $i), $key, true);
$res = $tmp;
for ($j = 1; $j < $count; $j++) {
$tmp = hash_hmac($algo, $tmp, $key, true);
$res ^= $tmp;
}
$result .= $res;
}
return substr($result, 0, $length);
}
