protected static function splitKeys(Key $master, string $salt = '', Config $config = null) : array
{
$binary = $master->getRawKeyMaterial();
return [Util::hkdfBlake2b($binary, \Sodium\CRYPTO_SECRETBOX_KEYBYTES, $config->HKDF_SBOX, $salt), Util::hkdfBlake2b($binary, \Sodium\CRYPTO_AUTH_KEYBYTES, $config->HKDF_AUTH, $salt)];
}