public function serialize(Signature $signature, $length)
    {
        $r = str_pad($this->adapter->decHex((string) $signature->getR()), $length, '0', STR_PAD_LEFT);
        $s = str_pad($this->adapter->decHex((string) $signature->getS()), $length, '0', STR_PAD_LEFT);

        return pack('H*', $r.$s);
    }