private function _compareMac($a, $b) {
        $result = "\x00";

        for ($i = 0; $i < $this->_macByteCount; $i++) {
            $result |= substr($a, $i, 1) ^ substr($b, $i, 1);
        }
        
        /* \x00 if equal, nonzero otherwise */
        return ($result === "\x00");
    }