protected function decb64($num)
    {
        $bytes = array();
        while($num) {
            array_unshift($bytes, $this->mod($num, 256));
            $num = $this->div($num, 256);
        }
        
        if($bytes && $bytes[0] > 127) {
            array_unshift($bytes,0);
        }
        
        array_unshift($bytes, 'C*');
        
        return base64_encode(call_user_func_array('pack', $bytes));
    }