Checksum in C++

Question

I need help with writing a code in C++ to do a 16 bit checksum.

The idea is to get data input from user (a string), convert it to binary form, then calculate checksum and put it in my data packet. I don't have any checksum type specification... I thought XORing the 16 bits would be a good idea. I have the following class packet given:

class packet
{
private:
    string message;
    unsigned int checksum;  // this is a 16 bit checksum
    unsigned int seqNum;

public:
    packet(string str, unsigned int seq);
    void setMessage(string str);
    void setSeqNum(unsigned int num);
    string getMessage();
    int getLength();
    unsigned int calculateChecksum();
    int getChecksum();
    bool getSeqNum();
};

I have to calculate my checksum here given the above:

packet::packet(string str, unsigned int seq)
{
    this->message = str;
    this->seqNum = seq;
    // calculate checksum here
    checksum = calculateChecksum(); //HOW TO CODE THIS FUNCTION?
}

Please help. I am new to C++ and C.

Answer 1

您可以使用 IPv4 标头中使用的校验和，它是 16 位：

http://en.wikipedia.org/wiki/IPv4_header_checksum

或查看 CRC16：http : //en.wikipedia.org/wiki/Crc16

如果你在谷歌上搜索一下，你可以找到更多的算法：

http://en.wikipedia.org/wiki/List_of_hash_functions

http://en.wikipedia.org/wiki/Category:Checksum_algorithms

要实现您的功能，只需首先依次转换您的每对字符String message并将其提供给校验和算法。

如果您有奇数个字节，则必须正确填充最后一个字节（检查您选择的单个算法的文档，或者只计算最后一对的 16 位值，因为最后一个缺少的字符为零）。

处理完消息后，将序列号添加到 has 并将校验和结果存储在checksum变量中。

如果您想测试您是否在此过程中犯了错误，许多算法（循环）在对包括其 CRC 在内的整个消息进行哈希处理后确实会输出零结果。如果您得到零，则表示消息没有损坏，或者您的算法在您测试时正常工作。