base64编码简介

什么是base64编码？

• Base64是一种基于64个可打印字符来表示二进制数据的表示方法。由于 2^6=64 ，所以每6个比特为一个单元，对应某个可打印字符。3个字节有24个比特，对应于4个Base64单元，即3个字节可由4个可打印字符来表示。
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• 如果要编码的字节数不能被3整除，最后会多出1个或2个字节，那么可以使用下面的方法进行处理：先使用0字节值在末尾补足，使其能够被3整除，然后再进行Base64的编码。在编码后的Base64文本后加上一个或两个=号，代表补足的字节数。也就是说，当最后剩余两个八位字节（2个byte）时，最后一个6位的Base64字节块有四位是0值，最后附加上两个等号；如果最后剩余一个八位字节（1个byte）时，最后一个6位的base字节块有两位是0值，最后附加一个等号。 参考下表：

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Base64编码与解码的C++实现


#include <iostream>
#include <string>
#include <unistd.h>

using namespace std;
 
class Base64
{
public:
    int Encode(string &s);
    int Decode(string &s);
    int EncodeWeb(string &s);
    int DecodeWeb(string &s);
private:
    const unsigned char Base64EncodeMap[64] =
    {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
        'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
        'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
        'w', 'x', 'y', 'z', '0', '1', '2', '3',
        '4', '5', '6', '7', '8', '9', '+', '/'
    };
    const unsigned char Base64DecodeMap[256] =
    {
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0x3E, 0xFF, 0xFF, 0xFF, 0x3F,
        0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3A, 0x3B,
        0x3C, 0x3D, 0xFF, 0xFF, 0xFF, 0x00, 0xFF, 0xFF,
        0xFF, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06,
        0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E,
        0x0F, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16,
        0x17, 0x18, 0x19, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20,
        0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28,
        0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E, 0x2F, 0x30,
        0x31, 0x32, 0x33, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
    };
    string EncodeEach(char s[3]);
    string s_processed;
    int DecodeCheck(const string &s, bool isWebSafe);
};
int Base64::Encode(string &s)
{
    char str_to_operate[3] = { 0 };
    for (size_t i = 0; s.size() >= 3; ++i)
    {
        for (size_t j = 0; j != 3; ++j)
            str_to_operate[j] = s[j];
        s = s.substr(3);
        s_processed += EncodeEach(str_to_operate);
    }
    switch (s.size())
    {
    case 0:break;
    case 1:
        str_to_operate[0] = s[0];
        str_to_operate[1] = 0;
        str_to_operate[2] = 0;
        break;
    case 2:
        str_to_operate[0] = s[0];
        str_to_operate[1] = s[1];
        str_to_operate[2] = 0;
        break;
    }
    s_processed += EncodeEach(str_to_operate);
    s = s_processed;
    return 0;
}
 
string Base64::EncodeEach(char s[3])
{
    string tmp;
    tmp.clear();
    if (s[0] != 0)
    {
        tmp += Base64EncodeMap[s[0] >> 2];
        if (s[1] != 0)
        {
            tmp += Base64EncodeMap[((s[0] & 0x03) << 4) | (s[1] >> 4)];
            if (s[2] != 0)
            {
                tmp += Base64EncodeMap[((s[1] & 0x0F) << 2) | (s[2] >> 6)];
                tmp += Base64EncodeMap[s[2] & 0x3F];
            }
            else
            {
                tmp += Base64EncodeMap[(s[1] & 0x0F) << 2];
                tmp += "=";
            }
        }
        else
        {
            tmp += Base64EncodeMap[((s[0] & 0x03) << 4)];
            tmp += "==";
        }
    }
    return tmp;
}
 
int Base64::EncodeWeb(string &s)
{
    Encode(s);
    for (string::size_type i = 0; i != s.size(); ++i)
    {
        if (s[i] == '+')
            s[i] = '-';
        else if (s[i] == '/')
            s[i] = '_';
    }
    return 0;
}
 
int Base64::DecodeCheck(const string &s, bool isWebSafe)  //检查待解码的字符串是否是Base64编码而来
{
    if (isWebSafe == true)
    {
        for (string::const_iterator it = s.begin(); it != s.end(); ++it)
            if (!isalnum(*it) && *it != '+' && *it != '/')
                return -1;
    }
    else
    {
        for (string::const_iterator it = s.begin(); it != s.end(); ++it)
            if (!isalnum(*it) && *it != '-' && *it != '_')
                return -1;
    }
    return 0;
}
 
int Base64::Decode(string &s)
{
    if (DecodeCheck(s, false) != -1)
    {
        char str_to_operate[4];
        for (size_t i = 0; s.size() >= 4; ++i)
        {
            for (size_t j = 0; j != 4; ++j)
            {
                str_to_operate[j] = Base64DecodeMap[s[j]];
            }
            s = s.substr(4);
            s_processed += (str_to_operate[0] << 2) | (str_to_operate[1] >> 4);
            s_processed += (str_to_operate[1] << 4) | (str_to_operate[2] >> 2);
            s_processed += (str_to_operate[2] << 6) | str_to_operate[3];
        }
        s = s_processed;
    }
    else return -1;
    return 0;
}
int Base64::DecodeWeb(string &s)
{
    if (s.size() % 4 == 0)
    {
        if (!DecodeCheck(s, true))
        {
            for (string::size_type i = 0; i != s.size(); ++i)
            {
                if (s[i] == '-')
                    s[i] = '+';
                else if (s[i] == '_')
                    s[i] = '/';
            }
            Decode(s);
            return 0;
        }
        else return -1;
    }
    else return -1;
 
}
 
int main()
{
     cout<<"1.coding\n2.decoding\noperating(1 or 2):"<<ends;
    char input;
    string s;
    Base64 a;
    cin >> input;
    if (input == '1')
    {
        cout << "Enter the string to be encrypted:" << endl;
        cin >> s;
        a.Encode(s);
        cout << "Encoding result:\n" << s << endl;
    }
    else if (input == '2')
    {
        cout << "Enter the string to be decrypted:" << endl;
        cin >> s;
        a.Decode(s);
        cout << "Decoded result:\n" << s << endl;
    }
    else
        cout << "input error" << endl;

    pause();
    return 0;
}