# Base 64

# 概述

这是一个将二进制的数据可视化的过程,通过 6 位的字符,作为索引在 64 个字符中获取字符,组成全部为可视化字符。

# 加密

unsigned char *base64_encode(unsigned char *str)
{
    long len;
    long str_len;
    unsigned char *res;
    int i, j;
    //定义base64编码表
    unsigned char *base64_table = 
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    //计算经过base64编码后的字符串长度
    str_len = strlen(str);
    if (str_len % 3 == 0)
        len = str_len / 3 * 4;
    else
        len = (str_len / 3 + 1) * 4;

    res = malloc(sizeof(unsigned char) * len + 1);
    res[len] = '\0';

    //以3个8位字符为一组进行编码
    for (i = 0, j = 0; i < len - 2; j += 3, i += 4)
    {
        //取出第一个字符的前6位并找出对应的结果字符
        res[i] = base64_table[str[j] >> 2];
        //将第一个字符的后位与第二个字符的前4位进行组合并找到对应的结果字符           
        res[i + 1] = base64_table[(str[j] & 0x3) << 4 | (str[j + 1] >> 4)];
        //将第二个字符的后4位与第三个字符的前2位组合并找出对应的结果字符   
        res[i + 2] = base64_table[(str[j + 1] & 0xf) << 2 | (str[j + 2] >> 6)]; 
        //取出第三个字符的后6位并找出结果字符
        res[i + 3] = base64_table[str[j + 2] & 0x3f];                           
    }

    switch (str_len % 3)
    {
    case 1:
        res[i - 2] = '=';
        res[i - 1] = '=';
        break;
    case 2:
        res[i - 1] = '=';
        break;
    }

    return res;
}

# 解密

unsigned char *base64_decode(unsigned char *code)
{
    //根据base64表,以字符找到对应的十进制数据
    int table[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
                   0, 0, 0, 0, 0, 0, 0, 62, 0, 0, 0,
                   63, 52, 53, 54, 55, 56, 57, 58,
                   59, 60, 61, 0, 0, 0, 0, 0, 0, 0, 0,
                   1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
                   13, 14, 15, 16, 17, 18, 19, 20, 21,
                   22, 23, 24, 25, 0, 0, 0, 0, 0, 0, 26,
                   27, 28, 29, 30, 31, 32, 33, 34, 35,
                   36, 37, 38, 39, 40, 41, 42, 43, 44,
                   45, 46, 47, 48, 49, 50, 51};
    long len;
    long str_len;
    unsigned char *res;
    int i, j;

    //计算解码后的字符串长度
    len = strlen(code);
    //判断编码后的字符串后是否有=
    if (strstr(code, "=="))
        str_len = len / 4 * 3 - 2;
    else if (strstr(code, "="))
        str_len = len / 4 * 3 - 1;
    else
        str_len = len / 4 * 3;

    res = malloc(sizeof(unsigned char) * str_len + 1);
    res[str_len] = '\0';

    //以4个字符为一位进行解码
    for (i = 0, j = 0; i < len - 2; j += 3, i += 4)
    {
        //取出第一个字符对应base64表的十进制数的前6位
        //与第二个字符对应base64表的十进制数的后2位进行组合
        res[j] = ((unsigned char)table[code[i]]) << 2 |
             (((unsigned char)table[code[i + 1]]) >> 4);        
        //取出第二个字符对应base64表的十进制数的后4位
        //与第三个字符对应bas464表的十进制数的后4位进行组合   
        res[j + 1] = (((unsigned char)table[code[i + 1]]) << 4) 
            | (((unsigned char)table[code[i + 2]]) >> 2); 
        //取出第三个字符对应base64表的十进制数的后2位与第4个字符进行组合
        res[j + 2] = (((unsigned char)table[code[i + 2]]) << 6) 
            | ((unsigned char)table[code[i + 3]]);        
    }

    return res;
}