base64编解码的两个函数-白红宇

base64编解码的两个函数

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发布时间：2019-03-24

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base64编码解码函数实现

base64是一种基于ASCII字符的传输编码方法，广泛应用于数据的压缩和保留。以下是base64编码和解码的实现代码，以及详细说明。

base64编码函数

unsigned char *base64_encode(unsigned char *str, long* lpBufLen)
{
    const unsigned char *base64_table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    long str_len = strlen((char*)str);
    
    // 计算编码后的长度
    long len = (str_len % 3 == 0) ? str_len / 3 * 4 : (str_len / 3 + 1) * 4;
    
    // 提示用户分配内存
    if (lpBufLen != NULL)
    {
        *lpBufLen = len;
    }
    
    // 分配编码结果
    unsigned char *res = (unsigned char*)malloc(len + 1);
    res[len] = '\0';
    
    // 径向过程
    for (long i = 0, j = 0; i < len - 2; j += 3, i += 4)
    {
        // 取出三字节进行编码
        unsigned char a = str[j], b = str[j+1], c = str[j+2];
        
        // 取出每组的六个二进制位
        unsigned char v = (c >> 6) | (b >> 2) | (a >> 0);
        unsigned char v1 = (c >> 2) | (b >> 4);
        unsigned char v2 = (c << 2) | a;
        unsigned char v3 = c >> 6;
        
        res[i] = base64_table[v3];
        res[i+1] = base64_table[v1];
        res[i+2] = base64_table[(v2 & 0b00001111)];
        
        // 添加等号填充
        if (i == len - 4)
        {
            res[i+4] = '=';
            res[i+5] = '=';
        }
    }
    
    // 处理最后的填充
    if (str_len % 3 != 0)
    {
        res[len-2] = '=';
        res[len-1] = '=';
    }
    
    return res;
}

base64解码函数

unsigned char *base64_decode(unsigned char *code, long* lpBufLen)
{
    int table[] = [ ... ]; // 伪代码示例
    
    long len = strlen((char*)code);
    long str_len = len;
    
    // 计算解码后的长度
    str_len = (str_len % 4 == 2 ? str_len : (str_len % 4 == 1 ? str_len + 2 : str_len));
    
    if (lpBufLen != NULL)
    {
        *lpBufLen = str_len;
    }
    
    unsigned char *res = (unsigned char*)malloc(str_len + 1);
    res[str_len] = '\0';
    
    // 解码过程
    for (long i = 0; i < len - 3; i += 4)
    {
        unsigned char v1 = code[i], v2 = code[i+1], v3 = code[i+2], v4 = code[i+3];
        
        unsigned char a = (v3 & 0b100000) | (v2 >> 4) | (v1 >> 6);
        unsigned char b = (v3 & 0b011111) | (v2 << 2);
        long c = ((v1 & 0b111) << 8) | v4;
        unsigned char ch = table[c / 4 * 62 + 26 + 1];
        unsigned char ch2 = table[c % 4 * 62 + 26];
        
        if (i == len - 4)
        {
            res[i+2] = ch = '=';
            res[i+3] = ch2 = '=';
        }
    }
    
    return res;
}