c实现base64

版权声明：本文为博主原创文章，转载请注明原文地址 https://blog.csdn.net/Ivan_zgj/article/details/51296340

今天给自己的android项目写了一个加解密算法。base64的原理如下：

内存1个字符占8位

转前： s 1 3

先转成ascii：对应 115 49 51

2进制： 01110011 00110001 00110011

6个一组（4组） 011100110011 000100110011

然后才有后面的 011100 110011 000100 110011

然后计算机是8位8位的存数 6不够，自动就补两个高位0了

所有有了 高位补0

科学计算器输入 00011100 00110011 00000100 00110011

得到 28 51 4 51

查对下照表 c z E z

c实现的代码：

#include <malloc.h>
#include <string.h>
#include "base64.h"

static const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

char *base64_encode(const char* data, int data_len)
{
    //int data_len = strlen(data);
    int prepare = 0;
    int ret_len;
    int temp = 0;
    char *ret = NULL;
    char *f = NULL;
    int tmp = 0;
    char changed[4];
    int i = 0;
    ret_len = data_len / 3;
    temp = data_len % 3;
    if (temp > 0)
    {
        ret_len += 1;
    }
    ret_len = ret_len*4 + 1;
    ret = (char *)malloc((size_t) ret_len);

    if ( ret == NULL)
    {
        return ("No enough memory.\n");
    }
    memset(ret, 0, (size_t) ret_len);
    f = ret;
    while (tmp < data_len)
    {
        temp = 0;
        prepare = 0;
        memset(changed, '\0', 4);
        while (temp < 3)
        {
            //printf("tmp = %d\n", tmp);
            if (tmp >= data_len)
            {
                break;
            }
            prepare = ((prepare << 8) | (data[tmp] & 0xFF));
            tmp++;
            temp++;
        }
        prepare = (prepare<<((3-temp)*8));
        for (i = 0; i < 4 ;i++ )
        {
            if (temp < i)
            {
                changed[i] = 0x40;
            }
            else
            {
                changed[i] = (char) ((prepare>>((3-i)*6)) & 0x3F);
            }
            *f = base[changed[i]];
            //printf("%.2X", changed[i]);
            f++;
        }
    }
    *f = '\0';

    return ret;

}
/* */
static char find_pos(char ch)
{
    char *ptr = strrchr(base, ch);//the last position (the only) in base[]
    return (char)(ptr - base);
}
/* */
char *base64_decode(const char *data, int data_len)
{
    int ret_len = (data_len / 4) * 3;
    int equal_count = 0;
    char *ret = NULL;
    char *f = NULL;
    int tmp = 0;
    int temp = 0;
    char need[3];
    int prepare = 0;
    int i = 0;
    if (*(data + data_len - 1) == '=')
    {
        equal_count += 1;
    }
    if (*(data + data_len - 2) == '=')
    {
        equal_count += 1;
    }
    if (*(data + data_len - 3) == '=')
    {//seems impossible
        equal_count += 1;
    }
    switch (equal_count)
    {
        case 0:
            ret_len += 4;//3 + 1 [1 for NULL]
            break;
        case 1:
            ret_len += 4;//Ceil((6*3)/8)+1
            break;
        case 2:
            ret_len += 3;//Ceil((6*2)/8)+1
            break;
        case 3:
            ret_len += 2;//Ceil((6*1)/8)+1
            break;
        default:
            break;
    }
    ret = (char *)malloc((size_t) ret_len);
    if (ret == NULL)
    {
        return NULL;
    }
    memset(ret, 0, (size_t) ret_len);
    f = ret;
    while (tmp < (data_len - equal_count))
    {
        temp = 0;
        prepare = 0;
        memset(need, 0, 4);
        while (temp < 4)
        {
            if (tmp >= (data_len - equal_count))
            {
                break;
            }
            prepare = (prepare << 6) | (find_pos(data[tmp]));
            temp++;
            tmp++;
        }
        prepare = prepare << ((4-temp) * 6);
        for (i=0; i<3 ;i++ )
        {
            if (i == temp)
            {
                break;
            }
            *f = (char)((prepare>>((2-i)*8)) & 0xFF);
            f++;
        }
    }
    *f = '\0';
    return ret;
}