/
Base64是一種基于64個可打印字符來表示二進制數據的表示方法。三個字節有24個比特，對應于4個Base64單元，即3個字節需要用4個可打印字符來表示。
當最后剩余一個八位字節(一個byte)時，最后一個6位的base64字節塊有四位是0值，最后附加上兩個等號；
如果最后剩余兩個八位字節(2個byte)時，最后一個6位的base字節塊有兩位是0值，最后附加一個等號。/

include <stdio.h>
include "string.h"
include "stdlib.h"
const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
static char find_pos(char ch);
char base64_encode(const char data, int data_len,int len);
char base64_decode(const char data, int data_len,int len);
/*
 找到ch在base中的位置
 /
static char find_pos(char ch)

{

    //the last position (the only) in base[]
    char ptr = (char*)strrchr(base, ch); 
    return (ptr - base);

}
/*
 BASE64編碼
 /
char base64_encode(const char data, int data_len,int len)

{

    int prepare = 0;

    int ret_len;

    len=0;
    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;

    }

    //最后一位以''結束 
    ret_len = ret_len4 + 1;

    ret = (char *)malloc(ret_len);

if ( ret == NULL)  
{  
    printf("No enough memory.n");  
    exit(0);  
}  
memset(ret, 0, ret_len);  
f = ret;
//tmp記錄data中移動位置  
while (tmp < data_len)  
{  
    temp = 0;  
    prepare = 0;  
    memset(changed, 0, 4);  
    while (temp < 3)  
    {     
        if (tmp >= data_len)  
        {  
            break;  
        } 
        //將data前8*3位移入prepare的低24位 
        prepare = ((prepare << 8) | (data[tmp] & 0xFF));  
        tmp++;  
        temp++;  
    }  
    //將有效數據移到以prepare的第24位起始位置
    prepare = (prepare<<((3-temp)*8));  

    for (i = 0; i < 4 ;i++ )  
    {  
        //最后一位或兩位
        if (temp < i)  
        {  
            changed[i] = 0x40;  
        }  
        else
        {  
            //24位數據
            changed[i] = (prepare>>((3-i)*6)) & 0x3F;  
        }  
        *f = base[changed[i]];  

        f++;
        (*len)++; 
    }  
}  
*f = '';  

return ret;  


}
/*
 BASE64解碼
 /
char base64_decode(const char data, int data_len,int len)

{

    int ret_len = (data_len / 4)  3+1;

    int equal_count = 0;

    char ret = NULL;

    char f = NULL;  
    len=0;
    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;

    }

ret = (char *)malloc(ret_len);  
if (ret == NULL)  
{  
    printf("No enough memory.n");  
    exit(0);  
}  
memset(ret, 0, 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++;
        (*len)++;  
    }  
}
*f = '';
if(data[data_len-1]=='=')
{
    (*len)--;
}
/*
while(*(--f)=='')
{
  (*len)--;

}
*/
return ret;  

}
int main(){
    char former = "hello";
    int len1,len2;
    printf("%sn",former);
    char after = base64_encode(former, 5,&len1);
    printf("%d %sn",len1,after);
    former = base64_decode(after, len1,&len2);
    printf("%d %sn",len2,former);
}</pre>