Base64 编解码C语言实现

这里给出的代码是整个完整的编码和解码，将加密之后的串解密得到原来的字符串。

具体代码如下：

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#include <stdio.h>   
const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";   
char* base64_encode(const char* data, int data_len);   
char *base64_decode(const char* data, int data_len);   
static char find_pos(char ch);   
int main(int argc, char* argv[])   
{   
    char *t = "那个abcd你好吗，哈哈，ANMOL";   
    int i = 0;   
    int j = strlen(t);   
    char *enc = base64_encode(t, j);   
    int len = strlen(enc);   
    char *dec = base64_decode(enc, len);   
    printf("\noriginal: %s\n", t);   
    printf("\nencoded : %s\n", enc);   
    printf("\ndecoded : %s\n", dec);   
    free(enc);   
    free(dec);   
    return 0;   
}   
/* */ 
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(ret_len);   
       
    if ( ret == NULL)   
    {   
        printf("No enough memory.\n");   
        exit(0);   
    }   
    memset(ret, 0, 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));   
        //printf("before for : temp = %d, prepare = %d\n", temp, prepare);   
        for (i = 0; i < 4 ;i++ )   
        {   
            if (temp < i)   
            {   
                changed[i] = 0x40;   
            }   
            else 
            {   
                changed[i] = (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 = (char*)strrchr(base, ch);//the last position (the only) in base[]   
    return (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;   
    }   
    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++;   
        }   
    }   
    *f = '\0';   
    return ret;   
} 
#include <stdio.h>
const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
char* base64_encode(const char* data, int data_len);
char *base64_decode(const char* data, int data_len);
static char find_pos(char ch);
int main(int argc, char* argv[])
{
char *t = "那个abcd你好吗，哈哈，ANMOL";
int i = 0;
int j = strlen(t);
char *enc = base64_encode(t, j);
int len = strlen(enc);
char *dec = base64_decode(enc, len);
printf("\noriginal: %s\n", t);
printf("\nencoded : %s\n", enc);
printf("\ndecoded : %s\n", dec);
free(enc);
free(dec);
return 0;
}
/* */
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(ret_len);

if ( ret == NULL)
{
   printf("No enough memory.\n");
   exit(0);
}
memset(ret, 0, 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));
   //printf("before for : temp = %d, prepare = %d\n", temp, prepare);
   for (i = 0; i < 4 ;i++ )
   {
    if (temp < i)
    {
     changed[i] = 0x40;
    }
    else
    {
     changed[i] = (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 = (char*)strrchr(base, ch);//the last position (the only) in base[]
return (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;
}
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++;
   }
}
*f = '\0';
return ret;
}

这里添加了两个方法find_pos和base64_decode。前者是寻找给定字符在base数组中的位置的，使用了strrchr函数，寻找字符在字符串中最后一次的位置，由于总会存在并且仅存在一次，所以函数结果直接使用。base64_decode函数里边跟encode函数差不多，我没有作注释。

对于代码中给定的串，运行结果如下：

original: 那个abcd你好吗，哈哈，ANMOL

encoded : xMe49mFiY2TE47rDwvCjrLn+uf6jrEFOTU9M

decoded : 那个abcd你好吗，哈哈，ANMOL