搬运自：https://blog.csdn.net/qq_26093511/article/details/78836087

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#include<stdio.h>
#include<stdlib.h>
#include<string.h>

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)  
    {  
        res[i]=base64_table[str[j]>>2]; 
		//取出第一个字符的前6位并找出对应的结果字符  
        res[i+1]=base64_table[(str[j]&0x3)<<4 | (str[j+1]>>4)]; 
		//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+2]=base64_table[(str[j+1]&0xf)<<2 | (str[j+2]>>6)]; 
		//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+3]=base64_table[str[j+2]&0x3f]; 
		//取出第三个字符的后6位并找出结果字符  
    }  
  
    switch(str_len % 3)  
    {  
        case 1:  
            res[i-2]='=';  
            res[i-1]='=';  
            break;  
        case 2:  
            res[i-1]='=';  
            break;  
    }  
  
    return res;  
}  
  
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)  
    {  
        res[j]=((unsigned char)table[code[i]])<<2 | (((unsigned char)table[code[i+1]])>>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+1]=(((unsigned char)table[code[i+1]])<<4) | (((unsigned char)table[code[i+2]])>>2); 
		//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+2]=(((unsigned char)table[code[i+2]])<<6) | ((unsigned char)table[code[i+3]]); 
		//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  
    }  
  
    return res;  
  
}  
int main(int argc,char **argv)  
{  
    unsigned char *buf =NULL;
    if(strcmp(argv[1],"-d") == 0) 
    { 
	buf = base64_decode(argv[2]);
        printf("%s\n",buf);  
    }
    else  
    {
	buf = base64_encode(argv[1]);
        printf("%s\n",buf);   
    }
 
    free(buf);
  
    return 0;  
}