C语言实现base64编码

base64编码，基于C语言实现；

为了直观表现原理，使用位操作符实现；

base64编码原理：

每个字节都是由8位二进制组成，base64编码是将3个8位的字节转换成4个6位的base64码；

base64码表：



实现方式：

q=01110001

w=01110111

e=01100101

char buf[4]="qwe";

buf前三位的二进制编码为：

buf=01110001 01110111 01100101
    (q)
   (w)     (e)

base64  b1,b2,b3,b4

b1=011100  = 28

b2=010111  = 23

b3=011101  = 29

b4=100101  = 37

对照base64码表：

b1=c

b2=X

b3=d

b4=l

如果源码buf的字节数不能被3整除会做如下处理：

一.如果余数是1，即buf最后有一个字节剩余,后两个字节会补0，形成24位二进制；但是后两个字节在编码的时候，会将它编译成 '=';

1. buf=00110000

b1=001100

b2=000000 因为第一个字节最后会剩下两个0 ，所以b2的实际编码是0；即base64表里面的 'A'

b3='='
b4='='

2. buf=00110001
b1=001100
b2=010000
b3='='
b4='='

二。如果余数是2，即buf最后有两个字节剩余：

1. buf=00110000 00000000
b1=001100
b2=000000  =='A'
b3=000000  =='A'

b4= '='

2. buf=00110001 00000100
b1=001100
b2=010000
b3=010000

b4= '='

代码：

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

char Base[65]={};

void ch_to_base64(char src[])
{

char dest[64]={};
int s_len=strlen(src);
int remainder = s_len%3;
int times = (s_len-remainder)/3;
int pos = 0;
int i = 0,j = 0;
int parr[5];
while(times){
unsigned int tmp1 = (unsigned int) src[pos++];
unsigned int tmp2 = (unsigned int) src[pos++];
unsigned int tmp3 = (unsigned int) src[pos++];
parr[0] = tmp1>>2;
parr[1] = ((tmp1-(tmp1>>2<<2))<<4) + (tmp2>>4);
parr[2] = ((tmp2-(tmp2>>4<<4))<<2) +(tmp3>>6);
parr[3] = tmp3-(tmp3>>6<<6);

for(i,j=0;j<4;i++,j++){
dest[i] = Base[parr[j]];
}
times--;
}
if(remainder == 1){
unsigned int re1 = (unsigned int)src[pos];
if((re1&3) == 0) {
parr[0] = re1>>2;
dest[i++]=Base[parr[0]];
dest[i++]=Base[0];
dest[i++]='=';
dest[i++]='=';

}else{
parr[0] = re1>>2;
parr[1] = (re1&3)<<4;
dest[i++]=Base[parr[0]];
dest[i++]=Base[parr[1]];
dest[i++]='=';
dest[i++]='=';
}
}
if(remainder == 2){
unsigned int re1 = (unsigned int)src[pos++];
unsigned int re2 = (unsigned int)src[pos];
if((re2&15) == 0){
parr[0] = re1>>2;
parr[1] = ((re1 - (re1>>2<<2))<<4) + (re2>>4);
dest[i++]=Base[parr[0]];
dest[i++]=Base[parr[1]];
dest[i++]=Base[0];
dest[i++]='=';
}else{
parr[0] = re1>>2;
parr[1] = ((re1 - (re1>>2<<2))<<4) + (re2>>4);
parr[2] = (re2-(re2>>4<<4))<<2 ;
dest[i++] = Base[parr[0]];
dest[i++] = Base[parr[1]];
dest[i++] = Base[parr[2]];
dest[i++]='=';
}
}
strcpy(src,dest);
}

int main(int argc,char **argv)
{
strcpy(Base,"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/");
if(argc !=2) {
printf("error: param num is wrong!\n");
return -1;
}
ch_to_base64(argv[1]);
printf("buf = %s\n",argv[1]);
}