c语言实现base64_c语言 base64

Base64字符包括 A-Z a-z 0-9 + / 共计64个字符，对应0-63

将原始字符串按6bit拆分，前面补两个0组成一个新的字节

最后一位不满8位补0  ，在末尾用=标记补了几个0，补两个0加一个= ，补四个0加俩=

C语言实现如下：

#include "stdio.h"
#include "string.h"
#include "stdlib.h"
 
char* 	base64_decode(const char* str){
	unsigned char imap[79]={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};
	int rest_count =0;
	int buf_idx=0;
	int len = strlen(str);
	int flen = len;
	while(str[flen-1]=='=')
		flen--;
	int buflen = flen*6/8+1;
	unsigned char* buf = (unsigned char*) malloc(buflen);
	memset(buf,'\0',buflen);
	for(int i=0;i<flen;i++){
		unsigned char c=imap[str[i]-43];
		int mcount = rest_count ==0?-2:6-rest_count;
		unsigned char c0 = (mcount>0?c>>mcount:c<<-mcount);
		buf[buf_idx] = buf[buf_idx] | c0;
		rest_count= mcount <0 ? -mcount : 8-mcount;
		if(mcount>=0)
			buf[++buf_idx]=c<<rest_count;
	}
	return buf; 	
}	
 
char*	base64_encode(const char* str){
	char map[65]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
	int len =strlen(str);
	int buflen = len * 8 / 6 + 4;//缺位补齐1 俩等号2 最后0-1位
	unsigned char* buf = (unsigned char*)malloc(buflen);
	memset(buf,'\0',buflen);
	char a [4];
	int rest_count = 0;
	int index = 0;
	int buf_idx =0 ;
	unsigned char last  = 0;
	for(int i=0;i<len;i++){
		unsigned char c = str[i];
		a[index]= (last<<(6-rest_count))&63;
		int get_count = 6-rest_count;
		a[index] = a[index] |  ( c >>(8-get_count));
		a[index]=map[a[index]];
		rest_count = 8-get_count;
		last = c;
		index = index==2?0:index+1;
		if(index==0){
			a[3]=map[c&63];
			memcpy(buf+buf_idx,a,4);
			buf_idx+=4;
			rest_count=0;
			last = 0;
		}
	}
	if(index!=0){
		a[index]= last<<(6-rest_count) & 63;
		a[index]=map[a[index]];
		memcpy(buf+buf_idx,a,index+1);
		buf[buf_idx+index+1]='=';	
		buf[buf_idx+index+2]= rest_count ==2 ? '=':0;	
	}
	return buf;
}
int main()
{
	char* c = base64_encode("abcdefgh");
	printf("decode:%s\n",c);
	char* d = base64_decode(c);
	printf("encode:%s\n",d);
	free(c);
	free(d);
}	

输出结果如下：