Base64编码解码的C语言实现代码

#include<stdio.h>
#include<string.h>
//base64表，用于编码
static const char Base64Code[64] = 
{
    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
    'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
    'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
    'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
    'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
    'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
    'w', 'x', 'y', 'z', '0', '1', '2', '3',
    '4', '5', '6', '7', '8', '9', '+', '/'
};
//base64逆表，用于解码
static const char Base64Back[128] = 
{
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1,  0, -1, -1,
    -1,  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, -1, -1, -1, -1, -1, 
    -1, 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, -1, -1, -1, -1, -1,
};

//编码过程，其中in待编码字符，out为编码后字符
void base64(char* in,char* out)
{
	int i,j=0,w,m;
	w=strlen(in)/3*3;
	m=strlen(in)%3;
	
	for(i = 0;i < w;i+=3){
			out[j++] = Base64Code[in[i]>>2];
			out[j++] = Base64Code[(in[i]<<4 & 0x3f) | in[i+1]>>4];
			out[j++] = Base64Code[(in[i+1]<<2 & 0x3f) | in[i+2]>>6];
			out[j++] = Base64Code[in[i+2] & 0x3f];
	}
	switch(m){
		case 0:	out[j] = '\0';
				return;
		case 1:	out[j++] = Base64Code[in[i]>>2];
				out[j++] = Base64Code[(in[i]<<4 & 0x3f)];
				out[j++] = '=';
				out[j++] = '=';
				out[j] = '\0';
				return;
		default:out[j++] = Base64Code[in[i]>>2];
				out[j++] = Base64Code[(in[i]<<4 & 0x3f) | in[i+1]>>4];
				out[j++] = Base64Code[(in[i+1]<<2 & 0x3f)];
				out[j++] = '=';
				out[j] = '\0';
				return;
	}
}

//解码函数，in为代解码字符，out为解码后的字符
void base64_d(char* in,char* out)
{
	int i,j=0,w;
	w=strlen(in);
	for(i = 0;i < w;i+=4){
		out[j++] = Base64Back[in[i]]<<2 | Base64Back[in[i+1]]>>4;
		out[j++] = Base64Back[in[i+1]]<<4 | Base64Back[in[i+2]]>>2;
		out[j++] = Base64Back[in[i+2]]<<6 | Base64Back[in[i+3]];
	}
	
	if(in[i+2] == '='){			//倒数第二位为'='
		out[j++] = Base64Back[in[i]]<<2 | Base64Back[in[i+1]]>>4;
		out[j]='\0';
		return;
	}
	else if(in[i+3] == '=' ){	//最后一位为'='
		out[j++] = Base64Back[in[i]]<<2 | Base64Back[in[i+1]]>>4;
		out[j++] = Base64Back[in[i+1]]<<4 | Base64Back[in[i+2]]>>2;
		out[j++] = '\0';
	}
	else{
		out[j++] = Base64Back[in[i]]<<2 | Base64Back[in[i+1]]>>4;
		out[j++] = Base64Back[in[i+1]]<<4 | Base64Back[in[i+2]]>>2;
		out[j++] = Base64Back[in[i+2]]<<6 | Base64Back[in[i+3]];
		out[j] = '\0';
		return;
	}
}

做一个简单的测试：

int main()
{
	char s[100]="hello world!";
	char bs[100];
	base64(s,bs);
	printf("s:%s\n",s);
	printf("bs:%s\n\n",bs);
	
	base64_d(bs,s);
	printf("s:%s\n",s);
	printf("bs:%s\n",bs);
	return 0;
}