Base64加密解密C语言实现

void encodeBase64(char* str,int len,char** in){
	char base64[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
	//读取3个字节zxc，转换为二进制01111010 01111000 01100011
    //转换为4个6位字节，011110 100111 100001 100011
    //不足8位在前补0，变成00011110 00100111 00100001 00100011
    //若剩余的字节数不足以构成4个字节，补等号
	int encodeStrLen = 1 + (len/3)*4 ,k=0;
	encodeStrLen += len%3 ? 4 : 0;
    char* encodeStr = (char*)(malloc(sizeof(char)*encodeStrLen));
    for(int i=0;i<len;i++){
    	if(len - i >= 3){
            encodeStr[k++] = base64[(unsigned char)str[i]>>2];
    		encodeStr[k++] = base64[((unsigned char)str[i]&0x03)<<4 | (unsigned char)str[++i]>>4];
    		encodeStr[k++] = base64[((unsigned char)str[i]&0x0f)<<2 | (unsigned char)str[++i]>>6];
            encodeStr[k++] = base64[(unsigned char)str[i]&0x3f];
    	}else if(len-i == 2){
            encodeStr[k++] = base64[(unsigned char)str[i] >> 2];
            encodeStr[k++] = base64[((unsigned char)str[i]&0x03) << 4 | ((unsigned char)str[++i] >> 4)];
            encodeStr[k++] = base64[((unsigned char)str[i]&0x0f) << 2];
            encodeStr[k++] = '=';
    	}else{
    		encodeStr[k++] = base64[(unsigned char)str[i] >> 2];
            encodeStr[k++] = base64[((unsigned char)str[i] & 0x03) << 4];                                                                                                              //末尾补两个等于号
            encodeStr[k++] = '=';
            encodeStr[k++] = '=';
    	}
    }
    encodeStr[k] = '\0';
    *in = encodeStr;
}

/**
* 解码既编码的逆过程，先找出编码后的字符在编码之前代表的数字
* 编码中将3位个字符变成4个字符，得到这4个字符的每个字符代表的原本数字
* 因为在编码中间每个字符用base64码表进行了替换，所以这里要先换回来
* 在对换回来的数字进行位运算使其还原成3个字符
*/
void decodeBase64(char* str,int len,char** in){
	char base64[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    char ascill[129];
    int k = 0;
    for(int i=0;i<64;i++){
        ascill[base64[i]] = k++;
    }
	int decodeStrlen = len / 4 * 3 + 1;
	char* decodeStr = (char*)malloc(sizeof(char)*decodeStrlen);
	k = 0;
	for(int i=0;i<len;i++){
        decodeStr[k++] = (ascill[str[i]] << 2) | (ascill[str[++i]] >> 4);
		if(str[i+1] == '='){
			break;
		}
        decodeStr[k++] = (ascill[str[i]] << 4) |  (ascill[str[++i]] >> 2);
		if(str[i+1] == '='){
			break;
		}
        decodeStr[k++] = (ascill[str[i]] << 6) | (ascill[str[++i]]);
	}
	decodeStr[k] = '\0';
	*in = decodeStr;
}