Base64编码与解码

Base64是什么

Base64是一种将二进制编码成64个可见字符的编码方式。

Base64编码把3个8位字节转化为4个6位的字节。如果不足3个字节，则用0填充，输出字符末尾补’=’，因此编码后输出的文本末尾可能会出现1或2个’=’。为了保证所输出的编码位可读字符，Base64制定了一个编码表，以便进行统一转换。编码表的大小为2^6=64，这也是Base64名称的由来。

比如加密字符A

字符A的ASCII码是65，二进制为‭01000001‬

不足24位，末尾补0，变为01000001‬ 00000000 00000000

由于添加了2个8位，所以最终结果要补充2个’=’

划分为4个6位，变为010000 01 0000 000000 000000

查表可得010000对应’Q’，因此最终结果为”QQ==”

实现代码

#ifndef BASE64_H
#define BASE64_H

class Base64
{
public:
Base64();
bool Encode(const unsigned char* bytes, int size, char* str, int maxLen);
bool Decode(unsigned char* bytes, int& size, const char* str, int maxSize);
int SizeToLen(int size);
int LenToSize(int len);
virtual ~Base64();
protected:
private:
static const char* table;
int getIdx(char chr);
};

#endif // BASE64_H

#include "Base64.h"

Base64::Base64() { }
Base64::~Base64() { }

const char* Base64::table =  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int Base64::SizeToLen(int size) {
return (size+1)*8%6;
}

int Base64::LenToSize(int len) {
return len*6/8;
}

bool Base64::Encode(const unsigned char* bytes, int size, char* str, int maxLen) {
unsigned long bit = 0;
unsigned long idx[4];
int len = 0;
int add = 0;
for (int i = 0; i < size; i += 3) {
bit = 0;
for (int j = 0; j < 3; j++) {
bit <<= 8;
if (i + j < size) bit |= bytes[i + j];
else add++;
}
for (int j = 3; j >= 0; j--) {
if (add) {
idx[j] = '=';
add--;
}
else idx[j] = table[bit & 63];
bit >>= 6;
}
for (int j = 0; j < 4; j++) {
if (len >= maxLen) {
str[len] = '\0';
return false;
}
str[len++] = idx[j];
}
}
str[len] = '\0';
return true;
}

bool Base64::Decode(unsigned char* bytes, int& size, const char* str, int maxSize) {
unsigned long bit = 0;
unsigned long idx[3];
size = 0;
int add = 0;
for (;*str;) {
bit = 0;
for (int j = 0; j < 4; j++) {
bit <<= 6;
if (*str == '\0') continue;
if (*str != '=') bit |= getIdx(*str);
else add++;
str++;
}
for (int j = 2; j >= 0; j--) {
idx[j] = bit & 255;
bit >>= 8;
}
for (int j = 0; j < 3 - add; j++) {
if (size >= maxSize) {
bytes[size] = 0;
return false;
}
bytes[size++] = idx[j];
}
}
bytes[size] = 0;
size--;
return true;
}

int Base64::getIdx(char chr) {
if (chr >= 'A' && chr <= 'Z') return chr - 'A';
if (chr >= 'a' && chr <= 'z') return chr - 'a' + 26;
if (chr >= '0' && chr <= '9') return chr - '0' + 52;
if (chr == '+') return 62;
if (chr == '/') return 63;
return -1;
}