base64加解密

//base64 加解密
//by as1138 2013-05-18
#include <iostream>
#include <string>
#include <map>
using namespace std;

string base64_encode(const char* pSrc, size_t nLen);
string base64_decode(const char* pSrc, size_t nLen);

char chMap[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','+','/'};

map<char, int> nMap;

int main()
{
	//init map
	for (int i=0; i<64; ++i)
	{
		nMap.insert(make_pair(chMap[i],i));
	}
	string str = "asd";
	string sTmp = base64_encode(str.c_str(), str.length());
	cout << sTmp << endl;
	cout << base64_decode(sTmp.c_str(), sTmp.length()) << endl;
}

string base64_decode(const char* pSrc, size_t nLen)
{
	string strRes;
	strRes.reserve(nLen/3*2);
	size_t nNum = (pSrc[nLen-1] == '=')?4:0;

	for (int i=0; i<(nLen-nNum);i+=4)// 4 bytes -> 3 bytes
	{
		unsigned char chArr[4];
		for (int j=0; j<4; ++j)
		{
			chArr[j] = nMap[pSrc[i+j]];
		}
		char ch;
		ch = ((chArr[0]<<2)&0xFC)|((chArr[1]>>4)&0x03);//first byte
		strRes.push_back(ch);
		ch = ((chArr[1]<<4)&0xF0)|((chArr[2]>>2)&0x0F);//second byte
		strRes.push_back(ch);
		ch = ((chArr[2]<<6)&0xC0)|chArr[3];//third byte
		strRes.push_back(ch);
	}

	if (nNum == 4)//the end of '='
	{
		if (pSrc[nLen-1] == pSrc[nLen-2]) // "=="
		{
			unsigned char chArr[2] = {nMap[pSrc[nLen-4]],nMap[pSrc[nLen-3]]};
			char ch;
			ch = ((chArr[0]<<2)&0xFC)|((chArr[1]>>4)&0x03);
			strRes.push_back(ch);
		}
		else                              //'='
		{
			unsigned char chArr[3] = {nMap[pSrc[nLen-4]],nMap[pSrc[nLen-3]],nMap[pSrc[nLen-2]]};
			char ch;
			ch = ((chArr[0]<<2)&0xFC)|((chArr[1]>>4)&0x03);
			strRes.push_back(ch);
			ch = ((chArr[1]<<4)&0xF0)|((chArr[2]>>2)&0x0F);
			strRes.push_back(ch);
		}
	}
	return strRes;
}

string base64_encode(const char* pSrc, size_t nLen)
{
	size_t resNum = nLen % 3;
	string strRes;
	strRes.reserve(nLen*3/2);

	for (int i=0; i < (nLen-resNum); i+=3)//3 bytes -> 4 bytes
	{
		unsigned char ch;
		ch = (pSrc[i]>>2)&0x3F;//first byte
		strRes.push_back(chMap[ch]);
		ch = ((pSrc[i]<<4)&0x30)|((pSrc[i+1]>>4)&0x0F);//second byte
		strRes.push_back(chMap[ch]);
		ch = ((pSrc[i+1]<<2)&0x3C)|((pSrc[i+2]>>6)&0x03);//third byte
		strRes.push_back(chMap[ch]);
		ch = pSrc[i+2]&0x3F;//fourth byte
		strRes.push_back(chMap[ch]);
	}

	//less than 3 byte
	switch(resNum)
	{
	     case 1://1 byte -> 2 bytes and 2 bytes = "=="
		     {
				unsigned char ch;
				ch = (pSrc[nLen-1]>>2)&0x3F;//first byte
				strRes.push_back(chMap[ch]);
				ch = (pSrc[nLen-1]<<4)&0x30;//second byte
				strRes.push_back(chMap[ch]);
				strRes.append("==");// last 2 bytes
		     	break;
		     }
	     case 2://2 byte ->3 bytes and 1 byte = '='
		     {
				unsigned char ch;
				ch = (pSrc[nLen-2]>>2)&0x3F;//first byte
				strRes.push_back(chMap[ch]);
				ch = ((pSrc[nLen-2]<<4)&0x30)|((pSrc[nLen-1]>>4)&0x0F);//second byte
				strRes.push_back(chMap[ch]);
				ch = (pSrc[nLen-1]<<2)&0x3C;//third byte;
				strRes.push_back(chMap[ch]);
				strRes.push_back('=');//last 1 byte
			    break;
		     }
	     default:
		        break;
	}

	return strRes;
}//end fucntion base64_encode