//用更接近于编程的思维来说，编码的过程是这样的：
//第一个字符通过右移2位获得第一个目标字符的Base64表位置，根据这个数值取到表上相应的字符，就是第一个目标字符。
//然后将第一个字符与0x03(00000011)进行与(&)操作并左移4位,接着第二个字符右移4位，两者相或(|)，或者相加，即获得第二个目标字符。
//再将第二个字符与0x0f(00001111)进行与(&)操作并左移2位,接着第三个字符右移6位，两者相或(|)，或者相加，即获得第三个目标字符。
//最后将第三个字符与0x3f(00111111)进行与(&)操作，即获得第四个目标字符。//在以上的每一个步骤之后，再把结果与 0x3F 进行 AND 位操作，就可以得到编码后的字符了。

const char* Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxwz0123456789+/";
std::string Base64encode(const unsigned char* buf, unsigned int size)
{std::string sBase64;unsigned int i;for (i = 0; i < size/3*3; i += 3){sBase64 += Base64[buf[i] >> 2];sBase64 += Base64[(buf[i]<<4 | buf[i+1]>>4) & 0x3f];sBase64 += Base64[(buf[i+1]<<2 | buf[i+2]>>6) & 0x3f];sBase64 += Base64[buf[i+2] & 0x3f];}if (size % 3 == 1){sBase64 += Base64[buf[i] >> 2];sBase64 += Base64[buf[i]<<4 & 0x3f];sBase64 += '=';sBase64 += '=';}else if (size % 3 == 2){sBase64 += Base64[buf[i] >> 2];sBase64 += Base64[(buf[i]<<4 | buf[i+1]>>4) & 0x3f];sBase64 += Base64[buf[i+1]<<2 & 0x3f];sBase64 += '=';}return sBase64;
}
std::string Base64decode(std::string sBase64)
{std::string sRet;unsigned int nLen = sBase64.size();//nLen一定是4的倍数unsigned char* a = new unsigned char[nLen];int count = 0; // the number of '='.for (unsigned i = 0; i < nLen; ++i){if (sBase64[i] >= 'A' && sBase64[i] <= 'Z')a[i] = sBase64[i] - 'A';else if (sBase64[i] >= 'a' && sBase64[i] <= 'z')a[i] = sBase64[i] - 'a' + 26;else if (sBase64[i] >= '0' && sBase64[i] <= '9')a[i] = sBase64[i] - '0' + 52;else if ('+' == sBase64[i])a[i] = 62;else if ('/' == sBase64[i])a[i] = 63;else if ('=' == sBase64[i]){a[i] = 0;count++;}}sRet.resize(nLen/4*3);unsigned j = 0;for (unsigned i = 0; i < nLen; i += 4){sRet[j++] = a[i]<<2 | a[i+1]>>4;sRet[j++] = a[i+1]<<4 | a[i+2]>>2;sRet[j++] = a[i+2]<<6 | a[i+3];}delete[] a;assert(j == nLen / 4 * 3);if (count)sRet.resize(sRet.size() - count);return sRet;
}int main() {//std::string s("i\xb7\x1d\xfb\xef\xffi");//-> abcd++//aQ==//std::string s("\x00\x3f\xbf\xd7m",5);//-> AD+/120=std::string s("abcde"); // -> YWJjZGU=std::string r = Base64encode((const unsigned char*)s.c_str(), s.size());std::string sRet = Base64decode(r);return 0;
}

std::string Base64encode(const unsigned char* buf, unsigned int size)
{//方法二typedef   unsigned char       BYTE;std::string sBase64;std::basic_string<BYTE> s(buf, buf+size);unsigned count = 0;//不足3的倍数，补 \0 的个数，即编码后“=”个数。if (size % 3 == 1){s.push_back('\0');s.push_back('\0');count = 2;}else if (size % 3 == 2){s.push_back('\0');count = 1;}sBase64.resize((size+count)/3*4);//sBase64的大小一定是4的倍数。unsigned j = 0;for (unsigned i = 0; i < s.size(); i += 3){sBase64[j++] = Base64[s[i] >> 2];sBase64[j++] = Base64[(s[i]<<4 | s[i+1]>>4) & 0x3f];sBase64[j++] = Base64[(s[i+1]<<2 | s[i+2]>>6) & 0x3f];sBase64[j++] = Base64[s[i+2] & 0x3f];}while (count--)//此时 j = (size+count)/3*4sBase64[--j] = '=';return sBase64;
}