标签： base64

C++中Base64编码解码实现


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template<typename CharT>
class base64 {
public:
    //base64 加密
    static bool encode (std::basic_string<charT>& _old, std::basic_string<charT>& _new) {
        static charT _code [] = {
            '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', '+', '/',
        };
        ptrdiff_t i, len = _old.length ();
        _new.resize (((len % 3 ? 1 : 0) + len / 3) * 4);
        //主编码循环
        for (i = 0; i < len / 3; ++i) {
            _new [i * 4] = _code [_old [i * 3] >> 2];
            _new [i * 4 + 1] = _code [((_old [i * 3] & 0x03) << 4) | (_old [i * 3 + 1] >> 4)];
            _new [i * 4 + 2] = _code [((_old [i * 3 + 1] & 0x0F) << 2) | (_old [i * 3 + 2]) >> 6];
            _new [i * 4 + 3] = _code [_old [i * 3 + 2] & 0x3F];
        }
        //长度不为3的倍数
        if (len % 3) {
            //第一字节肯定不会溢出，故不检查边界
            _new [i * 4] = _code [_old [i * 3] >> 2];
            _new [i * 4 + 1] = _code [((_old [i * 3] & 0x03) << 4) | (i * 3 + 1 < len ? _old [i * 3 + 1] >> 4 : 0)];
            //倒数第二位可能为'='
            if (1 == len % 3)
                _new [i * 4 + 2] = '=';
            else
                _new [i * 4 + 2] = _code [(i * 3 + 1 < len ? (_old [i * 3 + 1] & 0x0F) << 2 : 0)];
            //末位肯定为'='
            _new [i * 4 + 3] = '=';
        }
        return true;
    }
 
    //base64 解密
    static bool decode (std::string& _old, std::string& _new) {
        static charT _ucode [] = {
            0,  0,  0,  0,  0,  0,  0,  0,    0,  0,  0,  0,  0,  0,  0,  0,
            0,  0,  0,  0,  0,  0,  0,  0,    0,  0,  0,  0,  0,  0,  0,  0,
            0,  0,  0,  0,  0,  0,  0,  0,    0,  0,  0,  62, 0,  0,  0,  63,
            52, 53, 54, 55, 56, 57, 58, 59,   60, 61, 0,  0,  0,  64, 0,  0,
            0,  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, 0,  0,  0,  0,  0,
            0,  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, 0,  0,  0,  0,  0,
        };
        ptrdiff_t i, len = _old.length ();
        ptrdiff_t len2 = len / 4 * 3;
        //判断末位是否有等号、有几个等号
        if ('=' == _old [len - 2])
            len2 -= 2;
        else if ('=' == _old [len - 1])
            len2--;
        else if (len % 4)
            len2 += (len % 4 - 1);
        _new.resize (len2);
        //主解密循环
        for (i = 0; i < len2 / 3; ++i) {
            _new [i * 3] = (_ucode [_old [i * 4]] << 2) | (_ucode [_old [i * 4 + 1]] >> 4);
            _new [i * 3 + 1] = ((_ucode [_old [i * 4 + 1]] & 0x0F) << 4) | (_ucode [_old [i * 4 + 2]] >> 2);
            _new [i * 3 + 2] = ((_ucode [_old [i * 4 + 2]] & 0x03) << 6) | (_ucode [_old [i * 4 + 3]]);
        }
        //末位为一个或两个等号的情况，这时候不能通过主解密循环进行解密
        if (len2 % 3) {
            _new [i * 3] = (_ucode [_old [i * 4]] << 2) | (_ucode [_old [i * 4 + 1]] >> 4);
            if (2 == len % 3)
                _new [i * 3 + 1] = ((_ucode [_old [i * 4 + 1]] & 0x0F) << 4) | (_ucode [_old [i * 4 + 2]] >> 2);
        }
        return true;
    }
};