private string GetMD5Hash(string str)
        {
            string md5Str = "";
            byte[] buffer = Encoding.UTF8.GetBytes(str);
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            byte[] cryptBuffer = md5.ComputeHash(buffer);
            //把每一个字节 0-255，转换成两位16进制数
            for (int i = 0; i < cryptBuffer.Length; i++)
            {
                //大写X转换的是大写字母，小写X转换的是小写字母
                md5Str += cryptBuffer[i].ToString("X2");
            }
            return md5Str;
        }