PRivate static string CheckSum(string message){    char[] chars = message.ToCharArray();    int checksum = 0;    for (int i = 0; i < chars.Length; i++)    {        checksum += (int)chars[i];    }    checksum = (~checksum & 0xFFFF) + 0x0001;    return Convert.ToString(checksum, 16).ToUpper();}