private void submitButton_Click(object sender, EventArgs e)
        {
            String login    = loginTextBox.Text;
            String password = passwordTextBox.Text;

            if (firstClickLogin || firstClickPassword || login == "" || password == "")
            {
                MessageBox.Show("Insert login and password before", "Autentication failed");
                return;
            }
            byte   answer      = Connection.Authentication(login, password);
            String status      = "Authentication has done successfully. Now you can log in.";
            String statusTitle = "Authentication done";

            if (answer == Connection.AUTHENTICATION_FAILED_CODE)
            {
                status      = "This login is used already or password is incorrect. Try again.";
                statusTitle = "Authentication failed";
            }
            MessageBox.Show(status, statusTitle);
            if (answer == Connection.AUTHENTICATION_DONE_CODE)
            {
                this.Hide();
                MessageForm messageForm = new MessageForm(login);
                messageForm.ShowDialog();
                this.Close();
            }
        }