One time passwords in C#

private async void Form_Load(object sender, System.EventArgs e)
        {
            otp = new OneTimePassword(txtSecret.Text);
            tmrUpdate_Tick(null, null);

            for (int timeStep = 30; timeStep < 200; timeStep++)
            {
                var secret = GetRandomSecretKey();
                var otpGenerator = new OneTimePassword(secret) { Algorithm = OneTimePasswordAlgorithm.Sha512, Digits = 6, TimeStep = timeStep };
                var sp = Stopwatch.StartNew();
                Debug.WriteLine($"TimeStep: {timeStep}, Key: {secret}, Started: {DateTime.Now.ToString("hh:mm:ss.fff")}");
                var code = otpGenerator.GetCode();
                var isCodeValid = true;
                while (isCodeValid)
                {
                    await Task.Delay(1000);
                    var otpVerfier = new OneTimePassword(secret) { Algorithm = OneTimePasswordAlgorithm.Sha512, Digits = 6, TimeStep = timeStep };
                    isCodeValid = otpVerfier.IsCodeValid(code);
                    var wrongResult = (sp.Elapsed.TotalSeconds > timeStep) && isCodeValid;
                    Debug.WriteLine($"ValidCode: {isCodeValid}, {sp.Elapsed}{(wrongResult ? $", Not good {timeStep}" : "")}");
                }

                Debug.WriteLine($"TimeStep: {timeStep}, Key: {secret}, Ended: {DateTime.Now.ToString("hh:mm:ss.fff")}");
            }
        }

        Random _random = new Random();
        char GetRandomLetter()
        {
            // This method returns a random lowercase letter.
            // ... Between 'a' and 'z' inclusize.
            int num = _random.Next(0, 26); // Zero to 25
            char let = (char)('a' + num);
            return let;
        }

        string GetRandomSecretKey()
        {
            var firstSet = new string(new char[] { GetRandomLetter(), GetRandomLetter(), GetRandomLetter(), GetRandomLetter() });
            var secondSet = new string(new char[] { GetRandomLetter(), GetRandomLetter(), GetRandomLetter(), GetRandomLetter() });
            var thridSet = new string(new char[] { GetRandomLetter(), GetRandomLetter(), GetRandomLetter(), GetRandomLetter() });
            var fourthSet = new string(new char[] { GetRandomLetter(), GetRandomLetter(), GetRandomLetter(), GetRandomLetter() });
            return string.Join(" ", firstSet, secondSet, thridSet, fourthSet);
        }