RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
    //ASCIIEncoding ByteConverter = new ASCIIEncoding(); //128 byte hatası devam ediyor. Array can not be null.
    //UnicodeEncoding ByteConverter = new UnicodeEncoding(); //128 byte hatası devam ediyor. Array can not be null.
    UTF8Encoding ByteConverter = new UTF8Encoding(); //128 byte hatası devam ediyor. Array can not be null.
    byte[] dataToEncrypt;
    byte[] dataToDecrypt;
    byte[] encryptedData;
    byte[] decryptedData;


    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            dataToEncrypt = ByteConverter.GetBytes("ofof");
            RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
            encryptedData = RSAEncrypt(dataToEncrypt, RSA.ExportParameters(true), false);
            Label1.Text = ByteConverter.GetString(encryptedData);

            dataToDecrypt = encryptedData;
            decryptedData = RSADecrypt(dataToDecrypt, RSA.ExportParameters(true), false);
            if(decryptedData != null)
                Label2.Text = ByteConverter.GetString(decryptedData);

            Label2.Text += "<br/>";
            dataToDecrypt = ByteConverter.GetBytes(Label1.Text.Trim());
            decryptedData = RSADecrypt(dataToDecrypt, RSA.ExportParameters(true), false);
           //decryptedData hata oluşnca null geliyor.
            if(decryptedData != null)
                Label2.Text += ByteConverter.GetString(decryptedData);

            HttpCookie hc = new HttpCookie("Temp");
            hc.Values.Add("TempPassword", ByteConverter.GetString(encryptedData));
            hc.Expires = DateTime.Now.AddDays(30); //cookie 30 gün
            Response.Cookies.Add(hc);
        }
    }

    static public byte[] RSAEncrypt(byte[] DataToEncrypt, RSAParameters RSAKeyInfo, bool DoOAEPPadding)
    {
        try
        {
            RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
            RSA.ImportParameters(RSAKeyInfo);
            return RSA.Encrypt(DataToEncrypt, DoOAEPPadding);
        }
        catch (CryptographicException ex)
        {
            HttpContext.Current.Response.Write(ex.Message + "<br/>");
            return null;
        }
    }

    static public byte[] RSADecrypt(byte[] DataToDecrypt, RSAParameters RSAKeyInfo, bool DoOAEPPadding)
    {
        try
        {
            RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
            RSA.ImportParameters(RSAKeyInfo);
            return RSA.Decrypt(DataToDecrypt, DoOAEPPadding);
        }
        catch (CryptographicException ex)
        {
            HttpContext.Current.Response.Write(ex.Message + "<br/>");
            return null;
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
        HttpCookie hc = Request.Cookies["Temp"];
        dataToDecrypt = ByteConverter.GetBytes(hc.Values["TempPassword"]);
        //string a = ByteConverter.GetString(dataToDecrypt);
        Response.Write(dataToDecrypt.Length);
        dataToDecrypt = ByteConverter.GetBytes(hc.Values["TempPassword"]);

        decryptedData = RSADecrypt(dataToDecrypt, RSA.ExportParameters(false), false);
        try
        {
            Label2.Text = ByteConverter.GetString(decryptedData);
        }
        catch (CryptographicException ex)
        {
            Response.Write(ex.Message + "<br/>");
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message + "<br/>");
        }
    }

   
}