//let's say we have a list of strings
public static List<string> validations = new List<string> {"adfssssss","dfasssssss","dafsssssss","adfsssssss","adsffffff","adsffffff","adfasdfsadfsdf","ddsdsdfasdfsd"};

//forgive me for the random strings, but these could be loaded from a db, hardcoded in there etc...

//next let's do an MD5 Hash function
#region [MD5 Hash Code]
      
        private void btnValidate_Click(object sender, EventArgs e)
        {
            MD5 md5 = System.Security.Cryptography.MD5.Create();
            byte[] input =     System.Text.Encoding.ASCII.GetBytes(txtSerial.Text.ToString());
            byte[] hash = md5.ComputeHash(input);
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i < hash.Length; i++)
                sb.Append(hash[i].ToString("X2"));

            txtActivate.Text = sb.ToString();
            
            //see if validation is successful.
           foreach(stirng s in validations)
                if(s == sb.ToString()) MessageBox.Show("Validation Successful", "Validation Sucessful", MessageBoxButton.OK, MessageBoxImage.Information);
               else
                  MessageBox.Show("error", "error", MessagBox.OK, MessageBoxImage.Error);

        }
#endregion

protected void btnLogin_Click(object sender, EventArgs e)
    {
        AdminUser objAdminUser = new AdminUser();
        objAdminUser.UserName =txtUserName.Text.Trim();
        objAdminUser.Password = txtPassword.Text.Trim();
        if (objAdminUser.AdminLoginUserValidate() != -100)
        {
            if (objAdminUser.UserID > 0)
            {
                // Create the authentication ticket
                FormsAuthenticationTicket authTicket = new FormsAuthenticationTicket(1,
                    objAdminUser.UserID.ToString(),  // user id
                    DateTime.Now,               // creation
                    DateTime.Now.AddMinutes(60),// Expiration
                    false,                        // Persistent
                    objAdminUser.UserName + "," + objAdminUser.FirstName);    // UserName and Fill Name
                // Encrypt the ticket.
                string encryptedTicket = FormsAuthentication.Encrypt(authTicket);
                // Create a cookie and add the encrypted ticket to the cookie as data.
                HttpCookie authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
                // Add the cookie to the outgoing cookies collection.
                Response.Cookies.Add(authCookie);
                //if there is a previous url to redirect them to
                FormsAuthentication.RedirectFromLoginPage(objAdminUser.UserID.ToString(), false);
                //if (Request.QueryString["ReturnUrl"] == null)
                //{
                //    Response.Redirect("Welcome.aspx");
                //}
                //else
                //{
                //    Response.Redirect(Request.QueryString["ReturnUrl"].ToString());
                //}
            }
        }
        else
        {
            lblMsg.Text = "Invalid username and password.";
            ModalPopupExtender2.Show();
        }
    }


In Web.congig

<authentication mode="Forms">
            <forms name="SMGR" loginUrl="login.aspx"  defaultUrl="welcome.aspx" timeout="1" protection="All" path="/"/>
        </authentication>


Impliment in page:

bool IsAttribute = false;
    public void CheckAuthentication()
    {
        if (!Context.User.Identity.IsAuthenticated)
        {
            string strRedirectURL = Server.UrlEncode(Context.Request.ServerVariables["URL"]);
            Context.Response.Redirect("Login.aspx?ReturnUrl=" + strRedirectURL);
        }
    }
    protected void Page_Load(object sender, EventArgs e)
    {
        CheckAuthentication();
        

    }