Plz try ths for encryption...
System.Web.Security.FormsAuthenticationTicket EncPass = new FormsAuthenticationTicket(txtPass.Text.Trim(), false, 600);
System.Web.Security.FormsAuthenticationTicket EncUser = new FormsAuthenticationTicket(txtUser.Text.Trim(), false, 600);
string pass = FormsAuthentication.Encrypt(EncPass);
string User = FormsAuthentication.Encrypt(EncUser);
Response.Cookies["UName"].Value = User;
Response.Cookies["PWD"].Value = pass;
Response.Cookies["UName"].Expires = DateTime.Now.AddDays(30);
Response.Cookies["PWD"].Expires = DateTime.Now.AddDays(30);