[eluser]Rok Biderman[/eluser]
For very simple stuff I would do this: Download Ion_auth (my own preference, there are some others that are nice), copy library and model to its place, then take anything out of auth controller that i don't need. Take only login and logout and chuck them into my own controller (Don't fortget to include the library ofc).
Now, if it were me, I'd still leave create user and change/forgot password available cause accidents happen and the password in the database is encrypted and salted. If it's not, you're using the wrong library. As for your hidden link, that is no security at all, so don't consider it such.
If you're intent on writing your own authentication (I really advise you against it), then start at the CI tutorials on Nettuts (lesson 6 is authentication if i remember correctly.) Hope this is of some help to you.