Hello, thanks for your great plugin!
I added a Auth->logout method to delete the Auth session. If $this->Auth->logout('logoutBoth'), the script will BOTH delete the Auth session AND log out of SimpleSamlPHP IdP (+ redirect to the same page). I did not activated it by default because some may not want to log out the IdP.
I also added a Auth->login() method which does the exact same than your authenticate() but is closer to the original Auth->login($user) method. However, I did not manage to find a way to login in BOTH the Auth way and SimpleSamlPHP :(

These 2 functions work with me but I use SimpleSamlPHP with SQL instead of PHP sessions (because I couldn't make it work with CakePHP sessions...) and I did not try with other setups of SimpleSamlPHP.

Have a nice day,