Cron.php and CAS

Hello, sorry for my english...

If I check "Check to see if a user is allready logged in?" the test

 function cas_login_check() 
... 
if ($cas_force_login || $cas_check_first) {
... 

is already right... So when I want to run the cron with my crontab command :
0 */3 * * * root curl --silent --compressed http://dud.univmed.fr/cron.php

I have a error because it try to authenticated

Thks...