Situation: Client has TX licensing certification requirements that mandate that internet courses use a personal question authentication system. The questions are filled out during registration and then the student answers them again at various points throughout the course to confirm that the person sitting at the keyboard is the same one who registered.
The way I have it setup using the user profile custom field options works great except that the user can go back and see the answers to his authentication questions in his profile. Although prohibiting them from seeing their profile answers doesn't seem to be a requirement, it seems to violate the spirit of the idea.
You may want to check the following posting on the same subject. I had some problems with it, but I think it was my fault since I am not that knowledgable in programming with moodle. Good luck and if you are able to get it working let me know. Thanks.