MARIA: Ay, but you must confine yourself within the modest limits of order.
|--Twelfth Night, I, iii, 8–9.|
In 1976, Harrison, Ruzzo, and Ullman  proved that in the most general abstract case, the security of computer systems was undecidable and explored some of the limits of this result.
Models explore the most basic question of the art and science of computer security: under what conditions can a generic algorithm determine whether a system is secure? Understanding models and the results derived from them lays the foundations for coping with limits in policy and policy composition as well as applying the theoretical work.
Given a computer system, how can we determine if it is secure? More ...