  Given that individual defenses are rarely sufficient, defense-in-depth is
nearly universal and options for individual defensive layers abound. We develop
a simple mathematical theory that can help in selecting the type and quantity
of defenses for two different defense-in-depth strategies: Blockade and Delay.
This theoretical approach accounts for budgetary constraints and the number,
skill, and diversity of attackers. We find that defenders have several reasons
to be optimistic including that the number of required defenses increases more
slowly than the number of attackers, that similar attackers are defended more
easily than similar defenses are defeated, and that defenders do not
necessarily need to act as quickly as attackers.
