In this chapter we develop the theory of channel capacity. In Def. 5.11 we considered for a fixed prior π and gain function g the idea of comparing prior and posterior vulnerabilities. As we noted in §6.2 however, in general one cannot be sure of the π’s and g’s our channels might face in practice; and so it is worth asking “What is the worst leakage that we, as defenders, might have to deal with if we use this channel?”
