Can you explain this theorem in detail?
Suppose (P,C,K,E,D) is a cryptosystem where |K|=|C|=|P|.
Then the cryptosystem provides perfect secrecy if and only if every key is used with equal
probability (1)/(|)K|, and for every xinP and every yinC, there is a unique key K such that
e_(K)(x)=y.