1 Answer Sorted by: Highest score (default) ' Well, for perfect secrecy, we require that for all message distributions over M, all messages m E M, and all (possible) ciphertexts c it holds that 3 PT[M:m|C:c]:Pr[M=m] V In particular, that means, if we can find a single counterexample, i.e. a distribution over M, a V message m E M, and a ciphertexts c such that G PT[M=m|C:c] 7EPr[M=m] then, by our definition, the encryption scheme does not offer perfect secrecy. So, let's take the uniform distribution over M. For this distribution, we obviously have PT[M: c] = l/IMI. Now, given that 0\" is not a valid key, what is the probability Pr[M:c|C:c]? Are the two probabilities equal