1The astute reader will notice that Ek(Dk(c)) might differ from c for c not in the range of Ek. However, in
most of the cryptosystems we consider, the message space and ciphertext space
are the same. When that
is the case, the range of Ek is all of
, so every c
can be written as c = Ek(y) for some message y, and
Ek(Dk(c)) = Ek(Dk(Ek(y))) = Ek(y) = c.