Recall the substitution cipher encryption, and consider its variant which works on decimal digits instead of English alphabet letters. In other words, if D = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} then C = M = Dl for some integer l, each key k in key space K is a permutation on set D, i.e. it's a one-to-one function mapping D to itself (note that |K| = |D|! = 10!), and if k K and m M then Enc(k, m) = c s.t. ci = k(mi), i.e. permutation k applied to mi . 1