Method
Choose a random permutation of the letters, this is the alphabet mapping. For example,
ABCDEFGHIJKLMNOPQRSTUVWXYZ
PVJWDCHTSKZFNQEYORIGAUMLXB
means that A -> P, B -> V and so on. To decrypt, simply read the table backwards.
Complexity
In particular, since we can pick any permutation of letters, there are
possible Simple Substitution Ciphers.