To improve Caesar Cipher Encryption, we can assign each alphabet with another one in an non-sequential manner. For example A can be mapped to D while B can be mapped to R, and so are the rest being mapped to a different alphabet. Recall that Caesar Cipher has a key space of 25, Monoalphabetic Substitution can have a key space of 26 x 25 x 24 x…….x 1, i.e. 26! (This time we have a different assumption, we assume that each alphabet can be mapped to itself).

26! is roughly equal to 4.03291461 × 1026

So this improvement greatly increases the possible key combinations and hence the work factor of brute force attack. Unfortunately, since each alphabet is still tied to another alphabet in the encryption process, so Monoalphabetic Subsitution is still susceptible to Frequency Analysis Attack

Can we do anything more to improve it?