Python Programming in Context, 3rd Edition

8.4 Cracking the Substitution Cipher

The substitution cipher is much more difficult to crack than the rail fence cipher. As we mentioned previously, 26! or roughly 4 × 10²⁶ different possible arrangements of the alphabet could be used as a key. Clearly, the brute-force strategy for finding the key would take an extremely long time. However, the substitution cipher does have a fatal flaw that we can exploit.

The flaw in the substitution cipher allows us to take advantage of patterns in the English language that can help us deduce letters in the key. The first pattern we will exploit is that some letters in the English language are used more frequently than others. If we count the number of times each letter occurs in a document such as this ...