Substitution Cipher

A more sophisticated cipher is one that replaces the letters of a plain text by any permutation of the English alphabet.

• First, mix the English alphabet letters in any order, and
• Second, replace A by the 1st letter in that order, B by the 2nd letter, etc.

Example: a random permutation of the alphabet is: IQCYTWDVJLGEZHAMNKUPRSFBOX, so every instance of A will be replaced by I, B will be replaced by Q, C will be replaced by C, etc.

Fun Class Activity: How many permutations exist? Hint: choose one of 26 letters for the 1st place, one of the remaining 25 letters for the 2nd place, one of the remaining 24 letters for the 3rd place, etc.

The number of permutations, as the above exercise shows, is insanely large. However, a substitution cipher has a significant weakness: some English letters are more frequent than others, so one can perform a frequency analysis to crack the encryption.