The monoalphabetic cipher simply substitutes each letter of the alphabet with a symbol, so that A might always be replaced with +, and B with 8, and so on. The letter substitutions remain the same throughout a message. This cipher was secure for centuries, until codebreakers noticed that each letter has an average frequency and no matter how the letter is disguised the new symbol will take on the frequency of the letter it represents. The most common letter in English is E, so if a coded message contains lots of Ys, then Y probably represents E. The earliest known description of this codebreaking technique (frequency analysis) dates back to 9th century Baghdad.

In contrast, the polyalphabetic cipher works by using switching the rules of substitution – hence, the ‘poly’. For example, if E appears as the 1st, 3rd, 5th letter in a message then it is substituted by F, but in the even positions it is substituted for K. In the other positions, F might represent Z, and K might represent Q. Although F and K represent E, they are not incredibly common in the encrypted message because they share the frequency of E and at other times they represent the rare letters Q and Z.