I found two interesting examples of cryptography in history.

Firstly, the creation and use of enigma machines by Germany in World War 2, and the efforts by the allies to break the codes.  Discussion of this example can be centred around the difficult and time-consuming efforts required to decrypt complex codes, and how the addition of one more variable can exponentially increase that difficulty.

Secondly, the polyalphabetic cipher developed in the 15^th century is a good example of a major progression in cryptography to thwart decryption efforts.