This course introduces students to the history of sending secret messages from the times of the Ancient Greeks using the Polybius square to the invention of machine ciphers such as Purple and the Enigma through World War II. As people learned to decipher messages throughout history, new ways to encipher were created. Over time, these methods became increasingly more mathematical, and this course will discuss the mathematics behind enciphering messages, and how to use mathematics to break ciphers. Famous historical topics will include Julius Caesar and the Roman Empire, Mary Queen of Scots’ casket letters, the American Revolution, WWI (including the Zimmerman Telegram), WWII (including the Navajo code talkers), and of course the breaking of the Enigma at Bletchley Park. Mathematical topics include frequency analysis, primes, probability, statistics, modular arithmetic, and inverses. Ciphers discussed include Polybius squares, substitution ciphers, shift ciphers, affine ciphers, transposition ciphers, polyalphabetic ciphers, grilles, wheels, WWI field ciphers, and machine ciphers. Note: Students who have taken MTH 2250 Introduction to Cryptology may not take the course for math credit. Does not count toward the math major.