Cryptology Before 1500—A Bit of Magic

Cryptology was well established in ancient times, with both Greeks and Romans practicing different forms of cryptography. With the fall of the Roman Empire, cryptology was largely lost in the West until the Renaissance, but it flourished in the Arabic world. The Arabs invented the first reliable tool for cryptanalysis, frequency analysis. With the end of the Middle Ages and the increase in commerce and diplomacy, cryptology enjoyed a renaissance of its own in the West. This chapter examines the most common cipher of the period, the monoalphabetic substitution cipher, and then looks at the technique of frequency analysis that is used to break monoalphabetically enciphered messages. An extended example is given to illustrate the use of frequency analysis to break a monoalphabetic.