1. Introduction -- 2. From Julius Caesar to simple substitution -- 3. Polyalphabetic systems -- 4. Jigsaw ciphers -- 5. Two-letter ciphers -- 6. Codes -- 7. Ciphers for spies -- 8. Producing random ...

At the US Central Intelligence Agency’s headquarters in Langley, Virginia, there is a monument to secrecy that has vexed both professional and amateur codebreakers for over two decades. Erected in ...