In general, you want to avoid writing down sensitive information and just keeping it in your head but that's not always an option. If you have to write down a password, or some other sensitive ...

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 ...