Secret Key Cryptography

5 Substitution ciphers

This chapter covers

• Simple substitution and polyalphabetic substitution ciphers
• Solving polyalphabetic ciphers using the Kasiski test and the index of coincidence
• Autokey and running key ciphers, and methods for solving them
• Simulating rotor-based cipher machines

We are now ready to explore the basic tools described in the preceding chapter in greater depth. Before I begin describing all the various ciphers, let me explicitly state the goals that these ciphers are trying to achieve. The Dutch linguist and polymath Auguste Kerckhoffs first expressed these principles in a pair of articles in Journal des Sciences Militaires in 1883:

1. The cipher should be unbreakable in practice, even if not in theory.

2. This should be true ...