Cryptology Part I. Classic Ciphers

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Contents

0. Cryptology as entertainment (literature and puzzles)
1. Monoalphabetic ciphers
2. Polyalphabetic ciphers
3. Some statistical properties of languages (after Friedman, Sinkov, and Kullback)
4. Cylinder ciphers (mechanical cipher devices)
5. Rotor machines (machine ciphers)
6. The Enigma
7. Aperiodic polyalphabetic ciphers (running-text and autokey ciphers)
8. Transpositions
9. Linear ciphers
10. Theoretical security

Motivational Hints

Classical cryptography considers ciphers in use up to the 1970's, that is, in the precomputer era. Today no one seriously uses these ciphers. Why does it make sense to deal with them?

• We get a feeling for the security of the basic encryption steps that are in use as components of the more complex ciphers of today.
• The complexity of modern techniques becomes perspicuous.
• Most of the mathematical foundations are relevant also for modern cryptologic techniques.
• We may learn a lot from the failures of the past—many of the commonly accepted principles of cryptology arose a long time ago. In short: The algorithms are out-of-date, the methods and principles are up-to-date.
• Classical cryptology makes a good part of general education, not only for mathematicians or computer scientists. In particular it provides many interesting project ideas for undergraduates or even school children.
• Classical cryptology provides intellectual challenges—better than chess, poker, or war games [:-)]. The puzzle corners of journals often contain puzzles (link target is in german) whose cryptological background is easily recognized.
• And last but not least: occupation with classical cryptology is fun.

Elonka Dunin's web site Famous Unsolved Codes and Ciphers has an overview over unsolved »historic« cryptograms.

The Secret Code Breaker (Bob Reynard) has a lot of elementary material that's also great for kids.

CrypTool also contains a lot of educational material and challenges.

CrypTool online contains lots of classic ciphers which are explained and executable in a browser or on a smartphone.

MysteryTwister C3, abbreviated MTC3, is a crypto cipher contest with currently more than 180 challenges created by more than 40 authors and used by more than 5000 solvers. The website has a moderated forum. The challenges are distributed in 4 different levels.

Klaus Schmeh has a blog with the latest news in classical cryptology and many unsolved ciphers (German only).

Conventions

In order to not get lost in less relevant and nasty details most examples in this Part I of the lecture notes follow the model:

• Ciphertexts are written in uppercase letters (usually without word boundaries), employing the 26 letter alphabet A...Z.
• Plaintexts are written in upper-, lower-, or mixed-case letters, with or without word boundaries and punctuation.

Gender Mainstreaming

It is common use in modern cryptology to staff the scenarios with men and women alternately, see FAQ:

• Alice and Bob are communicating partners,
• Eve is the eavesdropper,
• Mallory is the »man in the middle«.