Part I. Classic Ciphers

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  1. Cryptology as entertainment (literature and puzzles)
  2. Monoalphabetic ciphers
  3. Polyalphabetic ciphers
  4. Some statistical properties of languages (after Friedman, Sinkov, and Kullback)
  5. Cylinder ciphers (mechanical cipher devices)
  6. Rotor machines (machine ciphers)
  7. The Enigma
  8. Aperiodic polyalphabetic ciphers (running-text and autokey ciphers)
  9. Transpositions
  10. Linear ciphers
  11. Theoretical security
The mathematical parts of the sections can be downloaded in parts or as a single large PDF file (1.3 MB).

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?

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).


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:

In contrast the mathematical complements try to be as general as possible with respect to the used alphabet.

Gender Mainstreaming

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

In classical cryptology the role of the cryptanalyst corresponds to the eavesdropper. For this reason in the following we consider the cryptanalyst as female and honor the famous cryptanalysts Elizebeth Friedman, Joan Clarke, and Mavis Lever.

Author: Klaus Pommerening, 1997-Apr-09; last change: 2021-Jan-17.