Part I. Classic Ciphers
The mathematical parts of the sections can be downloaded in parts or as a single
large PDF file (1.3 MB).
- Cryptology as entertainment
(literature and puzzles)
- Monoalphabetic ciphers
- Polyalphabetic ciphers
- Some statistical properties of languages
(after Friedman, Sinkov, and Kullback)
- Cylinder ciphers (mechanical cipher devices)
- Rotor machines (machine ciphers)
- The Enigma
- Aperiodic polyalphabetic ciphers
(running-text and autokey ciphers)
- Linear ciphers
- Theoretical security
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
- 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In short: The algorithms are out-of-date, the methods and principles are
- 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.
contains lots of classic ciphers which are explained
and executable in a browser or on a smartphone.
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.
- 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.
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
and Mavis Lever.
- Alice and Bob are communicating partners,
- Eve is the eavesdropper,
- Mallory is the »man in the middle«.
Author: Klaus Pommerening, 1997-Apr-09;
last change: 2021-Jan-17.