Introduction to Cryptography with Coding Theory, 3rd Edition

Chapter 2 Classical Cryptosystems

Methods of making messages unintelligible to adversaries have been important throughout history. In this chapter we shall cover some of the older cryptosystems that were primarily used before the advent of the computer. These cryptosystems are too weak to be of much use today, especially with computers at our disposal, but they give good illustrations of several of the important ideas of cryptology.

First, for these simple cryptosystems, we make some conventions.

• plaintext will be written in lowercase letters and CIPHERTEXT will be written in capital letters (except in the computer problems).

• The letters of the alphabet are assigned numbers as follows:

  $$\begin{array}{c}\begin{array}{cccccccccccccccc}a& b& c& d& e& f& g& h& i& j& k& l& m& n& o& p\\ 0& 1& 2& 3& 4& 5& 6& 7& 8& 9& 10& 11& 12& 13& 14& 15\end{array}\\ \begin{array}{cccccccccc}q& r& s& t& u& v& w& x& y& z\\ 16& 17& \mathrm{18\; ...}\end{array}\end{array}$$