Classical Ciphers and Their Cryptanalysis

In this chapter, the authors survey some historical ciphers which have been used since antiquity. They discuss the Vigenere cipher and show how it can be broken by finding the length of the keyword and then the keyword itself. The idea behind the use of frequency analysis in cryptanalysis is that all human languages have underlying statistical patterns and redundancies that can be exploited to help break a variety of ciphers. The authors also give a detailed but accessible description of the famous Enigma system used in World War II both from the mechanical and the mathematical point of view. With the advent and ubiquity of computer‐based encryption systems, cryptanalysis has shifted the emphasis from attacks based purely on the ciphering scheme to attacks on other aspects of cryptosystems, such as authentication, key exchanges, and digital signatures.