TABLE OF CONTENTS Content
Foundation: history, vocabulary, transpositions, substitutions
Basic ciphers: simple substitution, Vigen re, Vernam
Modern settings: digital communications, Kerckhoffs principles
?The Shannon Theory of secrecy: entropy, encryption model, perfect secrecy
Cryptography distinguishes itself from coding theory in the sense that the presence of random noise in the latter is replaced by malicious adversaries in the former. It uses a specific vocabulary and relies on some fundamental principles which are presented in this chapter. We also expose historical cases which are relevant for our analysis, and survey the Shannon Theory of secrecy which was an attempt to make cryptography rely on information theory. Unfortunately, the goals of achieving "perfect" secrecy were too ambitious and impossible to achieve in practice at a reasonable cost.
As an easy introduction, we can say that, strictly speaking, cryptography begins with history: with the origin of language writing. As an example, the Egyptians were able to communicate by written messages with hieroglyph. This code was the secret of a selected category of people: the scribes. Scribes used to transmit the secret of writing hieroglyph from father to son until the society collapsed. It was only several millenia later that this secret code was broken by Champollion.
Although Egyptian codes are quite anecdotal, history includes many other cryptographic usages. [1] Communication with secret codes was commonly required
for diplomacy: governments had to communicate with their remote embassies in suspicious environments,
during...
