0 Introduction

(0.1) The basic model of communication is that of sending information between communication partners, Alice and Bob say, who communicate through some channel, which might be anything as, for example, a telephone line, radio, an audio compact disc, a keyboard, and so on. This leads to the following questions: Exercise (11.1) • Information theory. What is information? How much information can be sent through a channel per time unit? • Coding theory. The channel might be noisy, that is information might be changed randomly when sent through the channel. How is it possible to recover a sufficiently large fraction of the original information from distorted data? • Cryptography. The channel might be insecure, that is information which is intended to be kept private to Alice and Bob might be caught by an opponent, Oscar say, or even be changed deliberately by Oscar. How can this be prevented?