In the next few chapters we are going to talk about security of communication. First, you will be introduced to some main conceptes related to security of data transmission over computer networks in general. We will discuss what the term "security" means, what is required for data to be secure and how that can be achieved. We will introduce and describe such concepts as "digital signature", "certificate", "encryption" and the like and show how they are achieved and to what purpose. Having presented some background, we will use those building blocks and concepts and focus on problems of communication between two computers (or, say, a smartphone and a web server) and then move towards a connection of a client station and the cloud in the end.

On finishing reading these notes you will have a general knowledge of security issues related to communication in a computer network. You will learn about communication protocols most widely used in the Internet and how they can introduce security. You will also know where and why this is needed and why it is so hard to achieve.