Chapter 1
Introduction

It is now a well-known fact that, despite all the benefits, the digital revolution with its omnipresent networking of information systems also involves some risks. This book looks at a specific category of risks, the category of risks that evolve as a result of eavesdropping and the manipulation of data transmitted in communication networks and the vulnerability of the communication infrastructure itself. In particular, measures are discussed that can be taken to minimise them.

Mankind1 very early on recognised the need to protect information that was being transferred or stored, and so the desire to protect information from unauthorised access is probably as old as writing itself. For example, reliable early records on protective measures describe a technique used by the Spartans around 400 BC. The technique entailed writing messages on a leather strip that was wrapped around a stick of a specific diameter. Before the message was delivered, the leather strip was removed from the stick, and a potential attacker who did not have a stick with the same diameter, because he did not know the diameter or anything about the technique, could not read the message. In a sense this was an implementation of the first 'analogue' encryption.

In2 the fourth century BC, the Greek Polybius developed a table of bilateral substitution that defined how to encode characters into pairs of symbols and their corresponding reinstatement, thereby specifying the first 'digital' encryption method. Of the Romans we know that they often protected their tactical communication by using simple monoalphabetic substitution methods. The most widely known one was probably the 'Caesar cipher', named after its creator Julius Caesar, in which each character of the alphabet is shifted upwards by three characters. Thus, 'A' becomes 'D', 'B' becomes 'E', etc.

The3 Arabs were the first people to develop a basic understanding of the two fundamental principles of substitution, that is, pure character replacement, and transposition, that is, changing the sequence of the characters of a text. When they evaluated a method they also considered how a potential attacker might analyse it. They were therefore aware of the significance of relative letter frequency in a language for the analysis of substitution ciphers because it gave some insight into substitution rules. By the beginning of the fifteenth century, the Arabic encyclopaedia 'Subh al-a'sha' already contained an impressive treatment and analysis of cryptographic methods.

In Europe, cryptology originated during the Middle Ages in the papal and Italian city-states. The first encryption algorithms merely involved vowel substitution, and therefore offered at least some rudimentary protection from ignorant attackers who may not have come up with the idea of trying out all the different possible vowel substitutions.

Not4 wanting to turn the entire development of cryptology into a scientific discipline at this juncture, we can deduce from the developments mentioned that special importance has always been given to protecting information. However, a second category of risks is increasingly becoming a major priority in the age of omnipresent communication networks. These risks actually affect communication infrastructures rather than the data being transmitted. With the development and expansion of increasingly complex networks, and the growing importance of these networks not only to the economic but also to the social development of the modern information society, there is also a greater demand for ways to secure communication infrastructures from deliberate manipulation. For economic operation it is important to ensure that the services provided by communication networks are available and functioning properly as well as that the use of these services can be billed correctly and in a way that everyone can understand.

1.1 Content and Structure of this Book

In this book equal treatment is given to the two task areas in network security mentioned: security of transmitted data and security of the communication infrastructure. We start by introducing central terms and concepts and providing an overview of the measures available for information security.

Building on this introductory information5, the rest of the chapters in Part 1 deal with the fundamental principles of data security technology. Chapter 2 uses basic concepts to introduce cryptology. Chapter 3 covers the use and functioning of symmetric ciphering schemes, whereas Chapter 4 is devoted to asymmetric cryptographic algorithms. Chapter 5 introduces cryptographic check values for the detection of message manipulation. Generating secure, non-predictable random numbers is the subject of Chapter 6. In a sense, the algorithms in these four chapters constitute the basic primitives of data security technology upon which the cryptographic protection mechanisms of network security are based. Chapter 7 discusses cryptographic protocols and introduces the authentication and key exchange protocols that are central to network security. Chapter 8 enlarges the topic in the context of scenarios with group communication. This deeper discussion may be skipped in an introductory course without impairing the understanding of further book chapters. Part 1 concludes with Chapter 9, which provides an introduction to the principles of access control.

Part 26 of this book focuses on the architectures and protocols of network security. It starts with Chapter 10, which examines general issues relating to the integration of security services in communication architectures. Chapter 11 discusses security protocols of the data link layer, Chapter 12 examines the security architecture for the Internet protocol IPsec and Chapter 13 closes Part 2 by describing security protocols for the transport layer.

Part7 3 of the book presents the field of secure wireless and mobile communication. Chapter 14 differentiates the additional security aspects that arise in mobile communications compared with conventional fixed networks, and presents approaches of a more conceptual nature for maintaining the confidentiality of the current location area of mobile devices. The other chapters in this part examine concrete examples of systems. Chapter 15 deals with the security functions of the IEEE 802.11 standard for wireless local networks and includes an in-depth discussion of the weaknesses of former versions of the standard. Chapter 16 introduces the security functions for the current standards for mobile wide-area networks, that is, GSM, UMTS and LTE.

While8 Parts 1 to 3 of the book mainly concentrate on the security of communication processes between end systems, the fourth and last part of the book deals with protection of large networks and the communication infrastructure. Chapter 17 first describes the basic problem of protecting systems in open networks and provides a short overview of systematic threat analysis. It also discusses the problem of protecting end systems as a requirement for secure network operation. Chapter 18 deals with denial-of-service attacks, which affect end systems as well as the communication infrastructure. Chapters 19 and 20 cover the security of fundamental communication infrastructure services: routing and name resolution. Internet firewalls as the main means for realising subnet-related access control are introduced in Chapter 21. Since attacks cannot always be prevented through the proactive security measures described in these chapters, it often makes sense to introduce additional control through intrusion detection systems and/or intrusion prevention systems. The principles of such systems and existing techniques are introduced in Chapter 22. Finally, Chapter 23 deals with difficulties in the management of large security infrastructures.

Before9 our attentive and inquisitive readers get too involved in the further content of this book, they should be made aware that the field of network security has developed into a very active field during the last few years. Consequently, extensive improvements are constantly being made to existing security protocols and new protocols are being developed and introduced. Doing justice to the speed of this development in a textbook thus becomes a very difficult if not impossible undertaking. We therefore ask for the reader's understanding if a detail or two has already been resolved in a way that deviates from our interpretation in a particular chapter or totally new protocols have established themselves in the meantime and are not dealt with in this book. It is precisely because of the rapid developments in this field that the priority of this book is to provide the reader with a fundamental understanding of the central principles presented and to describe them on the basis of concrete and relevant sample protocols.

1.2 Threats and Security Goals

The terms threat and security goal play an important role in assessing the risks in communication networks, therefore they will first be defined in general terms.

Definition 1.1

A threat in a communication network is a potential event or series of events that could result in the violation of one or more security goals....