Part One: Symmetric Ciphers

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Cryptography is probably the most important aspect of communications security and is becoming increasingly important as a basic building block for computer security.

Computers at Risk: Safe Computing in the Information Age, National Research Council, 1991

The increased use of computer and communications systems by industry has increased the risk of theft of proprietary information. Although these threats may require a variety of countermeasures, encryption is a primary method of protecting valuable electronic information.

Communications Privacy: Federal Policy and Actions, General Accounting Office Report GAO/OSI-94-2, November 1993

By far the most important automated tool for network and communications security is encryption. Two forms of encryption are in common use: conventional, or symmetric, encryption and public-key, or asymmetric, encryption. Part One provides a survey of the basic principles of symmetric encryption, looks at widely used algorithms, and discusses applications of symmetric cryptography.

Road Map for Part One

Chapter 2: Classical Encryption Techniques

Chapter 2 describes classical symmetric encryption techniques. It provides a gentle and interesting introduction to cryptography and cryptanalysis and highlights important concepts.

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Chapter 3: Block Ciphers and the Data Encryption Standard

Chapter 3 introduces the principles of modern symmetric cryptography, with an emphasis on the most widely used encryption technique, the Data Encryption Standard (DES). The chapter includes a discussion of design considerations and cryptanalysis and introduces the Feistel cipher, which is the basic structure of most modern symmetric encryption schemes.

Chapter 4: Finite Fields

Finite fields have become increasingly important in cryptography. A number of cryptographic algorithms rely heavily on properties of finite fields, notably the Advanced Encryption Standard (AES) and elliptic curve cryptography. This chapter is positioned here so that concepts relevant to AES can be introduced prior to the discussion of AES. Chapter 4 provides the necessary background to the understanding of arithmetic over finite fields of the form GF(2ⁿ).

Chapter 5: Advanced Encryption Standard

The most important development in cryptography in recent years is the adoption of a new symmetric cipher standard, AES. Chapter 5 provides a thorough discussion of this cipher.

Chapter 6: More on Symmetric Ciphers

Chapter 6 explores additional topics related to symmetric ciphers. The chapter begins by examining multiple encryption and, in particular, triple DES. Next, we look at the concept of block cipher modes of operation, which deal with ways of handling plaintext longer than a single block. Finally, the chapter discusses stream ciphers and describes RC4.

Chapter 7: Confidentiality Using Symmetric Encryption

Beyond questions dealing with the actual construction of a symmetric encryption algorithm, a number of design issues relate to the use of symmetric encryption to provide confidentiality. Chapter 7 surveys the most important of these issues. The chapter includes a discussion of end-to-end versus link encryption, techniques for achieving traffic confidentiality, and key distribution techniques. An important related topic, random number generation, is also addressed.