Cryptography provides many mechanisms on which security techniques and technologies are built. Part 4 reviews the basics of cryptography and its use in computer security.

Chapter 9, “Basic Cryptography,” reviews the principle elements of cryptosystems. It presents some classical ciphers, public key ciphers, and cryptographic hash functions. This material is the basis for Chapters 10, 11, and 12.

Chapter 10, “Key Management,” discusses the basics of managing cryptographic keys. Authentication and key exchange protocols are often combined, so this chapter includes authentication protocols that provide the ability to generate a common cryptographic key for future use. Generating keys is an intricate subject, full of traps for the unwary, and some basic principles and techniques improve the strength of the keys generated. Distributing fixed keys throughout the infrastructure leads to protocols for public key interchange, including certificates and key escrow and recovery protocols. This also suggests the notion of cryptography for digital signatures.

Chapter 11, “Cipher Techniques,” builds on the previous two chapters to present cryptographic protocols used in practice. It includes a discussion of stream and block ciphers and presents three Internet protocols as examples of how protocols work.

Chapter 12, “Authentication,” discusses authentication in a single system (although the system may be a remote one). As networking authentication protocols increase in importance, and as smart cards and other hardware become more common, the use of passwords as the sole authenticators is deprecated. However, many of the protocols and mechanisms require a PIN or other authentication data. This shifts the problem of authentication to a new level. Even there, the systems require the principles of how to choose a good password and how to identify oneself to the device or system from which the protocol is launched.