Network Security: Private Communication in a Public World, Second Edition

3. Secret Key Cryptography

3.1. Introduction

This chapter describes how secret key cryptographic algorithms work. It describes in detail the DES and IDEA algorithms. These algorithms take a fixed-length block of message (64 bits in the case of both DES and IDEA), a fixed-length key (56 bits for DES and 128 bits for IDEA) and generate a block of output (the same length as the input). In general, a message won't happen to be 64 bits long. In §4.2 Encrypting a Large Message we'll discuss how to convert the basic fixed-length block encryption algorithm into a general message encryption algorithm.

3.2. Generic Block Encryption

A cryptographic algorithm converts a plaintext block into an encrypted block. It's fairly obvious that if the key length is ...