Chapter 3. Secret Key Cryptography

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.

Generic Block Encryption

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

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