7 Freshness

7.1 Introduction

A message replay attack was introduced briefly in Section 3.4. In this attack a message, or a portion of a message, is recorded and replayed at some later date. For example, an instruction to a bank to transfer a sum of money from account A to account B could be recorded and replayed some time later to fool the bank into making a second payment to account B. The incorporation of a freshness indicator in the message is a means of thwarting attacks of this kind. In this section I introduce three methods for indicating freshness: time stamps, sequence numbers and nonces.