Chapter 3. Protocols

It is impossible to foresee the consequences of being clever.

— Christopher Strachey

Every thing secret degenerates, even the administration of justice; nothing is safe that does not show how it can bear discussion and publicity.

— Lord Acton


If security engineering has a deep unifying theme, it is the study of security protocols. We've come across a few protocols informally already — I've mentioned challenge-response authentication and Kerberos. In this chapter, I'll dig down into the details. Rather than starting off with a formal definition of a security protocol, I will give a rough indication and then refine it using a number of examples. As this is an engineering book, I will also give many examples of how protocols fail.

A typical security system consists of a number of principals such as people, companies, computers and magnetic card readers, which communicate using a variety of channels including phones, email, radio, infrared, and by carrying data on physical devices such as bank cards and transport tickets. The security protocols are the rules that govern these communications. They are typically designed so that the system will survive malicious acts such as people telling lies on the phone, hostile governments jamming radio, or forgers altering the data on train tickets. Protection against all possible attacks is often too expensive, so protocols are typically designed under certain assumptions about the threats. For example, the logon protocol ...

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