Security Engineering, 3rd Edition

CHAPTER 4Protocols

It is impossible to foresee the consequences of being clever.

– CHRISTOPHER STRACHEY

If it's provably secure, it probably isn't.

– LARS KNUDSEN

4.1 Introduction

Passwords are just one example of a more general concept, the security protocol. If security engineering has a core theme, it may be the study of security protocols. They specify the steps that principals use to establish trust relationships. They are where the cryptography and the access controls meet; they are the tools we use to link up human users with remote machines, to synchronise security contexts, and to regulate key applications such as payment. We've come across a few protocols already, including challenge-response authentication and Kerberos. In this chapter, I'll dig down into the details, and give many examples of how protocols fail.

A typical security system consists of a number of principals such as people, companies, phones, computers and card readers, which communicate using a variety of channels including fibre, wifi, the cellular network, bluetooth, 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 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 protocol designs ...