first looks at basic concepts, starting with the central concept of a security protocol, and going on to the human-computer interface, access controls, cryptology and distributed system issues. It does not assume any particular technical background other than basic computer literacy. It is based on an ‘Introduction to Security’ course which we teach to second year undergraduates.
The second part looks in much more detail at a number of important applications such as military communications, medical record systems, cash machines, mobile phones and pay-TV. These are used to introduce more of the advanced technologies and concepts. It also considers information security from the viewpoint of a number of different interest groups such as companies, consumers, criminals, the police and spies. This material is drawn from my senior course on security, from research work, and from experience consulting.
The third part looks at the organizational and policy issues: how computer security interacts with law, with evidence, and with corporate politics; how we can gain confidence that a system will perform as intended; and how the whole business of security engineering can best be managed.
I believe that building systems which continue to perform robustly in the face of malice is one of the most important, interesting, and difficult tasks facing engineers in the twenty-first century.
Ross Anderson
Cambridge, January 2001
For my daughter, and other lawyers…
The tricks taught in this book are intended only to enable you to build better systems. They are not in any way given as a means of helping you to break into systems or do anything else illegal. So where possible I have tried to give case histories at a level of detail that illustrates the underlying principles without giving a ‘hacker's cookbook’.
Governments fought to restrict knowledge of cryptography until the turn of the century, and there may still be people who believe that the knowledge contained in this book should not be published.
Their fears were answered in the first book in English that discussed cryptology, a 1641 treatise on optical and acoustic telegraphy written by Oliver Cromwell's cryptographer and son-in-law John Wilkins [2025]. He traced scientific censorship back to the Egyptian priests who forbade the use of alphabetic writing on the grounds that it would spread literacy among the common people and thus foster dissent. As he said:
‘It will not follow that everything must be suppresst which may be abused… If all those useful inventions that are liable to abuse should therefore be concealed there is not any Art of Science which may be lawfully profest.’
The question was raised again in the nineteenth century, when some well-meaning people wanted to ban books on locksmithing. In 1853, a contemporary writer replied [1899]:
‘Many well-meaning persons suppose that the discussion respecting the means for baffling the supposed safety of locks offers a premium for dishonesty, by showing others how to be dishonest. This is a fallacy. Rogues are very keen in their profession, and already know much more than we can teach them respecting their several kinds of roguery. Rogues knew a good deal about lockpicking long before locksmiths discussed it among themselves … if there be harm, it will be much more than counterbalanced by good.’
Thirty years later, in the first book on cryptographic engineering, Auguste Kerckhoffs explained that you must always assume that the other side knows the system, so security must reside in the choice of a key.
His wisdom has been borne out by long experience since. The relative benefits of ‘Open’ versus ‘Closed’ security systems have also been studied by researchers applying the tools of dependability analysis and security economics. We discuss their findings in this book.
In short, while some bad guys will benefit from a book such as this, they mostly know it already – and the good guys benefit much more.
Ross Anderson
Cambridge, November 2020
Foreword
In a paper he wrote with Roger Needham, Ross Anderson coined the phrase ‘programming Satan's computer’ to describe the problems faced by computer-security engineers. It's the sort of evocative image I've come to expect from Ross, and a phrase I've used ever since.
Programming a computer is straightforward: keep hammering away at the problem until the computer does what it's supposed to do. Large application programs and operating systems are a lot more complicated, but the methodology is basically the same. Writing a reliable computer program is much harder, because the program needs to work even in the face of random errors and mistakes: Murphy's computer, if you will. Significant research has gone into reliable software design, and there are many mission-critical software applications that are designed to withstand Murphy's Law.
Writing a secure computer program is another matter entirely. Security involves making sure things work, not in the presence of random faults, but in the face of an intelligent and malicious adversary trying to ensure that things fail in the worst possible way at the worst possible time … again and again. It truly is programming Satan's computer.
Security engineering is different from any other kind of programming. It's a point I made over and over again: in my own book, Secrets and Lies, in my monthly newsletter Crypto-Gram, and in my other writings. And it's a point Ross makes in every chapter of this book. This is why, if you're doing any security engineering … if you're even thinking of doing any security engineering, you need to read this book. It's the first, and only, end-to-end modern security design and engineering book ever written.
And it comes just in time. You can divide the history of the Internet into three waves. The first wave centered around mainframes and terminals. Computers were expensive and rare. The second wave, from about 1992 until now, centered around personal computers, browsers, and large application programs. And the third, starting now, will see the connection of all sorts of devices that are currently in proprietary networks, standalone, and non-computerized. By 2003, there will be more mobile phones connected to the Internet than computers. Within a few years we'll see many of the world's refrigerators, heart monitors, bus and train ticket dispensers, burglar alarms, and electricity meters talking IP. Personal computers will be a minority player on the Internet.
Security engineering, especially in this third wave, requires you to think differently. You need to figure out not how something works, but how something can be made to not work. You have to imagine an intelligent and malicious adversary inside your system (remember Satan's computer), constantly trying new ways to subvert it. You have to consider all the ways your system can fail, most of them having nothing to do with the design itself. You have to look at everything backwards, upside down, and sideways. You have to think like an alien.
As the late great science fiction editor John W. Campbell, said: “An alien thinks as well as a human, but not like a human.” Computer security is a lot like that. Ross is one of those rare people who can think like an alien, and then explain that thinking to humans. Have fun reading.
Bruce Schneier
January 2001
PART I
In the first section of the book, I cover the basics. The first chapter sets out to clarify concepts and terminology by describing the secure distributed systems commonly found in four environments:
