Security Engineering. Ross Anderson

many ways, and changed the shape of the security industry. Terrorism is not just about risk, but about the perception of risk, and about the manipulation of perception. This adds psychology and politics to the mix. Security engineers also have a duty to contribute to the political debate. Where inappropriate reactions to terrorist crimes have led to major waste of resources and unforced policy errors, we have to keep on educating people to ask a few simple questions: what are we seeking to prevent, and will the proposed mechanisms actually work?

      Ross Anderson

      Cambridge, January 2008

Preface to the First Edition

      For generations, people have defined and protected their property and their privacy using locks, fences, signatures, seals, account books, and meters. These have been supported by a host of social constructs ranging from international treaties through national laws to manners and customs.

      This is changing, and quickly. Most records are now electronic, from bank accounts to registers of real property; and transactions are increasingly electronic, as shopping moves to the Internet. Just as important, but less obvious, are the many everyday systems that have been quietly automated. Burglar alarms no longer wake up the neighborhood, but send silent messages to the police; students no longer fill their dormitory washers and dryers with coins, but credit them using a smartcard they recharge at the college bookstore; locks are no longer simple mechanical affairs, but are operated by electronic remote controls or swipe cards; and instead of renting videocassettes, millions of people get their movies from satellite or cable channels. Even the humble banknote is no longer just ink on paper, but may contain digital watermarks that enable many forgeries to be detected by machine.

      How good is all this new security technology? Unfortunately, the honest answer is ‘nowhere near as good as it should be.’ New systems are often rapidly broken, and the same elementary mistakes are repeated in one application after another. It often takes four or five attempts to get a security design right, and that is far too many.

The media regularly report security breaches on the Internet; banks fight their customers over ‘phantom withdrawals’ from cash machines; VISA reports huge increases in the number of disputed Internet credit card transactions; satellite TV companies hound pirates who copy their smartcards; and law enforcement agencies try to stake out territory in cyberspace with laws controlling the use of encryption. Worse still, features interact. A mobile phone that calls the last number again if one of the keys is pressed by accident may be just a minor nuisance – until someone invents a machine that dispenses a can of soft drink every time its phone number is called. When all of a sudden you find 50 cans of Coke on your phone bill, who is responsible, the phone company, the handset manufacturer, or the vending machine operator? Once almost every electronic device that affects your life is connected to the Internet – which Microsoft expects to happen by 2010 – what does ‘Internet security’ mean to you, and how do you cope with it?

      As well as the systems that fail, many systems just don't work well enough. Medical record systems don't let doctors share personal health information as they would like, but still don't protect it against inquisitive private eyes. Zillion-dollar military systems prevent anyone without a “top secret” clearance from getting at intelligence data, but are often designed so that almost everyone needs this clearance to do any work. Passenger ticket systems are designed to prevent customers cheating, but when trustbusters break up the railroad, they cannot stop the new rail companies cheating each other. Many of these failures could have been foreseen if designers had just a little bit more knowledge of what had been tried, and had failed, elsewhere.

      Security engineering is the new discipline that is starting to emerge out of all this chaos.

      Although most of the underlying technologies (cryptology, software reliability, tamper resistance, security printing, auditing, etc.) are relatively well understood, the knowledge and experience of how to apply them effectively is much scarcer. And since the move from mechanical to digital mechanisms is happening everywhere at once, there just has not been time for the lessons learned to percolate through the engineering community. Time and again, we see the same old square wheels being reinvented.

      The industries that have managed the transition most capably are often those that have been able to borrow an appropriate technology from another discipline. Examples include the reuse of technology designed for military identify-friend-or-foe equipment in bank cash machines and even prepayment gas meters. So even if a security designer has serious expertise in some particular speciality – whether as a mathematician working with ciphers or a chemist developing banknote inks – it is still prudent to have an overview of the whole subject. The essence of good security engineering is understanding the potential threats to a system, then applying an appropriate mix of protective measures – both technological and organizational – to control them. Knowing what has worked, and more importantly what has failed, in other applications is a great help in developing judgment. It can also save a lot of money.

The purpose of this book is to give a solid introduction to security engineering, as we understand it at the beginning of the twenty-first century. My goal is that it works at four different levels:

      1 as a textbook that you can read from one end to the other over a few days as an introduction to the subject. The book is to be used mainly by the working IT professional who needs to learn about the subject, but it can also be used in a one-semester course in a university;

      2 as a reference book to which you can come for an overview of the workings of some particular type of system (such as cash machines, taxi meters, radar jammers, anonymous medical record databases or whatever);

      3 as an introduction to the underlying technologies, such as crypto, access control, inference control, tamper resistance, and seals. Space prevents me from going into great depth; but I provide a basic road map for each subject, plus a reading list for the curious (and a list of open research problems for the prospective graduate student);

      4 as an original scientific contribution in which I have tried to draw out the common principles that underlie security engineering, and the lessons that people building one kind of system should have learned from others. In the many years I have been working in security, I keep coming across these. For example, a simple attack on stream ciphers wasn't known to the people who designed a common anti-aircraft fire control radar so it was easy to jam; while a trick well known to the radar community wasn't understood by banknote printers and people who design copyright marking schemes, which led to a quite general attack on most digital watermarks.

      I have tried to keep this book resolutely mid-Atlantic. A security engineering book has to be, as many of the fundamental technologies are American, while many of the interesting applications are European. (This isn't surprising given the better funding of US universities and research labs, and the greater diversity of nations and markets in Europe.) What's more, many of the successful European innovations – from the smartcard to the GSM mobile phone to the pay-per-view TV service – have crossed the Atlantic and now thrive in the Americas. Both the science, and the case studies, are necessary.

This book grew out of the security engineering courses I teach at Cambridge University, but I have rewritten my notes to make them self-contained and added at least as much material again. It should be useful to the established professional security manager or consultant as a first-line reference; to the computer science professor doing research in cryptology; to the working police detective trying to figure out the latest computer scam; and to policy wonks struggling with the conflicts involved in regulating cryptography and anonymity. Above all, it is aimed at Dilbert. My main audience is the working programmer or engineer who is trying to design real systems that will keep on working despite the best efforts of customers, managers, and everybody else.

      This book is divided into three parts.

       The 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