Commonsense in the Crisis

Since the dreadful attacks on the World Trade Center and the Pentagon, many companies and government agencies have been pushing security technologies that could cost a lot of money and, in some cases, seriously erode our liberties. Many of them won't do much, if anything, to make bin Laden's job harder; some of them he might even welcome.

We've been subjected to sales pitches for smart ID cards containing encoded fingerprints or iris scans, universal CCTV surveillance, government management of airport security, knowledge-based data mining of passenger lists, and significantly increased police control of Internet service providers. Some of the salesmen help propagate juicy urban myths, such as the claim that al-Qaida hides its electronic communications in pictures on pornographic web sites. The image of the mad mullah as cryptopornographer may be irresistible to a certain type of newspaper - but what's the truth?

I've been asked for comments on security engineering questions by quite a few journalists. Here, I gather together some of the points that I've made, and offer a few observations on why some of the common reactions to the attacks are not appropriate.


I'm one of the people who's supposed to understand security technology a little. My day job is leading the security group at the Cambridge University Computer Laboratory, where I'm on the faculty. Our professional interests cover things like cryptography, biometrics and smartcards. I also teach courses on security to undergraduates, and the lecture notes I developed for these courses are available as a book. If this had been read more widely, we would hear much less rubbish about security.

In the wake of the tragedy of the 11th September, we saw a publicity stampede by security vendors, consultancy companies, and intelligence agencies hustling for money and power. This seemed to be in rather bad taste, and showed a lack of respect for the dead. Now that the world is getting back to something like business as usual, it's time to debunk the rubbish that's being peddled.

Ross Anderson

Historical Precedents

Newspapers commonly describe shocking events as `unprecedented', but this is rarely so. The history of piracy offers some intriguing parallels. For example, after Ferdinand and Isabella drove the last Muslims from Spain in 1492, the hundreds of thousands of educated, unemployed refugees dumped in North Africa supported piracy against European shipping. They not only took cargoes, but also sold captured Christian sailors as slaves. (The pirates viewed this as `jihad'; European governments saw it as a revolting crime.) The equivalent of the World Trade Center attack was the seizure, near Elba in 1504, by the elder Barbarossa, of Pope Julius II's two treasure galleons, using only four rowing boats of fanatical Mujahideen.

This exploit made Barbarossa a hero to Muslim youth. Ferdinand led a punitive expedition, blockaded North Africa and sacked its cities. After his death, piracy flared up again ... and again. Then, as now, it was usually convenient for the rulers of states north of the Mediterranean to compromise with those to the south, and turn a blind eye to many violent substate activities that are nowadays called `terrorism' but were then known as piracy or privateering. Such semi-privatised warfare was conducted later in the Caribbean and elsewhere. The problem was not `fixed' until governments ended privateering, in effect nationalising warfare, by the Declaration of Paris in 1856.

For the intervening dozen generations, pirate bands repeatedly grew to be a threat to states, then either got destroyed in battle, got co-opted by governments, or even set up their own states. There is much history on what worked and what didn't, and how piracy (and its suppression) got entangled with the other great issues such as slavery and colonialism. (The standard excuse for a colonial adventure was the suppression of piracy and slavery.) There were many pure `terrorist' incidents, such as when unpopular Caribbean governors got swung from their flagpoles and their towns razed. There were also romantic aspects: successful pirates became national heroes.

Since the six-day war in 1967, the old destructive pattern appears to be re-emerging. Air piracy may have replaced sea piracy as the means of jihad, but neither the West nor the Muslim world should be keen to slip back into the old pattern of low-grade proxy conflict. Last time round, this kept the Muslim world impoverished, leading ultimately to its dismemberment and colonisation. It was not risk-free to the northen powers either. When a storm helped Pasha Hassan defeat Charles V in the Battle of Algiers in 1541, so many Spaniards were captured that the price of slaves collapsed: a Christian was `scarcely fair exchange for an onion'. Oh, and some `Christian' powers also harboured pirates. In one case, the King of Spain sent a fleet to crush one of these upstart rogue states. The loss of the resulting sea battle, in 1588, led ultimately to the collapse of the Spanish empire.

(The above points were picked up in an article in the Daily Telegraph.)


Punitive reprisals didn't provide a magic solution to the problem of piracy/terrorism, but what about technology? One of the `solutions' that people have been hawking since the 11th September is biometrics - devices that can recognise people. The Malaysian government is apparently going to issue all its citizens with an ID card that contains a machine readable fingerprint; it's been suggested that iris scanning be used to identify all UK citizens; and Visionics has seen its stock price treble after it held out a technology to scan the faces of people in crowds for known terrorists.

The Visionics system has since been thoroughly debunked in an article in the New York Times which revealed that their system, used in some UK towns, relies on the placebo effect; it manages to reduce crime because criminals believe it works, even though from the technical viewpoint it doesn't really. (I already remarked on this in my book at page 265.)

Biometrics suffer from a number of problems. These include high error rates; a typical fingerprint scanner will get it wrong about 1% of the time, and most other technologies are worse. The result is that in many applications the false alarm rate is unacceptable. In mass screening, there will be so many more false alarms than real ones that the `boy who cried wolf' effect will discredit the system in the eyes of the public and of its operators.

Scanners can be tuned to give fewer false alarms, but only at the most of more missed alarms. How this works out depends on the application; in banking, for example, the requirement is for a false alarm (`insult') rate of 0.01% and a missed alarm (`fraud') rate of 1%; currently the only technology that can meet this is iris scanning. (That's held back by marketing issues: people are reluctant to gaze into a scanner in case it malfunctions and blinds them.)

Other factors include scale (the more subjects you have, the more likely you are to have two subjects the system can't tell apart), forgery (is the subject using a rubber fingerpad, or wearing a contact lens with someone else's iris printed on it?), standards (if one particular biometric becomes the standard, then the Mafia will know yours from the time you ate in one of their retaurants and used it to authorise a credit card transaction), and social exclusion (fingerprint scanners work less well with the elderly and with manual workers, while iris scanners work less well with people whose eyes are dark, such as Arabs). Oh, and there are religious objections too: see Revelation 13:16-18. You don't want to have to fight both the Muslim fundamentalists and the Christian fundamentalists at the same time, do you?

The executive summary is that biometric systems are generally less suited for mass application. They can work well for attended applications involving small numbers of people. For example, I tried to get iris scanners for entry control to our new computer lab building. It would have meant I didn't have to fumble for a door key while carrying an armful of books - just look at the lock. (The reason we didn't buy it is that the currently available products are expensive and haven't been integrated with commodity entry control software. However, give the market a year or two, and that should get fixed.)

In my book, the chapter on biometrics covers the engineering issues of using handwritten signatures, face recognition, fingerprints, iris codes and voice recognition, as well as system-level issues and the things that commonly go wrong. I don't know of any other sources that give a warts-and-all view of the whole field.

False Alarms versus Missed Alarms

Managing the tradeoff between false alarms and missed alarms is not just a fundamental problem for biometric systems, but is one of the most pervasive problems in security engineering. If your metal detector is too sensitive, you end up hand-searching most of your passengers; if it isn't, it becomes too easy to get a gun on board. And now that you can buy box cutters that contain less ferrous metal than a typical belt buckle, things are getting tricky.

The false alarm rate not only sets limits on what can be achieved with metal detectors and biometric identification systems. It leaves many perimeter defences extremely vulnerable to opponents who induce false alarms deliberately; the smart way of sneaking a bomb into an airport at night is to rattle the fence a few times until the guards decide that the alarm is malfunctioning and stop coming round to look. (Here is a sample chapter of my book that describes this.) It also limits what can be done with wiretapping. If, prior to September 11th, the NSA had been able to scan all transatlantic telephone calls automatically for the simultaneous occurrence of the words `bomb', `al-Qaida' and `bin Laden', they might have filtered out a quantity small enough to listen to; but the FBI now tells us that the terrorists used simple open codes such as referring to their chief as `the director'. If the scope of the system had been increased to pick out everyone speaking in a guarded way, it would most likely have selected so many calls that no-one could have listened to them.

The tradeoff between false alarms and missed alarms was first explored systematically by radar engineers during World War 2, and quite a lot is known about it. It is the fulcrum on which modern electronic warfare rests. It's not enough just to combine multiple sources of information, as people trying to sell `AI data mining' systems suggest; combining sensors usually reduces one of the two types of alarm but increases the other. Suppose you have an air defence battery with both radar and infrared sensors. If you launch a missile only when both agree there is a plane in range, then there will be fewer false alarms, but more missed alarms; if you launch on either signal, it will be the other way round. (And if the enemy is smart, and your system is too sensitive, he will bombard you with false alarms and blind you.) The mathematics of all this isn't too difficult, but ignorance of it leads to a huge amount of money being wasted.

ID Cards

It's not just money at stake, but civil liberties. It is now fifty years since the UK citizens abolished mandatory ID cards, and for most of these fifty years our civil service has been scheming to reintroduce them, using a wide variety of excuses (access to computerised health records, smartcards for secure digital signatures for electronic commerce, unified access to e-government services, you name it). So one of the first government reactions to the atrocities was to float a plan for ID cards; in the USA, both Larry Ellison and Scott McNealy have called for an ID card system. Fortunately, in both our countries, the ID industry appears to have been beaten off - for now.

In addition to the well known political arguments against identity cards, which you can find at sites like Privacy International, there are quite a few security engineering issues that are often overlooked.

Some of the most subtle have to do with naming. An ID card system might seem to be just a means of assigning a unique name to everyone in the world: their country of residence, followed by their name and date of birth, and/or the number of their ID card or passport. Yet a systems engineer knows that the purpose of a name is to facilitate sharing, and this gives us a useful conceptual framework to analyse exactly what we're trying to achieve.

Suppose that we wish to ensure that some information from the French secret police, e.g. that the Algerian student Achmed Boulihia was said by an informant in 1992 to be a Muslim fundamentalist who idolised the assassins of Anwar Sadat, gets shared with an airline checkin agent to whom the same man presents himself in America in December 2001. What can go wrong?

Well, names can change in all sorts of ways. There are cultural issues (Arab men often change their names to celebrate the birth of their first son) and linguistic issues (Achmed might now spell his name with a different romanisation of the Arabic, such as Ahmed Abu Lihya). There are lots of issues like these, and people who design systems that deal with the same individuals over periods of decades (such as land registries) have to do a lot of work to cope with them. So the checking agent would probably prefer to rely on a simple unique identifier such as a social security number. But here again he runs up against funny foreign ways. In Germany, for example, ID card numbers are a document number rather than a person number, and so you can change your ID number by losing your ID card and getting another one. My book goes into a lot more detail, but, basically, identity and registration systems are extremely hard to design even when everyone involved is honest and competent. (And there are many parts of the world where you can buy a genuine passport in any old name for a few hundred dollars.)

Identity theft is another issue. It appears, for example, that one of the hijackers used a passport stolen from a Saudi in the USA five years previously. This is a good example of what security engineers call the revocation problem - how do you get bad news quickly to people who need to know it? The standard example of a global revocation system is given by the lists of stolen credit cards that circulate round the world's banks and merchant terminals. This is big, and complex, and only works really well where the transactions are local or the amounts involved are large. It is just not economic to inform every banking terminal in Russia whenever a credit card is reported missing in Portugal. And if the US airline system can't pick up on a theft reported in the USA to the US authorities, what chance is there for something that works well internationally?

In short, there are inherent limitations due to system-level issues of scale and complexity. These limitations are very difficult to conceal from the bad guys, as they can try various types of identity document and see which of them set off what alarms. In fact, there's a whole criminal industry doing this, and using identity theft for all sorts of wicked purposes - from obtaining credit cards, through student loan fraud, to registering mobile phones in other people's names to feed premium-rate scams. So if you are building a system to carry intelligence data to airport checkin terminals, it's not all that bright an idea to make it depend completely on such an unreliable foundation as the international system of government-issue photo-ID. (In any case, you have much more serious things to worry about, such as whether the intelligence service providing your data is trying to blacklist its political enemies, whether its informant was mistaken or lying, and whether the checkin clerk works for the terrorist group's counterintelligence department. The error rate tradeoffs for all this get complicated and unpleasant enough.)

There is also the issue of whether binding humans strongly to unique names is solving the right problem anyway. In the UK, our experience is that the IRA use people who are unknown to authority for major operations. Even a perfectly dependable global ID/passport system would be of limited help against such attacks.

Control of Cryptography

Since the early 1990s, various security and intelligence agencies have been arguing that cryptography would be a potent weapon for terrorists, and asking for laws that would give them control of authentication on the internet. They painted a lurid picture of terrorists and child pornographers escaping justice because they could encrypt their emails. People involved in IT have generally seen this as a shameless attempt to creat a huge new bureaucratic empire that would get in everyone's way. Civil libertarians went further and built up the whole issue as a Manichean struggle between privacy and surveillance: encryption, some said, was the only way the individual could protect himself against overweening state surveillance.

In my own writings on the topic, such as this paper from 1995, I argued that encryption, law enforcement and privacy have little to do with each other. The real problem in police communications intelligence is not understanding the traffic, but selecting the messages of interest. Only a stupid criminal will encrypt his traffic, as it will bring him to the attention of authority; the technologies the police ought to have been worried about are not `encryption on the Internet' but things like prepaid mobile phones. Although senior policemen would agree in private, none would dare buck the official line (that I was foolish, wicked, naive, technologically clueless, and everything in between).

The events since September 11th have proved my view to be right, and the agencies to be wrong. Al-Qaida's communications security was fit for purpose, in that their attack took the agencies completely by surprise. It did not involve encryption. It did involve hiding messages - among the zillions of innocuous emails that pass across the Internet each day. Two separate FBI reports stated that the hijackers used throwaway webmail accounts, accessed from public libraries.

It is disgraceful (but predictable) that the agencies are using the tragedy for bureaucratic empire building, by reviving all the old policies of crypto control that everyone thought dead and buried. If legislators allow themselves to be panicked into giving them their way, the agencies will need huge budget increases, and acquire the ability to push lots more people around. It is much less clear that such policies will do anything of real value in the fight against al-Qaida. Agencies that react to visible policy failure by demanding that their old, failed policies be supported by even more money, and even more coercive power, should be disbanded and replaced with new organisations managed by new people.

Airport Security

There has been much press comment on poor airport security, and a lot of effort is going into `tightening' it. Of course, it was clear from the start to security professionals that this was solving the wrong problem. Preventing similar hijackings in the future is a matter for aircraft operating procedures, and the problem is now probably fixed - regardless of anything that governments do. Thanks to the huge publicity given to the attacks, it will be a long time before a pilot opens the cockpit door and rushes out, just because he's heard that one passenger is disembowelling another.

In the USA, the government is taking over responsibility for airport security. This may not be an entirely good thing. The standard government-security model works via perimeter controls: people have clearances, objects have classifications and places are in zones. These are related by a set of access rules and perimeter control (such as no Secret document may be left on a desk overnight' and `only an officer of rank colonel or above can declassify a Top Secret document'). This approach gets used in a rough and ready way in airports (`only staff with a red badge, and ticketed passengers who have gone through metal detectors, can go airside'), but implementing it any more thoroughly is likely to be problematic.

In a large airport, over ten thousand employees of over a thousand companies might have airside badges. Many of these have tools that can be used as weapons; many others are minimum-wage staff with at best cursory background checks. How do you stop a casual restaurant cleaner from stealing a chisel from a maintenance carpenter?

The standard government-security approach would be to divide the airport into a number of zones, in which there would be clear restrictions on personnel and objects, and between which there would be screening. I expect this would involve a large investment in redesigning the buildings. An alternative would be to use only cleared staff airside, or to limit and subdivide the airside zone by screening passengers and hand baggage at the departure gate - but both of these would also cost more. There is by now a large literature on the practical problems of clearances, managing security compartments and so on; I discuss some of the issues in chapters 7-8 of my book.

Bruce Schneier has collected some useful comments and links on airport screening issues in Cryptogram. A number of the points he makes, such as that airports go for visible protective measures (such as bullying customers) rather than effective ones (such as positive matching of bags to passengers at the loading gate) have long been obvious to people in the `trade'. But why does so much money get wasted?

Economics and Security

Many security systems fail for reasons that are more economic than technical. The people responsible for protecting a system are not usually the people who get hurt when the protection fails. Often this is because the system has changed subtly since it was first designed. During the 1980s, banks connected up their ATM systems into networks, and now instead of just preventing fraud by customers their security people have to worry about lawsuits from other banks. This makes everyone defensive and leads to customers who report a fraud being told they must be mistaken or lying. This in turn makes bank staff realise that they can swindle the system without getting caught, which in turn causes fraud to increase.

Often, security should not just be discussed in the language of ciphers, seals and biometrics, but of asymmetric information, adverse selection and moral hazard - in short, of microeconomics. I have written a paper on these issues, which are one of our current research topics. They are also discussed at length in my book.

Things also go wrong within organisations when tensions arise between the interests of staff and those of their employers. This leads, for example, for risk reduction measures being turned into due diligence routines: `so long as I do X, Y and Z, I won't get fired'. There are also adverse selection effects; a notorious example is that individuals who are particularly risk-averse often seek jobs in the public sector. The result, as economists have long known, is that most organizations (and especially public sector ones) are excessively cautious; they take many fewer risks than a rational economic agent would in similar circumstances. There's a nice article on this by John Adams, and an analysis on how the `tightening' of security in the USA following the TWA crash a few years ago probably cost about 60 lives a year by driving airline passengers to drive instead.

Although it might seem strange for a security engineer to say this, our societies spend far too much on `security'. Increasing this still further in knee-jerk response to terrorist incidents will hinder economic growth - and is letting the other side dictate the game. The response to terrorist incidents must include much better risk management: better information on the threats and on what the available countermeasures can actually achieve, getting the mathematics of the risk models right, diverting protection resources from show to substance and from due diligence to risk reduction, and - finally - educating the press and public about what can realistically be done, and what can't.

The USA may well not find a simple military solution to the Islamic fundamentalism problem, any more than Spain could find a lasting military solution to the Barbary pirate problem in 1504. But, as an academic, I do believe we know in principle how to fix those secondary problems that arise from ignorance. That's after all why I wrote my book.

My book

Although there are decent books on some security tools, such as burglar alarms, access controls and cryptography, there has been almost nothing on how to use them in real systems. But most security systems don't fail because the protection mechanisms were weak, but because the designers protected the wrong thing, or protected the right thing in the wrong way. If an extra one or two percent of gross world product is going to be spent on security engineering over the next few years, that amounts to absolutely colossal waste. Reading my book can help you avoid some of it.

`Security Engineering - a Guide to Building Dependable Distributed Systems' gives a fairly detailed tutorial on a number of security applications, such as automatic teller machines, burglar alarms, copyright protection mechanisms and electronic warfare systems. It uses these to introduce a wide range of security technologies, such as biometrics, tamper-resistant electronics and authentication protocols. This material is then used to bring out the system-level engineering issues, such as false alarm rates, protection versus resilience, naming, security usability, reliability, and assurance.

Although the book grew out of notes for security courses I teach at Cambridge, I've rewritten the material to ensure it's accessible to the working professional. The people on whom I tested the manuscript included not just security geeks but doctors, lawyers, accountants, retired soldiers and airmen, an economics professor, a software company CEO, and even my mum (a retired pharmacist). Check out the following:

Finally, here is the book's home page, which has ordering information.