In his latest bestseller, Data and Goliath, world-renowned security expert and author Bruce Schneier goes deep into the world of surveillance, investigating how governments and corporations alike monitor nearly our every move. In this excerpt, Schneier explains how we are fed a false narrative of how our surveillance state is able to stop terrorist attacks before they happen. In fact, Schneier argues, the idea that our government is able to parse all the invasive and personal data they collect on us is laughable. The data-mining conducted every day only seems to take valuable resources and time away from the tactics that should be used to fight terrorism.

The NSA repeatedly uses a connect-the-dots metaphor to justify its surveillance activities. Again and again — after 9/11, after the Underwear Bomber, after the Boston Marathon bombings — government is criticized for not connecting the dots.

However, this is a terribly misleading metaphor. Connecting the dots in a coloring book is easy, because they're all numbered and visible. In real life, the dots can only be recognized after the fact.

That doesn't stop us from demanding to know why the authorities couldn't connect the dots. The warning signs left by the Fort Hood shooter, the Boston Marathon bombers, and the Isla Vista shooter look obvious in hindsight. Nassim Taleb, an expert on risk engineering, calls this tendency the "narrative fallacy." Humans are natural storytellers, and the world of stories is much more tidy, predictable, and coherent than reality. Millions of people behave strangely enough to attract the FBI's notice, and almost all of them are harmless. The TSA's no-fly list has over 20,000 people on it. The Terrorist Identities Datamart Environment, also known as the watch list, has 680,000, 40% of whom have "no recognized terrorist group affiliation."

Data mining is offered as the technique that will enable us to connect those dots. But while corporations are successfully mining our personal data in order to target advertising, detect financial fraud, and perform other tasks, three critical issues make data mining an inappropriate tool for finding terrorists.

The first, and most important, issue is error rates. For advertising, data mining can be successful even with a large error rate, but finding terrorists requires a much higher degree of accuracy than data-mining systems can possibly provide.

Data mining works best when you're searching for a well-defined profile, when there are a reasonable number of events per year, and when the cost of false alarms is low. Detecting credit card fraud is one of data mining's security success stories: all credit card companies mine their transaction databases for spending patterns that indicate a stolen card. There are over a billion active credit cards in circulation in the United States, and nearly 8% of those are fraudulently used each year. Many credit card thefts share a pattern — purchases in locations not normally frequented by the cardholder, and purchases of travel, luxury goods, and easily fenced items — and in many cases data-mining systems can minimize the losses by preventing fraudulent transactions. The only cost of a false alarm is a phone call to the cardholder asking her to verify a couple of her purchases.

Similarly, the IRS uses data mining to identify tax evaders, the police use it to predict crime hot spots, and banks use it to predict loan defaults. These applications have had mixed success, based on the data and the application, but they're all within the scope of what data mining can accomplish.

Terrorist plots are different, mostly because whereas fraud is common, terrorist attacks are very rare. This means that even highly accurate terrorism prediction systems will be so flooded with false alarms that they will be useless.

The reason lies in the mathematics of detection. All detection systems have errors, and system designers can tune them to minimize either false positives or false negatives. In a terrorist-detection system, a false positive occurs when the system mistakenly identifies something harmless as a threat. A false negative occurs when the system misses an actual attack. Depending on how you "tune" your detection system, you can increase the number of false positives to assure you are less likely to miss an attack, or you can reduce the number of false positives at the expense of missing attacks.

Because terrorist attacks are so rare, false positives completely overwhelm the system, no matter how well you tune. And I mean completely: millions of people will be falsely accused for every real terrorist plot the system finds, if it ever finds any.

We might be able to deal with all of the innocents being flagged by the system if the cost of false positives were minor. Think about the full-body scanners at airports. Those alert all the time when scanning people. But a TSA officer can easily check for a false alarm with a simple pat-down. This doesn't work for a more general data-based terrorism-detection system. Each alert requires a lengthy investigation to determine whether it's real or not. That takes time and money, and prevents intelligence officers from doing other productive work. Or, more pithily, when you're watching everything, you're not seeing anything.

The US intelligence community also likens finding a terrorist plot to looking for a needle in a haystack. And, as former NSA director General Keith Alexander said, "you need the haystack to find the needle." That statement perfectly illustrates the problem with mass surveillance and bulk collection. When you're looking for the needle, the last thing you want to do is pile lots more hay on it. More specifically, there is no scientific rationale for believing that adding irrelevant data about innocent people makes it easier to find a terrorist attack, and lots of evidence that it does not. You might be adding slightly more signal, but you're also adding much more noise. And despite the NSA's "collect it all" mentality, its own documents bear this out. The military intelligence community even talks about the problem of "drinking from a fire hose": having so much irrelevant data that it's impossible to find the important bits.

We saw this problem with the NSA's eavesdropping program: the false positives overwhelmed the system. In the years after 9/11, the NSA passed to the FBI thousands of tips per month; every one of them turned out to be a false alarm. The cost was enormous, and ended up frustrating the FBI agents who were obligated to investigate all the tips. We also saw this with the Suspicious Activity Reports —or SAR — database: tens of thousands of reports, and no actual results. And all the telephone metadata the NSA collected led to just one success: the conviction of a taxi driver who sent $8,500 to a Somali group that posed no direct threat to the US — and that was probably trumped up so the NSA would have better talking points in front of Congress.

The second problem with using data-mining techniques to try to uncover terrorist plots is that each attack is unique. Who would have guessed that two pressure-cooker bombs would be delivered to the Boston Marathon finish line in backpacks by a Boston college kid and his older brother? Each rare individual who carries out a terrorist attack will have a disproportionate impact on the criteria used to decide who's a likely terrorist, leading to ineffective detection strategies.

The third problem is that the people the NSA is trying to find are wily, and they're trying to avoid detection. In the world of personalized marketing, the typical surveillance subject isn't trying to hide his activities. That is not true in a police or national security context. An adversarial relationship makes the problem much harder, and means that most commercial big data analysis tools just don't work. A commercial tool can simply ignore people trying to hide and assume benign behavior on the part of everyone else. Government data-mining techniques can't do that, because those are the very people they're looking for.

Adversaries vary in the sophistication of their ability to avoid surveillance. Most criminals and terrorists — and political dissidents, sad to say — are pretty unsavvy and make lots of mistakes. But that's no justification for data mining; targeted surveillance could potentially identify them just as well. The question is whether mass surveillance performs sufficiently better than targeted surveillance to justify its extremely high costs. Several analyses of all the NSA's efforts indicate that it does not.

The three problems listed above cannot be fixed. Data mining is simply the wrong tool for this job, which means that all the mass surveillance required to feed it cannot be justified. When he was NSA director, General Keith Alexander argued that ubiquitous surveillance would have enabled the NSA to prevent 9/11. That seems unlikely. He wasn't able to prevent the Boston Marathon bombings in 2013, even though one of the bombers was on the terrorist watch list and both had sloppy social media trails — and this was after a dozen post-9/11 years of honing techniques. The NSA collected data on the Tsarnaevs before the bombing, but hadn't realized that it was more important than the data they collected on millions of other people.

This point was made in the 9/11 Commission Report. That report described a failure to "connect the dots," which proponents of mass surveillance claim requires collection of more data. But what the report actually said was that the intelligence community had all the information about the plot without mass surveillance, and that the failures were the result of inadequate analysis.

Mass surveillance didn't catch underwear bomber Umar Farouk Abdulmutallab in 2006, even though his father had repeatedly warned the U.S. government that he was dangerous. And the liquid bombers (they're the reason governments prohibit passengers from bringing large bottles of liquids, creams, and gels on airplanes in their carry-on luggage) were captured in 2006 in their London apartment not due to mass surveillance but through traditional investigative police work. Whenever we learn about an NSA success, it invariably comes from targeted surveillance rather than from mass surveillance. One analysis showed that the FBI identifies potential terrorist plots from reports of suspicious activity, reports of plots, and investigations of other, unrelated, crimes.

This is a critical point. Ubiquitous surveillance and data mining are not suitable tools for finding dedicated criminals or terrorists. We taxpayers are wasting billions on mass-surveillance programs, and not getting the security we've been promised. More importantly, the money we're wasting on these ineffective surveillance programs is not being spent on investigation, intelligence, and emergency response: tactics that have been proven to work. The NSA's surveillance efforts have actually made us less secure.

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Excerpted from Data and Goliath: The Hidden Battles to Collect Your Data and Control Your World by Bruce Schneier. Copyright (c) 2015 by Bruce Schneier. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved. 

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