Why are we spending $7 billion on TSA?

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Bruce Schneier is a security technologist and chief technology officer of Resilient Systems Inc. His latest book is "Data and Goliath: The Hidden Battles to Collect Your Data and Control Your World." He blogs at schneier.com and tweets @schneierblog. The opinions expressed in this commentary are solely those of the author.

(CNN)News that the Transportation Security Administration missed a whopping 95% of guns and bombs in recent airport security "red team" tests was justifiably shocking. It's clear that we're not getting value for the $7 billion we're paying the TSA annually.

But there's another conclusion, inescapable and disturbing to many, but good news all around: We don't need $7 billion worth of airport security. These results demonstrate that there isn't much risk of airplane terrorism, and we should ratchet security down to pre-9/11 levels.
We don't need perfect airport security. We just need security that's good enough to dissuade someone from building a plot around evading it. If you're caught with a gun or a bomb, the TSA will detain you and call the FBI. Under those circumstances, even a medium chance of getting caught is enough to dissuade a sane terrorist. A 95% failure rate is too high, but a 20% one isn't.
    Bruce Schneier
    For those of us who have been watching the TSA, the 95% number wasn't that much of a surprise. The TSA has been failing these sorts of tests since its inception: failures in 2003, a 91% failure rate at Newark Liberty International in 2006, a 75% failure rate at Los Angeles International in 2007, more failures in 2008. And those are just the public test results; I'm sure there are many more similarly damning reports the TSA has kept secret out of embarrassment.
    Previous TSA excuses were that the results were isolated to a single airport, or not realistic simulations of terrorist behavior. That almost certainly wasn't true then, but the TSA can't even argue that now. The current test was conducted at many airports, and the testers didn't use superstealthy ninja-like weapon-hiding skills.
    This is consistent with what we know anecdotally: The TSA misses a lot of weapons. Pretty much everyone I know has inadvertently carried a knife through airport security, and some people have told me about guns they mistakenly carried on airplanes. The TSA publishes statistics about how many guns it detects; last year, it was 2,212. This doesn't mean the TSA missed 44,000 guns last year; a weapon that is mistakenly left in a carry-on bag is going to be easier to detect than a weapon deliberately hidden in the same bag. But we now know that it's not hard to deliberately sneak a weapon through.
    So why is the failure rate so high? The report doesn't say, and I hope the TSA is going to conduct a thorough investigation as to the causes. My guess is that it's a combination of things. Security screening is an incredibly boring job, and almost all alerts are false alarms. It's very hard for people to remain vigilant in this sort of situation, and sloppiness is inevitable.
    There are also technology failures. We know that current screening technologies are terrible at detecting the plastic explosive PETN -- that's what the underwear bomber had -- and that a disassembled weapon has an excellent chance of getting through airport security. We know that some items allowed through airport security make excellent weapons.
    The TSA is failing to defend us against the threat of terrorism. The only reason they've been able to get away with the scam for so long is that there isn't much of a threat of terrorism to defend against.
    Even with all these actual and potential failures, there have been no successful terrorist attacks against airplanes since 9/11. If there were lots of terrorists just waiting for us to let our guard down to destroy American planes, we would have seen attacks — attempted or successful — after all these years of screening failures. No one has hijacked a plane with a knife or a gun since 9/11. Not a single plane has blown up due to terrorism.
    Terrorists are much rarer than we think, and launching a