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.