Security is only as strong as its weakest link; three locks on the front door do little good if the back door is open. Likewise, the air transportation system is only as secure as the country's most insecure airport, because once someone passes through security at one location, they don't have to do so at another. Los Angeles International Airport is planning a redesign partly for security reasons, but the weakest-link principle posits that a terrorist could simply drive to San Diego and catch a commuter flight to LAX.

Many of the security measures we encounter on a daily basis aim pinpoint the bad guys by treating everyone as a suspect. The Department of Homeland Security counts on technology to come to our rescue. But that creates a problem similar to the one you see when airport security screeners waste their time frisking false alarms. Terrorists are so rare that any individual lead is almost certainly a false one. So billions of dollars are wasted with no assurance that any terrorist will be caught.

By Bruce Schneier at Wired.

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