Bruce Schneier’s Cryptogram newsletter points to a paper from MIT that outlines the “Carnival Booth: An Algorithm for Defeating the Computer-Assisted Passenger Screening System“:
What may be more alarming is that evidence from the September 11 investigation shows that Atta already knew the kernel idea behind this algorithm. Newsweek reported that in the weeks before September 11, Atta and his conspirators practiced their attack by boarding the exact same target flights they intended to later hijack (same planes, same times, same origins and destinations). They wanted to ensure that they didn’ raise any suspicions or red flags. This is a clear demonstration of Atta’s cleverness. Like Atta, terrorists are smart. They already know this algorithm. And they are already using it.
I’d like to point to more in that newsletter, but you should just go read the whole thing. Also, see this Atlantic Monthly profile of Bruce Schneier.