It's easy to see how CAPPS II might create as many problems as it solves. Commercial credit data, for instance, can be shoddy, subject to the whims of thousands of companies who have access to your record, or the actions of identity thieves. Your criminal record, too, can be wrong -- or irrelevant to the threat you may pose to an airliner.

"Remember, this isn't just about privacy, it's also about accountability," Tien says. "It's not just Orwell -- it's Kafka."

Kshirsagar, of EPIC, echoes that theme. "We really worry about the credit data -- if your name is mismatched, say, or your date of birth or address is wrong, you could get searched every time." That's alarming, he says, because "they're creating a new identification standard through these databases." Both Kshirsagar and Tien say TSA's use of commercial databases to identify people would be conceptually no different from the creation of a national I.D. card.

When asked how incorrect credit data would affect one's CAPPS II score, Rosenker, the TSA spokeswoman, said, "Well, first, the industry itself encourages the general public to make sure that [credit] information is correct, so that's something we should all be doing. But number two, should there ever be a problem, we will have an ombudsman program to deal with that. We will have a check and balance in that regard. If there's a problem, a passenger would literally be able to be on the phone with them then and clear it up." She said the experience would be not unlike having your credit card declined at a store: a bit unpleasant but easily corrected.

A bigger problem with CAPPS II, though, is that it may not work very well at finding terrorists. In May 2002, Samidh Chakrabarti and Aaron Strauss, two graduate students in computer science (and a few other disciplines) at MIT, decided to see if they could come up with an algorithm that terrorists might use to beat a profiling system like the current version of CAPPS. After studying everything that is publicly known about CAPPS, the pair determined that anyone with the will and not very many resources could easily get around the system. They concluded that airlines would be safer if, instead of profiling, they instead selected a portion of fliers at random and subjected them to more thorough searches for weapons. (Chakrabarti and Strauss wrote up their findings in a term paper for a class, but it was picked up by First Monday, a peer-reviewed academic journal on the Web.)

The first thing one has to understand about security in U.S. air travel is that a lot of people take airplanes. In 2001, about 640 million people passed through U.S. airports, and it would be impossible to subject each person to a thorough check. It is also unnecessary to do so, of course, since the overwhelming majority of people flying are not terrorists.

Profiling systems were developed to pick out the people considered most likely to pose a threat in order to give them closer attention. The trouble, say Chakrabarti and Strauss, is that every time someone is picked out by a profiling system, the scrutiny he's given -- a bag search, for example -- tips him off that he fits the profile. If this person is part of a terrorist group, he can use the information to develop a plan for a future attack.

"Transparency is the Achilles' Heel of CAPPS; the fact that individuals know their CAPPS status enables the system to be reverse engineered," Chakrabarti and Strauss write. "You ... know if your carry-ons have been manually inspected. You know if you've been questioned. You know if you're asked to stand in a special line. You know if you've been frisked. All of this open scrutiny makes it possible to learn an anti-profile to defeat CAPPS, even if the profile itself is always kept secret."

How would a terrorist go about defeating CAPPS? All you'd have to do is have everyone in your cell take several flights; the people who don't get searched don't fit the profile, meaning that they're the ones who should carry out the attack.

In an interview on Tuesday, Strauss said that the same problem would apply to CAPPS II. Unless it's a "magic formula," a system that could catch everyone with terrorist intentions, it can be fooled. A random search, on the other hand, does not have that flaw. Every time a terrorist tried to get on a plane, there would be a probability that he would be searched, a probability that would stay the same no matter what he did or which members of the cell were sent.

The problem with a random search, though, is that it would subject a lot of people who clearly do not fit the "profile" of a terrorist to extra scrutiny. However much the country changed after Sept. 11, the fact remains that nobody likes being searched at airports. It's not only humiliating, it's not only a hassle, but because you're innocent, it's also -- to you -- completely unnecessary. While they're rifling through your belongings, security agents are very possibly missing a real terrorist -- and, if you look around you at that moment, there'll surely be some people who, you think, deserve to be searched more than you.

One gets this sense of indignation from many of the complaint letters released by the TSA. "Did we really deserve that much of your valuable time and attention?" one couple, who describe themselves as "natural-born, patriotic citizens of the United States," ask the TSA. The couple says they were "obviously profiled and systematically harassed" on a trip, and they wonder, "What is your organization going to do to eliminate this type of harassment of citizens so that the focus can be placed on that element of the population from which the threat really exists?"

It's a good question, and for that couple -- people who were so bothered by the TSA that they decided to file a complaint with lawmakers -- perhaps a system like CAPPS II, one that goes after more "terrorist-like" people, is in order.

If you really want to be safer in the air, though, and especially if you'd rather not give the government the power to check into your records, perhaps it'd be best if you -- and everyone else -- accepted the possibility of being thoroughly searched at an airport every once in a while. "Maybe that's just the price you have to pay after Sept. 11," Strauss says.

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