Airline passenger screening

The so-called Carnival Booth algorithm shows the weakness of airline (and other) passenger screening.

Suppose that I’m a bad guy and I want to carry out an attack. I start with 100 possible attackers. I send them on flights around the country, in which they behave completely innocently. Anyone who ever gets pulled over for extra screening is removed from the pool.

Eventually my pool of 100 is reduced to a much smaller number, but I can have high confidence that the remaining members will not receive any extra screening when they travel. And now I can use them to carry out an attack, with high probability that they will not be given special consideration. (Of course, they will still receive normal screening, so my attack will have to involve mechanisms that are not normally detected.)

This is why randomization helps — if the criteria for extra screening are randomized slightly from hour to hour, or perhaps some people are selected completely at random, then I can never be sure that someone who hasn’t been selected yet, will not be selected next time, when they may be travelling less innocently.

The corollary to randomization is that some people who are “obviously” innocent will sometimes be given extra screening. People tend to see this as silliness or a waste of resources — but is is actually a good use of resources. This case needs to be explained to the public more clearly.

The Carnival Booth algorithm was first explained by some students from MIT.

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