What are no-fly lists for?

There’s a great deal of confusion in the discussion of airline security because several things are going on at once.

It’s sort of clear what the point of passenger screening is. The goal is to prevent anyone from carrying out an attack, either a hijacking or a bombing. The mechanism is to make sure that nobody can take the required tools or devices onto a flight.

Why is there an analysis component? Why not simply search everyone in exactly the same way, to make sure that they aren’t carrying anything they shouldn’t be? It’s a question of managing the costs and the risks. Analysing data about potential passengers allows them to be placed in categories. Each category consumes a different amount of resources to check; the amount is related to the perceived risk of people in that category.

Of course, there are advantages to not making these categories too rigid or predictable, as I discussed in the previous post.

This idea of risk management is a sensible one. But, given this framework, what is the point of a no-fly list? If I can be absolutely sure that a terrorist who gets on a plane does not have the ability to do anything different from the other passengers, is there any reason not to let him fly.

There are two reasons why a no-fly list might still be a good idea:

  1. It’s actually impossible to be sure that someone who gets on a plane cannot do anything destructive, no matter how much time is spent on checking beforehand. Even the Israelis, who are no slouches when it comes to airline security, and who’ve been doing it a long time, cannot guarantee that someone is actually innocuous. It’s not clear what the issues are, but it seems at least possible to make a swallowable IED, or for a group to take on board objects that are individually innocuous, but together could make something nasty.
    As a matter of practicality, it’s also the case that people who are highly motivated to carry out an attack are not worth the resources to screen completely. In other words, the point of passenger screening is to act as a safety net, catching people who aren’t known to be terrorists, either terrorists not yet discovered, or people who’ve suddenly snapped.
  2. A no-fly list puts barriers in the way of terrorists, making it hard for them to move and meet. This only makes sense for domestic travel, since borders already perform this function for international travel. In other words, once you become a terrorist you cut yourself off from moving freely around a country, because you can no longer use air transportation. Obviously, this matters more in large countries, and those with lots of islands or other physical barriers, than in small countries like those in Europe.

The biggest worry, and problem, with no-fly lists is their false positive rate, that is the number of innocent people who are mistakenly identified as terrorists and prevented from flying. This is partly a self-inflicted problem by governments which rushed to implement no-fly lists for largely cosmetic reasons, without taking the time to think about and implement reliable lists to begin with.

However, constructing and maintaining such lists is difficult because it requires making secret information widely available, which is not the way to keep it secret. (The list has to be at least partly secret because covert means might have been used to put some people on it; if they knew they were on it, they might be able to work out how.) There are technical ways to check whether someone is on a list without having to make the list public, using encryption techniques, but this idea has not, afaik, been used.

The second big problem with no-fly lists is that they are lists of identities, and it’s quite hard to robustly establish the identity of the person standing in front of you, if they are motivated not to make it easy. That’s why there’s such high levels of interest in biometrics — they provide a way to link some property of your physical presence to other data about you, and so to establish your identity. As a result, identity theft is big business — according to Bruce Schneier, a bigger business than drugs in the U.S.

Governments have also piggybacked law enforcement onto no-fly lists, so that people who are wanted for crimes of sufficient seriousness can be added to such lists. Whether or not this is a good idea is a complex subject; but it does make ordinary citizens suspicious about how much mission creep is a factor in government programs aimed at detecting and preventing terrorism. More another time, perhaps.

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