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Ever since we bought a hatchback car two years ago, I have found the method of protecting the contents of the trunk very amusing. Because there is no real trunk, the contents stored in the back are clearly visible through the rear window. In order to provide some measure of security, there is a plastic/vinyl tarp-like thing that has been installed that can be pulled down over whatever is in the back, shielding it from view. This is done so that whatever valuables are there cannot be seen from the outside.

Now this on the surface seems to be a pretty good idea. I wouldn't want my stereo attracting the eye of anybody walking by the car. The problem that I find with it, however, is that the system itself seems to be self-defeating. It seems to me that the presence of the tarp would cause a burglar to break into the car almost as much as seeing the content themselves. The reason why is this: the tarp exists to protect the contents of the trunk from being seen. People only use the tarp when there is something in the back that they don't want anybody to see (if the trunk were empty or contained say a pile of sticks, we wouldn't bother covering it) and they don't want it to be seen because they don't want it stolen. Therefore if the tarp is pulled over, there is something in the trunk that is worthy of being stolen. In effect, by covering the back with the tarp, the owner is in fact telling me that there is something to steal.

I have come to call this a Persistent Knowledge Situation because if there is something valuable in the back, there is nothing you can do to prevent me from learning of that fact. If the item is left visible, then I can tell that the item is valuable because I can see for myself. If it is concealed however, I still know that there is something valuable by the mere fact that it is being concealed. Either way, I am aware of its presence.

Now of course, this is not completely true, nor is it necessarily always the case (otherwise I assume there would be other protection). There are of course advantages to covering up whatever is in the trunk. First of all, I don't technically know that there is anything in the back. The trunk could be empty and still be concealed, and that is a risk I'd have to take. On the other hand, I don't think that too many people conceal their trunks if they have nothing to hide. The second, and most important reason why it is good to cover something up is because even if it does alert me to the fact that there is something in the trunk that the owner doesn't want stolen, it doesn't necessarily mean that whatever's there is valuable or worth the effort of breaking in. I don't know what is there - it could be a thousand-dollar stereo or it could be a box of used books. This makes it a big risk for me, and I imagine that it is this risk that makes the whole thing work. People are unwilling to go through the effort of breaking into a car if they don't know what the payoff will be.

I still find the whole situation very fascinating because in its generalized form, the method of preventing the knowledge in fact provides the same information. I saw another example of this recently when I saw the X-Men film. Magneto is taking over the Statue of Liberty and the X-Men set off to stop him. In order to shield their approach, the character Storm causes a dense fog to roll in, covering their plane. Magneto is not fooled by this however - he notices it and realizes that it is in fact the creation of the X-Men, alerting him to their presence. Storm's attempt to conceal their approach has instead alerted him to it!

The reason I think that both of these methods (trunk tarp and Storm's fog) are fallible is based on the probability of these things occurring normally. For the most part, a hatchback trunk will not be covered if there is nothing there, and a fog rolling in right as Magneto begins his attack is rather unlikely. Therefore, their presence is significant. The only way that I can see for the hatchback setup to truly work is if everybody covered up their trunk all the time. Then, the presence of the tarp would not be anything of the ordinary and would not implicitly signify anything.

A more high-tech instance of this problem is encrypted communications. If I'm worried that The Man will discover that lemuru and I are plotting to smash the state, we can exchange keys and communicate using strong encryption. We can make it practically impossible for anyone to find out what our emails say for the next several years. But it's substantially more difficult to disguise where a message originates, or where it's going. If carnivore (or a good old wiretap) uncovers the fact that we're taking some effort to make our email unreadable, that in itself can indicate that there's a conspiracy afoot. They can then begin investigating our activities through other means.

Edibleplastic touched on one solution to this problem earlier: if everyone had a tarp in their car, then the tarp would not be a useful source of information. The same would happen if everyone used strong encryption on their email. The spooks would have no idea whose mail to overclock their supercomputers cracking. Rather than trying to neutralize the persistent signal, we immerse it in undifferentiable noise.

This is the primary reason I use PGP.

Now, there's a substantial barrier to getting everyone to put a tarp in the back of their car. If we assume that there's a constant, unchanging number of thefts, then if everyone has a tarp the chances of having your car broken into (a major hassle, even if nothing is stolen) are completely random -- the number of thefts divided by the number of cars. But if only a few people have tarps, then those cars will get broken into, and the ones without will be left alone. It's like those no radio signs.

In this example, one flaw is that the number of thefts is not necessarily constant. If every car has a tarp, then theivery might become unrewarding enough to vanish completely. Then everyone benefits. But if there are even a few thefts, then one stands to benefit even more from having everyone else use a tarp, and leaving one's own car without one. Of course, if everyone thinks that way there will be no tarps. This is an illustration of a large-scale prisoner's dilemma, in which a few people who defect can cause widespread breakdown. A less abstract formulation is the free rider problem.

In the case of encryption, though, the situation is different. It's equally hard to break into a car with a tarp, but it's a heck of a lot harder to spy on encrypted email. And while those who encrypt run some risk of getting fingered by the NSA, they eliminate a much greater one that some 12 year-old reprobate with a packet sniffer will use the chance to giggle over their love letters. So, everyone who adopts encryption both gains an immediate, personal benefit and contributes to a decrease in the associated risks. This is also helped by the fact that internet users feel some altruism towards their community; the recent Jam Echelon Day, directed against the international espionage infrastructure ECHELON, was only the latest in a long line of attempts to overload keyword-based spying in a way that offers no personal benefits to the participants.

Communities can sometimes overcome the persistence of knowledge problem. The way to do it is to find a system that benefits individuals immediately, as well as helping out the group once it reaches critical mass.

(none of this really helps storm. her only option would have been to anticipate this contingency far in advance, and cause New York to fog over irregularly for a long period of time.)

note: Thanks to caseyhb for pointing out a lack of clarity in my discussion of similarities between defecting prisoners and free riders. It has been amended. I hope

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