TPMS and privacy

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Schneier expresses concern about the tire pressure monitoring system (TPMS). The way TPMS works is that each wheel contains a pressure sensor and a radio transmitter which transmits pressure data to a receiver in the car, which somehow alerts you if the pressure is too low. The alleged problem is that in order to allow distinguishing wheels from each other (and from those in adjoining cars), each wheel has a unique identifier, raising the possibility that one could build a radio receiver which would listen for these transmissions and track your car.

Obviously, this isn't that attractive a feature, as Hexview observes:

What problems exactly does the TPMS introduce? If you live in the United States, chances are, you have heard about the "traffic-improving" ideas where transportation authorities looked for the possibility to track all vehicles in nearly real time in order to issue speeding tickets or impose mileage-adjusted taxes. Those ideas caused a flood of privacy debates, but fortunately, it turned out that it was not technically of financially feasible to implement such a system within the next 5-10 years, so the hype quickly died out.

Guess what? With minor limitations, TPMS can be used for the very purpose of tracking your vehicle in real time with no substantial investments! TPMS can also be used to measure the speed of your vehicle. Similarly to highway/freeway speed sensors that measure traffic speed, TPMS readers can be installed in pairs to measure how quick your vehicle goes over a predefined distance. Technically, it is even plausible to use existing speed sensors to read TPMS data!


As every other tracking technology, the TPMS was introduced as a safety feature "for your protection". One might wonder why NTHSA (a government agency) would care so much about a small number of accidents related to under-pressurized tires. And why would it choose to mandate TPMS and not run-flat technology? Are we being tracked already? I hope not.

It's absolutely true that NHTSA required TPMS. It doesn't look to me, however, like NHTSA required this particular implementation, or any particular implementation. They just required that the car be able to detect that the car be able to detect loss of pressure by more than 25%. As Hexview observes, there is a simple implementation that dramatically reduces the privacy problem: encrypt the sensor readings, and as far as I can tell this would be quite compatible with the NHTSA requirements (this doesn't totally reduce the problem because of radio fingerprinting, but this is harder than just reading the ID out of the air). The good news is that since there's no need for my car to be able to read your car's tire pressure, it's quite possible for manufacturers to do the right thing without any kind of new standard.

Hexview implies that NHTSA may have required TPMS in order to enable them to monitor your whereabouts, but I find that somewhat unlikely. Certainly, when I was involved with DSRC/WAVE, privacy was foremost on everyone's minds, so it would be strange of NHTSA were to deliberately attempt to violate driver privacy. That said, the manufacturers were also pretty concerned about privacy, so if they have rolled out a system that enables tracking, that's a little surprising.


The transmitter would have to be a very low-power unit; I'd guess it would be powered by the wheel rotation. Wouldn't that be hard to pickup remotely? To be functional, it would only have to transmit a distance of maybe one centimeter.

would be interesting for NHTSA to do privacy impact assessments of different implementations of their requirements (then these could be an additional marketing lever for privacy-enhanced implementations).

Yeah. I think people who haven't worked in government sometimes get a very exagerated idea of how closely related and planned all the different government programs/initiatives are. Just because DHS might wish for ways to track you doesn't mean NHTSA is spending any time trying to set that up for them.

And I'll admit to being a bit skeptical about some of these privacy worries, given that all our cars have an identifier string bolted to their front and back, for all to see. That imposes a hard upper bound on how much difficulty anyone can have tracking cars that pass by their location, and it's not that high a bound. Speeding and red light cameras don't seem to have much trouble with capturing this identifier string, for example.

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