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The US National Highway Traffic Safety Administration (NHTSA) is proposing a requirement that every car should broadcast a cleartext message specifying its exact position, speed, and heading ten times per second. In comments filed in April, during the 90-day comment period, we (specifically, Leo Reyzin, Anna Lysyanskaya, Vitaly Shmatikov, Adam Smith, together with the CDT via Joseph Lorenzo Hall and Joseph Jerome) argued that this requirement will result in a significant loss to privacy. Others have aptly argued that the proposed system also has serious security challenges and cannot prevent potentially deadly malicious broadcasts, and that it will be outdated before it is deployed. In this post I focus on privacy, though I think security problems and resulting safety risks are also important to consider.

The basic summary of the proposal, known as Dedicated Short Range Communication (DSRC), is as follows. From the moment a car turns on and every tenth of a second until it shuts off, it will broadcast a so-called “basic safety message” (BSM) to within a minimum distance of 300m. The message will include position (with accuracy of 1.5m), speed, heading, acceleration, yaw rate, path history for the past 300m, predicted path curvature, steering wheel angle, car length and width rounded to 20cm precision, and a few other indicators. Each message will also include a temporary vehicle id (randomly generated and changed every five minutes), to enable receivers to tell whether they are hearing from the same car or from different cars.

Under the proposal, each message will be digitally signed. Each car will be provisioned with 20 certificates (and corresponding secret keys) per week, and will cycle through these certificates during the week, using each one for five minutes at a time. Certificates will be revocable; revocation is meant to guard against incorrect (malicious or erroneous) information in the broadcast messages, though there is no concrete proposal for how to detect such incorrect information.

It is not hard to see that if such a system were to be deployed, a powerful antenna could easily listen to messages from well over the 300m design radius (we’ve seen examples of design range being extended by two or three orders of magnitude through the use of good antennas with bluetooth and wifi). Combining data from several antennas, one could easily link messages together, figuring out where each car was parked, what path it took, and where it ended up. This information will often enable one to link the car to an individual–for example, by looking at the address where the car is parked at night.

The fundamental privacy problem with the proposal is that messages can be linked together even though they have no long-term ids. The linking is simplest, of course, when the temporary id does not change, which makes it easy to track a car for five minutes. When the temporary id changes, two consecutive messages can be easily linked using the high-precision position information they contain. One also doesn’t have to observe the exact moment that the temporary id changes: it is possible to link messages by a variety of so-called “quasi-identifiers,” such as car dimensions; position in relation to other cars; the relationship between acceleration, steering wheel angle, and yaw, which will differ for different models; variability in how different models calculate path history; repeated certificates; etc. You can read more about various linking methods in our comments; and in comments by the EFF.

Thus, by using an antenna and a laptop, one could put a neighborhood under ubiquitous real-time surveillance — a boon to stalkers and burglars. Well-resourced companies, crime bosses, and government agencies could easily surveill movements of a large population in real time for pennies per car per year.

To our surprise, the NHTSA proposal did not consider the cost of lost privacy in its cost-benefit analysis; instead, it considered only “perceived” privacy loss as a cost. The adjective “perceived” in this context is a convenient way to dismiss privacy concerns as figments of imagination, despite the fact that NHTSA-commissioned analysis found that BSM-based tracking would be quite easy.

What about the safety benefits of proposed technology? Are they worth the privacy loss? As the EFF and Brad Templeton (among others) have argued, the proposed mandate will take away money from other safety technologies that are likely to have broader applications and raise fewer privacy concerns. The proposed technology is already becoming outdated, and will be even more out of date by the time it is deployed widely enough to make any difference.

But, you may object, isn’t vehicle privacy already dead? What about license plate scanners, cell-phone-based tracking, or aerial tracking from drones? Indeed, all of these technologies are a threat to vehicle privacy. None of them, however, permits tracking quite as cheaply, undetectably, and pervasively. For example, license-plate scanners require visual contact and are more conspicuous that a hidden radio antenna would be. A report commissioned by NHTSA concluded that other approaches did not seem practical for aggregate tracking.

Moreover, it is important to avoid the fallacy of relative privation: even if there are other ways of tracking cars today, we should not add one more, which will be mandated by the government for decades to come. To fix existing privacy problems, we can work on technical approaches for making cell phones harder to track or on regulatory restrictions on the use of license plate scanners. Instead of creating new privacy problems that will persist for decades, we should be working on reducing the ones that exist.