Co-location and location privacy (PETS2014 in real time)

16 July 2014

The paper “Quantifying the Effect of Co-location Information on Location Privacy” presented by Alexandra-Mihaela Olteanu discusses the important problem of co-location on location privacy in mobile networks.

The work notes that co-location information is widespread: for example users tag a number of people on pictures in social networks; face recognition is getting better; and devices record the devices around them. Interestingly, the information an adversary may infer from this co-location information is not hidden by traditional location privacy mechanisms. In fact this is an instance of privacy technologies not doing a very good job at hiding information leaking from interactions between users. When correlations between user activity is observable by an adversary, the adversary may combine the information from both to increase the accuracy of their inferences. In fact the actions of one user may have serious consequences on the privacy of another (a co-target).

This work attempts to quantify the degree of privacy lost from such combined inferences. It uses the location privacy model by Shokri et al (S&P). Users locations and traces are modeled using a Markov model and the adversary observes co-locations. Tracking a single user is the traditional inferences of the hidden Markov model. With co-locations the attack is more complex since the constraints implied by co-locations need to be honored. This increases the information of the adversary, but also increases the computational complexity of the attack. As a result the authors also propose a heuristic attack, that ignores some of the information / constraints but tries to keep a certain set of observations consistent.

The evaluation was performed on the GeoLife (MSR Asia) dataset. Interestingly co-location information can be inferred from such raw traces, to simulate a number of scenarios. For example one may vary the fraction of co-location events observed by the adversary or the location privacy mechanism used. The result show, unsurprisingly, that the adversary observing co-locations has a significant impact on privacy. They also show that observing a target with low privacy settings may lead to the compromise of a co-target with high privacy settings.

This is a very nice work. As someone in the audience pointed out, some of the computational complexities may be tackled through using sampling based estimations of the posterior distribution that is otherwise intractable. This is probably a worthy space for follow-up research. Relaxing the Markov mobility assumption (which makes the posterior computations even more complex) is another avenue for future work.

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