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Jin, Hongyu, Papadimitratos, Panos.  2017.  Resilient Privacy Protection for Location-Based Services Through Decentralization. Proceedings of the 10th ACM Conference on Security and Privacy in Wireless and Mobile Networks. :253–258.

Location-based Services (LBSs) provide valuable features but can also reveal sensitive user information. Decentralized privacy protection removes the need for a so-called anonymizer, but relying on peers is a double-edged sword: adversaries could mislead with fictitious responses or even collude to compromise their peers' privacy. We address here exactly this problem: we strengthen the decentralized LBS privacy approach, securing peer-to-peer (P2P) interactions. Our scheme can provide precise timely P2P responses by passing proactively cached Point of Interest (POI) information. It reduces the exposure both to the honest-but-curious LBS servers and peer nodes. Our scheme allows P2P responses to be validated with very low fraction of queries affected even if a significant fraction of nodes are compromised. The exposure can be kept very low even if the LBS server or a large set of colluding curious nodes collude with curious identity management entities.

Khodaei, Mohammad, Papadimitratos, Panos.  2016.  Evaluating On-demand Pseudonym Acquisition Policies in Vehicular Communication Systems. Proceedings of the First International Workshop on Internet of Vehicles and Vehicles of Internet. :7–12.

Standardization and harmonization efforts have reached a consensus towards using a special-purpose Vehicular Public-Key Infrastructure (VPKI) in upcoming Vehicular Communication (VC) systems. However, there are still several technical challenges with no conclusive answers; one such an important yet open challenge is the acquisition of short-term credentials, pseudonym: how should each vehicle interact with the VPKI, e.g., how frequently and for how long? Should each vehicle itself determine the pseudonym lifetime? Answering these questions is far from trivial. Each choice can affect both the user privacy and the system performance and possibly, as a result, its security. In this paper, we make a novel systematic effort to address this multifaceted question. We craft three generally applicable policies and experimentally evaluate the VPKI system performance, leveraging two large-scale mobility datasets. We consider the most promising, in terms of efficiency, pseudonym acquisition policies; we find that within this class of policies, the most promising policy in terms of privacy protection can be supported with moderate overhead. Moreover, in all cases, this work is the first to provide tangible evidence that the state-of-the-art VPKI can serve sizable areas or domain with modest computing resources.

Khodaei, Mohammad, Noroozi, Hamid, Papadimitratos, Panos.  2018.  Privacy Preservation Through Uniformity. Proceedings of the 11th ACM Conference on Security & Privacy in Wireless and Mobile Networks. :279–280.

Inter-vehicle communications disclose rich information about vehicle whereabouts. Pseudonymous authentication secures communication while enhancing user privacy thanks to a set of anonymized certificates, termed pseudonyms. Vehicles switch the pseudonyms (and the corresponding private key) frequently; we term this pseudonym transition process. However, exactly because vehicles can in principle change their pseudonyms asynchronously, an adversary that eavesdrops (pseudonymously) signed messages, could link pseudonyms based on the times of pseudonym transition processes. In this poster, we show how one can link pseudonyms of a given vehicle by simply looking at the timing information of pseudonym transition processes. We also propose "mix-zone everywhere": time-aligned pseudonyms are issued for all vehicles to facilitate synchronous pseudonym update; as a result, all vehicles update their pseudonyms simultaneously, thus achieving higher user privacy protection.

Noroozi, Hamid, Khodaei, Mohammad, Papadimitratos, Panos.  2018.  VPKIaaS: A Highly-Available and Dynamically-Scalable Vehicular Public-Key Infrastructure. Proceedings of the 11th ACM Conference on Security & Privacy in Wireless and Mobile Networks. :302–304.
The central building block of secure and privacy-preserving Vehicular Communication (VC) systems is a Vehicular Public-Key Infrastructure (VPKI), which provides vehicles with multiple anonymized credentials, termed pseudonyms. These pseudonyms are used to ensure message authenticity and integrity while preserving vehicle (and thus passenger) privacy. In the light of emerging large-scale multi-domain VC environments, the efficiency of the VPKI and, more broadly, its scalability are paramount. In this extended abstract, we leverage the state-of-the-art VPKI system and enhance its functionality towards a highly-available and dynamically-scalable design; this ensures that the system remains operational in the presence of benign failures or any resource depletion attack, and that it dynamically scales out, or possibly scales in, according to the requests' arrival rate. Our full-blown implementation on the Google Cloud Platform shows that deploying a VPKI for a large-scale scenario can be cost-effective, while efficiently issuing pseudonyms for the requesters.
Vaas, Christian, Papadimitratos, Panos, Martinovic, Ivan.  2018.  Increasing Mix-Zone Efficacy for Pseudonym Change in VANETs Using Chaff Messages. Proceedings of the 11th ACM Conference on Security & Privacy in Wireless and Mobile Networks. :287–288.
Vehicular ad-hoc networks (VANETs) are designed to play a key role in the development of future transportation systems. Although cooperative awareness messages provide the required situational awareness for new safety and efficiency applications, they also introduce a new attack vector to compromise privacy. The use of ephemeral credentials called pseudonyms for privacy protection was proposed while ensuring the required security properties. In order to prevent an attacker from linking old to new pseudonyms, mix-zones provide a region in which vehicles can covertly change their signing material. In this poster, we extend the idea of mix-zones to mitigate pseudonym linking attacks with a mechanism inspired by chaff-based privacy defense techniques for mix-networks. By providing chaff trajectories, our system restores the efficacy of mix-zones to compensate for a lack of vehicles available to participate in the mixing procedure. Our simulation results of a realistic traffic scenario show that a significant improvement is possible.