In vehicular communication systems, cooperative awareness messages provide contextual information required for transportation safety and efficiency applications. However, without the appropriate design, these messages introduce a new attack vector to compromise passenger privacy. The use of ephemeral credentialspseudonyms-was therefore proposed, essentially to split a journey into unlinkable segments. To protect segment transitions, encrypted mix-zones provide regions where vehicles can covertly change their pseudonyms. While previous work focused on the placement, shape, and protocols for mixzones, attacks that correlate vehicles entering and existing these zones still remain a problem. Furthermore, existing schemes have only considered homogeneous traffic, disregarding variations in vehicle density due to differences in driver population, road layout, and time of day. Without realistic experimental results, any conclusion on real-world applicability is precarious. In this paper, we address this challenge and present a novel scheme that works independent of vehicles' mobility patterns. More precisely, our system generates fictive chaff vehicles when needed and broadcasts their traces, while it remains unobtrusive if sufficiently many vehicles are present. This greatly improves privacy protection in situations with inherently low traffic density, e.g., suburban areas, and during low traffic periods. Our scheme ensure that an external attacker cannot distinguish between real and chaff vehicles, while legitimate vehicles can recognize chaff messages; this is important, because chaff vehicles (and messages) must not affect the operation of safety applications. In our evaluation, we compare our chaff-based approach with an existing cryptographic mix-zone scheme. Our results under realistic traffic conditions show that by introducing fictive vehicles, traffic flow variations can be smoothed and privacy protection can be enhanced up to 76%.

Transparent authentication (TA) schemes are those in which a user is authenticated by a verifier without requiring explicit user interaction. By doing so, those schemes promise high usability and security simultaneously. The majority of TA implementations rely on the received signal strength as an indicator for the proximity of a user device (prover). However, such implicit proximity verification is not secure against an adversary who can relay messages over a larger distance.In this paper, we propose a novel approach for thwarting relay attacks in TA schemes: the prover permits access to authentication credentials only if it can confirm that it is near the verifier. We present STASH, a system for relay-resilient transparent authentication in which the prover does proximity verification by comparing its approach trajectory towards the intended verifier with known authorized reference trajectories. Trajectories are measured using low-cost sensors commonly available on personal devices. We demonstrate the security of STASH against a class of adversaries and its ease-of-use by analyzing empirical data, collected using a STASH prototype. STASH is efficient and can be easily integrated to complement existing TA schemes.

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