Mechanism for Privacy Preservation in VANETS

Introduction

A vehicular ad-hoc network (VANET) is a network of vehicles supported by fixed infrastructure. The vehicular networks are characterized by a highly dynamic topology with vehicles moving in restricted geographical strait jackets (roads). The vehicles exchange information via vehicle to vehicle and vehicle to road side infrastructure in both manners. The road side infrastructure acts as access points in vehicular communication are known as road side unit (RSU). A vehicle is equipped with on board unit (OBU). RSU are located along the roads at certain points. A bandwidth of 75 MHz has been allocated in the 5.850-5.925 GHz band for communication in such networks (Raya & Hubaux, 2005). In a VANET, an adversary can find the identity of the vehicle from message contents and to some extent its position through localization based on signal strength etc. (Raya & Hubaux, 2007; Dotzer, 2005). Over a time period, the physical vehicle and its communication identity can be related (Raya & Hubaux, 2005; Papadimitratos et al., 2007) to breach the location privacy of the user. This link can be used to disclose personal data of a user and would potentially dissuade a user from joining a VANET (Raya & Hubaux, 2007).

To obtain and sustain anonymity, a temporal identity, pseudonym, is used for communication. Pseudonyms allow a vehicle to interact with other vehicles anonymously. Pseudonyms are ephemeral and distinct pseudonyms hide their relation from each other and to the user's identity (Pfitzmann & Hansen, 2004). To preserve privacy, a pseudonym system must prevent credential forgeability and disallow usage of false pseudonym by a user. Moreover, the transaction of obtaining and the process of switching pseudonyms should not reveal the identity of the user or link pseudonyms to each other. Continually changing pseudonyms conceal the real identity of a vehicle by de-linking the source of signals to its original identity (Gerlach & Guttler, 2007). But, the relation between a communicating vehicle and its estimated location can reveal the identity of a vehicle. This vehicle can, then, be physically traced and switching pseudonyms would be meaningless (Sha et al., 2006). A vehicle can be under sustained observation and transmissions at different intervals of time with the same pseudonym can reveal the relation between physical vehicle and its current pseudonym if the vehicle is relatively isolated in a crowd. This relation can be established even when pseudonyms are updated when the time interval between transmission prior to and after the update is short (Sampigethaya et al., 2007). There is, moreover, one more challenge that needs to be addressed. When a vehicle under observation moves from one cluster and enters another cluster and changes its pseudonym, it can be spotted with high probability as soon as it transmits. This can happen if the number of vehicles from the previous cluster to the current cluster is small and the pseudonyms of vehicles belonging to current cluster are known a priori. The anonymity of the vehicle under observation is limited by the number of vehicles that join the current cluster from the previous cluster Sun (Zhang & Fang, 2007; Fonseca, 2007).