Abstract
The existing authentication protocols to securevehicular
ad hoc networks (VANETs) raise challenges such as certificate
distribution and revocation, avoidance of computation and
communication bottlenecks, and reduction of the strong reliance
on tamper-proof devices. This paper efficiently cope with these
challenges with a decentralized group authentication protocol
in the sense that the group is maintained by each RSU rather
than by a centralized authority as in most existing protocols
employing group signatures. In our proposal, we employ each
roadside unit (RSU) to maintain and manage an on-the-fly group
within its communication range. Vehicles entering the group can
anonymously broadcast vehicle-to-vehicle (V2V) messages, which
can be instantly verified by the vehicles in the same group (and
neighbor groups). Later, if the message is found to be false, a
third party can be invoked to disclose the identity of the message
originator. Our protocol efficiently exploits the specific features
of vehicular mobility, physical road limitations and properly
distributed RSUs. Our design leads to a robust VANET since,
if some RSUs occasionally collapse, only the vehicles driving in
those collapsed areas will be affected. Due to the numerous RSUs
sharing the load to maintain the system, performance does not
significantly degrade when more vehicles join the VANET; hence,
the system is scalable.
Index Terms: Information security; Vehicular ad hoc networks;
Protocol design; Conditional privacy
I. INTRODUCTION
Vehicular ad hoc networks (VANETs) are an instance of
mobile ad hoc networks with the aim to enhance the safety
and efficiency of road traffic. VANETs have a number of distinguishing
features and limitations related to the very nature
of wireless communications and the rapid movement of the
vehicles involved in those communications. Compared with
wired or other wireless networks, VANETs are very dynamic
and their communications are volatile. In such networks, nodes
are vehicles equipped with communication devices known as
on-board units (OBUs) and, depending on the applications,
OBUs are used to establish communications with other vehicles
or roadside units (RSUs) such as traffic lights or traffic
signs.
The specific properties of VANETs allow the development
of very attractive services such as the so-called comfort
services that include traffic information, weather information,
location of gas stations or restaurants, price information, and
interactive communication such as Internet access. Also, it is
possible to offer safety services such as emergency warnings,
lane-changing assistance, intersection coordination, traffic sign
violation warnings, and road-condition warnings [1]. However,
for those new services to make life easier rather than more
difficult, they should rely on secure and privacy-preserving
protocols that encourage users to participate without fear for
their safety or personal privacy.
Consequently, security and privacy are two critical concerns
for the designers of VANETs that, if forgotten, might lead to
the deployment of vulnerable VANETs. Unless proper measures
are taken, a number of attacks could easily be conducted,
namely message content modification, identity theft, false
information generation and propagation, etc. The following
are examples of some specific attacks:
² If message integrity is not guaranteed, a malicious vehicle
could modify the content of a message sent by another
vehicle to affect the behavior of other vehicles. By doing
so, the malicious vehicle could obtain many benefits
while keeping its identity unknown. Moreover, the vehicle
that originally generated the message would be made
responsible for the damage caused.
² If authentication is not provided, a malicious vehicle
might impersonate an emergency vehicle to surpass speed
limits without being sanctioned.
² A malicious vehicle could report a false emergency
situation to obtain better driving conditions (e.g. deserted
roads) and, if non-repudiation is not supported, it could
not be sanctioned even if discovered.
From the previous examples, it becomes apparent that message
authentication, integrity, and non-repudiation are primary
requirements in VANETs. There is a need for mechanisms
that provide VANETs with security, i.e., protocols, methods
and procedures that are able to: detect whether a message has
been modified by an attacker, determine who is the real sender
of a message, and avoid identity theft.

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