=Paper=
{{Paper
|id=Vol-397/paper-2
|storemode=property
|title=Invited talk extended abstract: Safety and Privacy in Vehicular Communications
|pdfUrl=https://ceur-ws.org/Vol-397/paper1.pdf
|volume=Vol-397
|dblpUrl=https://dblp.org/rec/conf/esorics/Domingo-FerrerW08
}}
==Invited talk extended abstract: Safety and Privacy in Vehicular Communications==
https://ceur-ws.org/Vol-397/paper1.pdf
Safety and Privacy in Vehicular Communications
Josep Domingo-Ferrer and Qianhong Wu
Universitat Rovira i Virgili, UNESCO Chair in Data Privacy, Dept. of Computer
Engineering and Mathematics, Av. Paı̈sos Catalans 26, E-43007 Tarragona, Catalonia
e-mail {josep.domingo,qianhong.wu}@urv.cat
Abstract. Vehicular ad hoc networks (VANETs) allow vehicles to dis-
seminate messages about road conditions to other vehicles. As long as
these messages are trustworthy, they can greatly increase traffic safety
and efficiency. Hence, care must be exerted to ensure that vehicle-generated
messages do not convey inaccurate or false content. A natural way to pro-
ceed is to request endorsement by nearby vehicles on the content of a
message originated by a certain vehicle. However, such a message gen-
eration and peer-to-peer endorsement should not result in any privacy
loss on the part of vehicles co-operating in it. We survey the available
solutions to this security-privacy tension and discuss their limitations.
We sketch a new privacy-preserving system which guarantees message
authentication through both a priori and a posteriori countermeasures.
1 Introduction
VANETs allow vehicles to broadcast messages to other vehicles in the
vicinity. It is suggested that each vehicle periodically send messages over
a single hop every 300ms within a distance of 10s travel time (which is a
distance range between 10 m and 300 m)[RH05]. This mechanism can be
used to improve safety and optimize traffic. However, malicious vehicles
can also make use of this mechanism by sending fraudulent messages for
their own profit or just to jeopardize the traffic system. Hence, the system
must be designed to ensure that the transmission comes from a trusted
source and has not been tampered with since transmission.
Another critical concern in VANETs is driving privacy or vehicle
anonymity. As noted in [Dot06], a lot can be inferred on the driver’s pri-
vacy if the whereabouts and the driving pattern of a car can be tracked.
However, it is possible for attackers to trace vehicles by using cameras or
physical tracking. But such physical attacks can only trace specific tar-
gets and are much more expensive than monitoring the communication
in VANETs. This paper addresses the latter attacks.
2 Countermeasures for securing VANETs
VANETs function to improve safety only if the messages sent by vehi-
cles are trustworthy. Dealing with fraudulent messages is a thorny issue
for safety engineers due to the self-organized property of VANETs. The
situation is deteriorated by the privacy requirements of vehicles since,
in a privacy-preserving setting, the message generators, i.e. the vehicles,
are anonymous in the sense that their identities are unknown. A num-
ber of schemes have been proposed to reduce fraudulent messages; such
proposals fall into two classes, namely a posteriori and a priori.
2.1 A posteriori countermeasures
A posteriori countermeasures consist of taking punitive action against
vehicles who have been proven to have originated fraudulent messages.
To be compatible with privacy preservation, these countermeasures re-
quire the presence of a trusted third party able to open the identities
of dishonest vehicles. Then the revoked vehicles can be expelled from
the system. Cryptographic authentication technologies have been exten-
sively exploited to offer a posteriori countermeasures. Some proposals use
regular digital signatures [RPH06,RH07,RPAJ07,AFWZ07]. In these pro-
posals, vehicle privacy is provided by a pseudonym mechanism, in which
certificate authorities (CAs) produce many pseudonyms for each vehi-
cle so that attackers cannot trace the vehicles producing signatures in
different periods with different pseudonyms, except if the CAs open the
identities of the vehicles. The pseudonym mechanism is not that efficient
due to the heavy overhead of pseudonym generation and storage. Other
schemes use sophisticated cryptographic technologies such as group signa-
tures [GBW07] or ring signatures [LSHS07,GGT06]. The latter methods
are more efficient, but those using ring signatures cannot trace malicious
vehicles due to the unconditional anonymity of ring signatures. Along this
research line, the scheme in [GBW07] seems the most efficient one that
can provide revokable anonymity.
2.2 A priori countermeasures
A priori countermeasures attempt to prevent the generation of fraud-
ulent messages. This approach is based on the assumption that most
users are honest and will not endorse any message containing false data.
Another implicit assumption is the usual common sense that, the more
people endorse a message, the more trustworthy it is. Along this research
line, the schemes in [GGS04,ODS07,PP05,RAH06,DDSV08] exploit the
assumption that there is a majority of honest vehicles in VANETs. Hence,
these schemes introduce some form of threshold mechanism: a message is
trusted if it has been verifiably endorsed by a number of vehicles above a
certain threshold. Among these schemes, the proposals in [DDSV08] may
be the most efficient while enabling anonymity of message originators.
But their scheme does not provide anonymity revocability, which may
not suit some applications in which anonymity must be revoked “for the
prevention, investigation, detection and prosecution of serious criminal
offences”[EP05].
3 Discussion
Unfortunately, neither a posteriori nor a priori countermeasures are solely
sufficient to secure VANETs. By taking strict punitive action, a posteri-
ori countermeasures can exclude some rational attackers producing bo-
gus messages to obtain benefits or pranks. However, they are ineffective
against irrational attackers such as terrorists. Even for rational attack-
ers, damage has already occurred when punitive action is taken. It seems
that a priori countermeasures function better in this case because they
prevent damage beforehand by letting the vehicles trust only messages
endorsed by a number of vehicles. Although the underlying assumption
that there is a majority of honest vehicles in VANETs generally holds, it
cannot be excluded that a number of malicious vehicles greater than or
equal to the threshold are present in specific locations, for instance. For
example, this is very plausible if some criminal organization undertakes
to divert traffic from a certain area by broadcasting messages informing
that a road is barred. Furthermore, for convenience in implementation,
existing schemes use an even stronger assumption that the number of
honest vehicles in all cases should be at least a preset threshold. But such
a universally valid threshold does not exist in practice. Indeed, the thresh-
old should somehow take the traffic density and the message scope into
account: a low density of vehicles calls for a lower threshold, whereas a
high density and a message relevant to the whole traffic of a city requires
a sufficiently high threshold.
The situation is aggravated by the anonymity technologies used some
proposals. A system preserves anonymity when it does not require the
identity of its users to be disclosed. Without anonymity, attackers can
trace all the vehicles by monitoring the communication in VANETs, which
in turn can enable the attackers to mount serious attacks against specific
targets. Hence, anonymity is a critical concern in VANETs. However,
anonymity can also weaken a posteriori and a priori countermeasures.
Indeed, attackers can send fraudulent messages without fear of being
caught due to anonymity, and as a result, no punitive action can be taken
against them. Furthermore, some proposals provide strong anonymity,
i.e. unlinkability. Unlinkability implies that a verifier cannot distinguish
whether two signatures come from the same vehicle or two vehicles. This
feature may enable malicious vehicles to mount the so-called Sybil attack:
a vehicle generates a fraudulent message and then endorses the message
herself by computing on it as many signatures as required by the thresh-
old in use; since signatures are unlinkable, no one can find out that all
of them come from the same vehicle. Hence, elegantly designed protocols
are required to secure VANETs when incorporating anonymity.
4 Towards a combination of a priori and a posteriori
countermeasures
Bearing in mind that enhancing safety and traffic efficiency is one of
the main thrusts behind VANETs, we propose a new efficient system
to balance public safety and vehicle privacy. Both a priori and a pos-
teriori countermeasures are resorted to in order to thwart attackers. To
the best of our knowledge, ours is the first system equipped with both
types of countermeasures. We achieve this goal by drawing on the novel
technology of message-linkable group signatures (MLGS). In an MLGS
scheme, a vehicle stays anonymous if it produces two signatures on two
different messages. However, if it produces two signatures on the same
message, then it will be identified, which effectively thwarts the Sybil at-
tack in a privacy-preserving system. This novel technology also enables us
to realize a threshold-adaptive authentication in which the threshold can
adaptively change in light of the context of messages, instead of having to
be preset during the system design stage. Furthermore, a fast batch ver-
ification method is presented to speed up the validation of authenticated
messages. Since vehicles periodically receive a large number of messages
to be validated, such a batch verification is critical to make authentica-
tion implementable in VANETs. Details on the new scheme will be given
in [WD08].
Acknowledgments and disclaimer
This work was partly supported by the Spanish Government through
projects CONSOLIDER INGENIO 2010 CSD2007-00004 “ARES” and
TSI2007-65406-C03-01 “E-AEGIS”, and by the Government of Catalonia
under grant 2005 SGR 00446. The authors are with the UNESCO Chair
in Data Privacy, but their views do not necessarily reflect the position of
UNESCO nor commit that organization.
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