Road safety has been drawing increasing attention in the public, and has been subject to extensive efforts from both industry and academia in mitigating the impact of traffic accidents. Recent advances in wireless technology promise new approaches to facilitating road safety and traffic management, where each vehicle (or referred to as On-board unit (OBU)) is allowed to communicate with each other as well as with Roadside units (RSUs), which are located in some critical sections of the road, such as a traffic light, an intersection, and a stop sign. With the OBUs and RSUs, a self-organized network, called Vehicular Ad Hoc Network (VANET), can thus be formed. Unfortunately, VANETs have faced various security threats and privacy concerns, which would jeopardize the public safety and become the main barrier to the acceptance of such a new technology. Hence, addressing security and privacy issues is a prerequisite for a market-ready VANET. Although many studies have recently addressed a significant amount of efforts in solving the related problems, few of the studies has taken the scalability issues into consideration. When the traffic density is getting large, a vehicle may become unable to verify the authenticity of the messages sent by its neighbors in a timely manner, which may result in message loss so that public safety may be at risk. Communication overhead is another issue that has not been well addressed in previously reported studies. Many efforts have been made in recent years in achieving efficient broadcast source authentication and data integrity by using fast symmetric cryptography. However, the dynamic nature of VANETs makes it very challenging in the applicability of these symmetric cryptography-based protocols. In this research, we propose a novel Secure and Efficient RSU-aided Privacy Preservation Protocol, called SERP^3, in order to achieve efficient secure and privacy-preserving Inter-Vehicle Communications (IVCs). With the commitments of one-way key chains distributed to vehicles by RSUs, a vehicle can effectively authenticate any received message from vehicles nearby even in the presence of frequent change of its neighborship. Compared with previously reported public key infrastructure (PKI)-based packet authentication protocols for security and privacy, the proposed protocol not only retains the security and privacy preservation properties, but also has less packet loss ratio and lower communication overhead, especially when the road traffic is heavy. Therefore, the protocol solves the scalability and communication overhead issues, while maintaining acceptable packet latency. However, RSU may not exist in some situations, for example, in the early stage deployment phase of VANET, where unfortunately, SERP^3 is not suitable. Thus, we propose a complementary Efficient and Cooperative Message Validation Protocol, called ECMVP, where each vehicle probabilistically validates a certain percentage of its received messages based on its own computing capacity and then reports any invalid messages detected by it. Since the ultimate goal of designing VANET is to develop vehicle safety/non-safety related applications to improve road safety and facilitate traffic management, two vehicle applications are further proposed in the research to exploit the advantages of vehicular communications. First, a novel vehicle safety application for achieving a secure road traffic control system in VANETs is developed. The proposed application helps circumvent vehicles safely and securely through the areas in any abnormal situation, such as a car crash scene, while ensuring the security and privacy of the drivers from various threats. It not only enhances traveler safety but also minimizes capacity restrictions due to any unusual situation. Second, the dissertation investigates a novel mobile payment system for highway toll collection by way of vehicular communications, which addresses all the issues in the currently existing toll collection technologies.

Road crashes, occurring at a high annual rate for many years, demand improvements in transportation systems to provide a high level of on-road safety. Implanting smart sensors, communication capabilities, memory storage and information processing units in vehicles are important components of Intelligent Transportation Systems (ITS). ITS should enable the communication between vehicles and allow cooperative driving and early warnings of sudden breaks and accidents ahead. The prompt availability of the emergency information will provide the driver a time to react in order to avoid possible accidents ahead. Hence, information delivery delay is an importance quality-of-service (QoS) metric in such applications. In this thesis, we focus on modeling the delay for emergency messaging in vehicular ad hoc networks (VANETs). VANETs consist of nodes moving with very high speeds, resulting in frequent topological changes. As a result, many existing models and packet forwarding schemes designed for general purpose mobile ad hoc networks (MANETs) cannot be directly applied to VANETs. In our system model, we consider mobility and traffic density of vehicles. We focus on studying the effect of the traffic flow density on the delay of emergency message dissemination. Hence, traffic flow theories developed by civil engineers form the base of our modeling. The common way of emergency message dissemination in VANETs is broadcasting. To overcome the broadcasting storm problem and improve scalability of such large networks, we adopt a node cluster based broadcasting mechanism. This research provides a realistic mathematical model for the broadcasting delay, which accounts for the randomness in user mobility and matches the highly dynamic nature of VANETs. An investigation on the minimum cluster size that achieves acceptable message delivery latency is provided. It is shown that network control and performance parameters are dependent on the traffic density. Experimental measurement data are used to demonstrate the accuracy of the mathematical modeling.

Numerous local incidents occur on road networks daily many of which may lead to congestion and safety hazards. If vehicles can be provided with information about such incidents or traffic conditions in advance, the quality of driving in terms of time, distance, and safety can be improved significantly. Vehicular Ad Hoc Networks (VANETs) have recently emerged as an effective tool for improving road safety through the propagation of warning messages among the vehicles in the network about potential obstacles on the road ahead. This research has presented an effective warning data dissemination scheme which deploys relay strategy and concept of Region of Interest (RoI). A warning data message is characterized as spatio-temporal, implying that both the location and the time of an incident must be considered. Factors such as the type of warning message, the layout of the road network, the traffic density and the capacity of alternative roads are influential in determining the RoI in which the warning message needs to be propagated. In the developed scheme, the type of warning message is taken into account for the determination of the RoI so that the more severe the incident, the wider the RoI. In the selection of the relay point, the border relay area in which the relay point is placed, is adapted to the traffic density so that the higher the traffic density , the narrower the relay area. Traffic statistics are used to calculate the RoI, which is then enclosed in the warning message so that the message is not retransmitted beyond the RoI. Also, the responsibility for retransmitting the message is assigned to the relay node. The data is then disseminated effectively so that vehicles in areas unrelated to the incident are not informed. The primary objective of this research is to provide better understanding of the dissemination of warning data in the context of a vehicular network with the ultimate goal of increasing the possibility of using VANETs for safety applications.

Vehicular Ad-hoc Networks (VANETs) have emerged as a new application scenario that is envisioned to revolutionize the human driving experiences, optimize traffic flow control systems, etc. Addressing security and privacy issues as the prerequisite of VANETs' development must be emphasized. To avoid any possible malicious attack and resource abuse, employing a digital signature scheme is widely recognized as the most effective approach for VANETs to achieve authentication, integrity, and validity. However, when the number of signatures received by a vehicle becomes large, a scalability problem emerges immediately, where a vehicle could be difficult to sequentially verify each received signature within 100-300 ms interval in accordance with the current Dedicated Short Range Communications (DSRC) protocol. In addition, there are still some unsolved attacks in VANETs such as Denial of Service (Dos) attacks, which are not well addressed and waiting for us to solve. In this thesis, we propose the following solutions to address the above mentioned security related issues. First of all, to address the scalability issues, we introduce a novel roadside unit (RSU) aided message authentication scheme, named RAISE, which makes RSUs responsible for verifying the authenticity of messages sent from vehicles and for notifying the results back to vehicles. In addition, RAISE adopts the k-anonymity property for preserving user privacy, where a message cannot be associated with a common vehicle. Secondly, we further consider the situation that RSUs may not cover all the busy streets of a city or a highway in some situations, for example, at the beginning of a VANETs' deployment period, or due to the physical damage of some RSUs, or simply for economic considerations. Under these circumstances, we further propose an efficient identity-based batch signature verification scheme for vehicular communications. The proposed scheme can make vehicles verify a batch of signatures once instead of one after another, and thus it efficiently increases vehicles' message verification speed. In addition, our scheme achieves conditional privacy: a distinct pseudo identity is generated along with each message, and a trust authority can trace a vehicle's real identity from its pseudo identity. In order to find invalid signatures in a batch of signatures, we adopt group testing technique which can find invalid signatures efficiently. Lastly, we identify a DoS attack, called signature jamming attack (SJA), which could easily happen and possibly cause a profound vicious impact on the normal operations of a VANET, yet has not been well addressed in the literature. The SJA can be simply launched at an attacker by flooding a significant number of messages with invalid signatures that jam the surrounding vehicles and prevent them from timely verifying regular and legitimate messages. To countermeasure the SJA, we introduces a hash-based puzzle scheme, which serves as a light-weight filter for excluding likely false signatures before they go through relatively lengthy signature verification process. To further minimize the vicious effect of SJA, we introduce a hash recommendation mechanism, which enables vehicles to share their information so as to more efficiently thwart the SJA. For each research solution, detailed analysis in terms of computational time, and transmission overhead, privacy preservation are performed to validate the efficiency and effectiveness of the proposed schemes.

Improving road safety and traffic efficiency has been a long-term endeavor for the government, automobile industry and academia. Recently, the U.S. Federal Communication Commission (FCC) has allocated a 75 MHz spectrum at 5.9 GHz for vehicular communications, opening a new door to combat the road fatalities by letting vehicles communicate to each other on the roads. Those communicating vehicles form a huge Ad Hoc Network, namely Vehicular Ad Hoc Network (VANET). In VANETs, a variety of applications ranging from the safety related (e.g. emergence report, collision warning) to the non-safety related (e.g., delay tolerant network, infortainment sharing) are enabled by vehicle-to-vehicle (V-2-V) and vehicle-to-roadside (V-2-I) communications. However, the flourish of VANETs still hinges on fully understanding and managing the challenging issues over which the public show concern, particularly, security and privacy preservation issues. If the traffic related messages are not authenticated and integrity-protected in VANETs, a single bogus and/or malicious message can potentially incur a terrible traffic accident. In addition, considering VANET is usually implemented in civilian scenarios where locations of vehicles are closely related to drivers, VANET cannot be widely accepted by the public if VANET discloses the privacy information of the drivers, i.e., identity privacy and location privacy. Therefore, security and privacy preservation must be well addressed prior to its wide acceptance. Over the past years, much research has been done on considering VANET's unique characteristics and addressed some security and privacy issues in VANETs; however, little of it has taken the social characteristics of VANET into consideration. In VANETs, vehicles are usually driven in a city environment, and thus we can envision that the mobility of vehicles directly reflects drivers' social preferences and daily tasks, for example, the places where they usually go for shopping or work. Due to these human factors in VANETs, not only the safety related applications but also the non-safety related applications will have some social characteristics. In this thesis, we emphasize VANET's social characteristics and introduce the concept of vehicular social network (VSN), where both the safety and non-safety related applications in VANETs are influenced by human factors including human mobility, human self-interest status, and human preferences. In particular, we carry on research on vehicular delay tolerant networks and infotainment sharing --- two important non-safety related applications of VSN, and address the challenging security and privacy issues related to them. The main contributions are, i) taking the human mobility into consideration, we first propose a novel social based privacy-preserving packet forwarding protocol, called SPRING, for vehicular delay tolerant network, which is characterized by deploying roadside units (RSUs) at high social intersections to assist in packet forwarding. With the help of high-social RSUs, the probability of packet drop is dramatically reduced and as a result high reliability of packet forwarding in vehicular delay tolerant network can be achieved. In addition, the SPRING protocol also achieves conditional privacy preservation and resist most attacks facing vehicular delay tolerant network, such as packet analysis attack, packet tracing attack, and black (grey) hole attacks. Furthermore, based on the ``Sacrificing the Plum Tree for the Peach Tree" --- one of the Thirty-Six Strategies of Ancient China, we also propose a socialspot-based packet forwarding (SPF) protocol for protecting receiver-location privacy, and present an effective pseudonyms changing at social spots strategy, called PCS, to facilitate vehicles to achieve high-level location privacy in vehicular social network; ii) to protect the human factor --- interest preference privacy in vehicular social networks, we propose an efficient privacy-preserving protocol, called FLIP, for vehicles to find like-mined ones on the road, which allows two vehicles sharing the common interest to identify each other and establish a shared session key, and at the same time, protects their interest privacy (IP) from other vehicles who do not share the same interest on the road. To generalize the FLIP protocol, we also propose a lightweight privacy-preserving scalar product computation (PPSPC) protocol, which, compared with the previously reported PPSPC protocols, is more efficient in terms of computation and communication overheads; and iii) to deal with the human factor -- self-interest issue in vehicular delay tolerant network, we propose a practical incentive protocol, called Pi, to stimulate self-interest vehicles to cooperate in forwarding bundle packets. Through the adoption of the proper incentive policies, the proposed Pi protocol can not only improve the whole vehicle delay tolerant network's performance in terms of high delivery ratio and low average delay, but also achieve the fairness among vehicles. The research results of the thesis should be useful to the implementation of secure and privacy-preserving vehicular social networks.

Vehicular ad hoc networks (VANETs) are instances of mobile ad hoc networks with the aim to enhance the safety and efficiency of road traffic. The basic idea is to allow arbitrary vehicles to broadcast ad hoc messages (e.g. traffic accidents) to other vehicles and remind drivers to change their route immediately or slow down to avoid dangers. However, some concerns of security and privacy are also raised in this environment. Messages should be signed and verified before they are trusted while the real identities of vehicles should not be revealed to guarantee the source privacy, but it still has to be traceable to prevent any abuse of VANETs (e.g. sending a fake message). Many related works have been presented in the literature so far. They can be generally divided into two constructions, where one is based on pseudonymous authentication and the other is based on group signatures. However, both of the two constructions have some drawbacks. Consequently, in this thesis, we come up with a provably secure and strong privacy preserving protocol based on the blind signature technique to guarantee privacy and fulfill other essential security requirements in the vehicular communication environment. Besides, compared with other similar works, we offer an efficient tracing mechanism to trace and revoke the vehicles abusing the VANETs. In addition, considering the real environment, we also provide simulation results to show that our scheme is more practical, efficient and suitable for VANETs under a real city street scenario with high vehicle density. Finally, we also demonstrate the security of the proposed protocol by formal proofs.

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published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

Opportunistic communication leverages communication opportunities arising by chance to provide significant performance benefit and even enable communication where it would be impossible otherwise. The goal of this dissertation is to optimize opportunistic communication to achieve good performance in wireless networks. A key challenge in optimizing opportunistic communication arises from dynamic and incidental nature of communication. Complicated wireless interference patterns, high mobility, and frequent fluctuations in wireless medium make the optimization even harder. This dissertation proposes a series of optimization frameworks that systematically optimizes opportunistic communication to achieve good performance in wireless mesh networks and vehicular networks. We make the following three major contributions: First, we develop novel algorithms, techniques, and protocols that optimize opportunistic communication of wireless mesh network to achieve good, predictable user performance. Our framework systematically optimizes end-to-end performance (e.g., total throughput). It yields significant improvement over existing routing schemes. We also show that it is robust against inaccuracy introduced by dynamic network conditions. Second, we propose a novel overlay framework to exploit inter-flow network coding in opportunistic routing. In this framework, an overlay network performs inter-flow coding to effectively reduce traffic imposed on the underlay network, and an underlay network uses optimized opportunistic routing to provide efficient and reliable overlay links. We show that inter-flow coding together with opportunistic routing and rate-limiting brings significant performance benefit. Finally, we develop a novel optimization framework in vehicular networks to effectively leverage opportunistic contacts between vehicles and access points (APs). We develop a new mobility prediction algorithm and an optimization algorithm to determine an efficient replication scheme that exploit the synergy among Internet connectivity, local wireless connectivity, mesh network connectivity, and vehicular relay connectivity. Based on our framework, we develop a practical system that enables high-bandwidth content distribution and demonstrate the effectiveness of our approach using simulation, emulation, and testbed experiments. / text