Suhas Mathur

Radio-telepathy: extracting a secret key from an unauthenticated wireless channel

Securing communications requires the establishment of cryptographic keys, which is challenging in mobile scenarios where a key management infrastructure is not always present. In this paper, we present a protocol that allows two users to establish a common cryptographic key by exploiting special properties of the wireless channel: the underlying channel response between any two parties is unique and decorrelates rapidly in space. The established key can then be used to support security services (such as encryption) between two users. Our algorithm uses level-crossings and quantization to extract bits from correlated stochastic processes. The resulting protocol resists cryptanalysis by an eavesdropping adversary and a spoofing attack by an active adversary without requiring an authenticated channel, as is typically assumed in prior information-theoretic key establishment schemes. We evaluate our algorithm through theoretical and numerical studies, and provide validation through two complementary experimental studies. First, we use an 802.11 development platform with customized logic that extracts raw channel impulse response data from the preamble of a format-compliant 802.11a packet. We show that it is possible to practically achieve key establishment rates of ~ 1 bit/sec in a real, indoor wireless environment. To illustrate the generality of our method, we show that our approach is equally applicable to per-packet coarse signal strength measurements using off-the-shelf 802.11 hardware.

ParkNet: drive-by sensing of road-side parking statistics

Urban street-parking availability statistics are challenging to obtain in real-time but would greatly benefit society by reducing traffic congestion. In this paper we present the design, implementation and evaluation of ParkNet, a mobile system comprising vehicles that collect parking space occupancy information while driving by. Each ParkNet vehicle is equipped with a GPS receiver and a passenger-side-facing ultrasonic range-finder to determine parking spot occupancy. The data is aggregated at a central server, which builds a real-time map of parking availability and could provide this information to clients that query the system in search of parking. Creating a spot-accurate map of parking availability challenges GPS location accuracy limits. To address this need, we have devised an environmental fingerprinting approach to achieve improved location accuracy. Based on 500 miles of road-side parking data collected over 2 months, we found that parking spot counts are 95% accurate and occupancy maps can achieve over 90% accuracy. Finally, we quantify the amount of sensors needed to provide adequate coverage in a city. Using extensive GPS traces from over 500 San Francisco taxicabs, we show that if ParkNet were deployed in city taxicabs, the resulting mobile sensors would provide adequate coverage and be more cost-effective by an estimated factor of roughly 10-15 when compared to a sensor network with a dedicated sensor at every parking space, as is currently being tested in San Francisco.

Information-Theoretically Secret Key Generation for Fading Wireless Channels

The multipath-rich wireless environment associated with typical wireless usage scenarios is characterized by a fading channel response that is time-varying, location-sensitive, and uniquely shared by a given transmitter-receiver pair. The complexity associated with a richly scattering environment implies that the short-term fading process is inherently hard to predict and best modeled stochastically, with rapid decorrelation properties in space, time, and frequency. In this paper, we demonstrate how the channel state between a wireless transmitter and receiver can be used as the basis for building practical secret key generation protocols between two entities. We begin by presenting a scheme based on level crossings of the fading process, which is well-suited for the Rayleigh and Rician fading models associated with a richly scattering environment. Our level crossing algorithm is simple, and incorporates a self-authenticating mechanism to prevent adversarial manipulation of message exchanges during the protocol. Since the level crossing algorithm is best suited for fading processes that exhibit symmetry in their underlying distribution, we present a second and more powerful approach that is suited for more general channel state distributions. This second approach is motivated by observations from quantizing jointly Gaussian processes, but exploits empirical measurements to set quantization boundaries and a heuristic log likelihood ratio estimate to achieve an improved secret key generation rate. We validate both proposed protocols through experimentations using a customized 802.11a platform, and show for the typical WiFi channel that reliable secret key establishment can be accomplished at rates on the order of 10 b/s.

Coalitions in Cooperative Wireless Networks

Cooperation between rational users in wireless networks is studied using coalitional game theory. Using the rate achieved by a user as its utility, it is shown that the stable coalition structure, i.e., set of coalitions from which users have no incentives to defect, depends on the manner in which the rate gains are apportioned among the cooperating users. Specifically, the stability of the grand coalition (GC), i.e., the coalition of all users, is studied. Transmitter and receiver cooperation in an interference channel (IC) are studied as illustrative cooperative models to determine the stable coalitions for both flexible (transferable) and fixed (non-transferable) apportioning schemes. It is shown that the stable sum-rate optimal coalition when only receivers cooperate by jointly decoding (transferable) is the GC. The stability of the GC depends on the detector when receivers cooperate using linear multiuser detectors (non-transferable). Transmitter cooperation is studied assuming that all receivers cooperate perfectly and that users outside a coalition act as jammers. The stability of the GC is studied for both the case of perfectly cooperating transmitters (transferrable) and under a partial decode-and-forward strategy (non-transferable). In both cases, the stability is shown to depend on the channel gains and the transmitter jamming strengths.

Coalitional Games in Gaussian Interference Channels

The formation of coalitions in a Gaussian interference channel where the receivers are allowed to cooperate is studied under the framework of coalitional game theory. Allowing any arbitrary sharing of the total rate achieved by a coalition between its member links, it is shown that the grand coalition (coalition of all links) maximizes spectrum utilization and is also stable, that is, the links in this coalition have no incentives to leave and form other coalitions. The issue of fairness in allocating rates to members of a grand coalition is addressed via a Nash bargaining solution where each link utility is modeled as the rate gained by being in a coalition relative to the rate achieved in the interference channel. Further, a rate allocation solution using proportional fairness is also presented and the results are illustrated with examples

ParkNet: a mobile sensor network for harvesting real time vehicular parking information

This paper describes the architecture and design of ParkNet, a mobile sensor network consisting of vehicles, which collects and disseminates real-time information about the availability of parking spaces in urban areas. We outline the broad challenges in real-time data collection and consumption by a mobile sensor network, as well as the design issues specific to ParkNet. Sensor nodes in ParkNet are a combination of privately owned and city owned vehicles and employ low cost ultrasonic sensors to detect the presence of vacant parking spots as they drive by. We present early results on the performance of our sensor platform.

Coalitional Games in Receiver Cooperation for Spectrum Sharing

The issue of sharing spectrum through receiver cooperation in wireless networks is studied under the framework of coalitional game theory. In particular, we consider two illustrative network models: (1) a Gaussian interference channel with receiver cooperation and (2) a multiple access channel (MAC) with linear multiuser detection. Allowing any arbitrary sharing of the sum-rate achieved by a coalition between member links in a Gaussian interference channel, it is shown that the grand coalition (coalition of all receivers) maximizes spectrum utilization and is also stable. For the linear MMSE multiuser detector, it is shown that the grand coalition is always stable and sum-rate maximizing, while for the decorrelating multiuser detector, the above observation is shown to be true only in the high SNR regime. Finally, transmitter cooperation in the context of a Gaussian interference channel is discussed, with focus on some open problems and a simplified framework for the resulting coalitional game is proposed.

Coalitional Games in Cooperative Radio Networks

The framework of coalitional game theory is used to study the formation of coalitions in an M-link wireless interference channel when either the transmitters or the receivers cooperate. It is shown that the stable coalition structure, i.e., the coalition structure for which users have no incentives to defect, depends upon the apportioning scheme chosen to distribute the cooperative rate gains between the coalition members. Under both a flexible (transferable) and fixed (non-transferable) apportioning scheme, the stable coalitions formed by cooperating receivers are presented. The problem of determining stable coalitions for the case of cooperating transmitters is discussed.

Towards large-scale mobile network emulation through spatial switching on a wireless grid

Experimentation with large mobile networks is notoriously tedious and expensive. We present the architecture and work-in-progress implementation of the m-ORBIT testbed, a mobility emulator using spatial switching, which facilitates mobile system experiments with 802.11a/b/g wireless network interfaces. The emulator does not require any physically moving parts---it emulates mobility by switching over an array of 128 spatially distributed radios. Instead of using hardware antenna switches, we implement spatial switching in software over Gigabit Ethernet links to the radio nodes. Preliminary results support the scaling of this approach to a large number of radios at relatively low cost. Packet error rate measurements also indicate that an experimenter can create multi-hop topologies by injecting additive white Gaussian noise into the environment. We demonstrate through an Ad hoc On Demand Distance Vector routing case study how this emulator enables mobile systems experiments and plan to make the emulator available for remote access by the research community.

BIT-TRAPS: Building Information-Theoretic Traffic Privacy Into Packet Streams

Sniffing encrypted data packets traveling across networks can often be useful in inferring nontrivial information about their contents because of the manner in which the transmission of such packets is handled by lower layers in the communications protocol stack. In this paper, we formally study the side-channel formed by variable packet sizes, and explore obfuscation approaches to prevent information leakage while jointly considering the practical cost of obfuscation. We show that randomized algorithms for obfuscation perform best and can be studied as well-known information-theoretic constructs, such as discrete channels with and without memory. We envision a separate layer called a Bit - Trap, that employs buffering and bit-padding as orthogonal methods for obfuscating such side channels. For streams of packets, we introduce the use of mutual-information rate as an appropriate metric for the level of obfuscation that captures nonlinear relationships between original and modified streams. Using buffering-delay and average bit-padding as the respective costs, a Bit - Trap formulates a constrained optimization problem with bounds on the average costs, to implement the best possible obfuscation policy. We find that combining small amounts of delay and padding together can create much more obfuscation than either approach alone, and that a simple convex trade-off exists between buffering delay and padding for a given level of obfuscation.