Cagatay Capar

Secret communication in large wireless networks without eavesdropper location information

We present achievable scaling results on the per-node secure throughput that can be realized in a large random wireless network of n legitimate nodes in the presence of m eavesdroppers of unknown location. We consider both one-dimensional and two-dimensional networks. In the one-dimensional case, we show that a per-node secure throughput of order 1/n is achievable if the number of eavesdroppers satisfies m = o(n/log n). We obtain similar results for the two-dimensional case, where a secure throughput of order 1/(√n log n) is achievable under the same condition. The number of eavesdroppers that can be tolerated is significantly higher than previous works that address the case of unknown eavesdropper locations. The key technique introduced in our construction to handle unknown eavesdropper locations forces adversaries to intercept a number of packets to be able to decode a single message. The whole network is divided into regions, where a certain subset of packets is protected from adversaries located in each region. In the one-dimensional case, our construction makes use of artificial noise generation by legitimate nodes to degrade the signal quality at the potential locations of eavesdroppers. In the two-dimensional case, the availability of many paths to reach a destination is utilized to handle collaborating eavesdroppers of unknown location.

Network coding for facilitating secrecy in large wireless networks

It is well known that network coding can improve the secrecy of a network that is represented by a graph with edges tapped by an eavesdropper. However, the application of such secure network coding to emerging problems in information-theoretic secrecy in wireless networks has been limited; in particular, the graph-based wiretap network approach does not readily extend to the physical wireless network. In this paper, we consider the impact of such an extension on scaling laws for wireless networks. We first employ simple examples to illustrate how an extension of the secure network coding approach can address difficult problems that have plagued wireless network security; for example, relaxing the known eavesdropper location assumption or avoiding the use of artificial noise generation. Based on this understanding, we then add secure network coding approaches to recent constructions for secrecy scaling in large wireless networks, most notably achieving secure pernode throughput in large wireless networks in the presence of an arbitrary number of non-collaborating eavesdroppers.

Broadcast Analysis for Extended Cooperative Wireless Networks

The capability of nodes to broadcast their message to the entire wireless network when nodes employ cooperation is considered. We employ an asymptotic analysis using an extended random network setting under an additive white Gaussian channel model with path loss, and show that the broadcast performance strongly depends on the path loss exponent of the medium. In particular, the probability of broadcasting in a 1-D infinite network is zero for path loss exponents larger than one, and is equal to a nonzero value for path loss exponents less than one. In 2-D infinite networks, the same behavior is observed for path loss exponents above and below two, respectively.

Signal-flow-based analysis of wireless security protocols

Abstract Security protocols operating over wireless channels can incur significant communication costs (e.g., energy, delay), especially under adversarial attacks unique to the wireless environment such as signal jamming, fake signal transmission, etc. Since wireless devices are resource constrained, it is important to optimize security protocols for wireless environments by taking into account their communication costs. Towards this goal, we first present a novel application of a signal-flow-based approach to analyze the communication costs of security protocols in the presence of adversaries. Our approach models a protocol run as a dynamic probabilistic system and then utilizes Linear System theory to evaluate the moment generating function of the end-to-end cost. Applying this technique to the problem of secret key exchange over a wireless channel, we quantify the efficiency of existing families of key exchange cryptographic protocols, showing, for example, that an ID-based approach can offer an almost 10-fold improvement in energy consumption when compared to a traditional PKI-based protocol. We then present a new key exchange protocol that combines traditional cryptographic methods with physical-layer techniques, including the use of “ephemeral” spreading codes, cooperative jamming, and role-switching. Utilizing signal flow analysis, we demonstrate that this new protocol offers performance advantages over traditional designs.

Cooperative jamming to improve the connectivity of the 1-D secrecy graph

Consider a one-dimensional wireless network with n nodes uniformly and independently distributed at random in the interval [0,1]. In addition, m eavesdropper nodes are uniformly and independently distributed in [0,1]. For a randomly selected source-destination pair, we consider the problem of securely delivering a message from the source to the destination and we present achievable results on the number of eavesdropper nodes that can be tolerated by the network. Our constructions make use of cooperative jamming, in which nodes located close to the eavesdroppers generate artificial noise. For the one-dimensional network case, our results provide an improvement to the connectivity properties of the recently-introduced secrecy graph which is disconnected for any positive number of eavesdroppers without cooperative jamming. We consider cases of both known and unknown eavesdropper locations. For known eavesdropper locations, we show that a message can be securely delivered from the source to the destination with probability one as the number of nodes n goes to infinity, for any number of independent eavesdroppers m(n) satisfying equation. For unknown eavesdropper locations, we present a construction which can tolerate m(n) = o(n/ log n) under the assumption of independent eavesdroppers, but which is fragile in the face of collaborating eavesdroppers.

Two-way secrecy schemes for the broadcast channel with internal eavesdroppers

The secrecy problem in a broadcast setting with one source and a number of users is considered, where the message to a given user must be kept secret from all of the other users. Multi-user diversity suggests an opportunistic approach that sends a message secretly to the user with the current best channel; however, the secrecy rate goes to zero in the limit of a large number of users. Here, channel reciprocity is exploited via a two-way secrecy scheme to provide a constant positive secrecy rate to the user with the best channel. Next, motivated by the desire to transmit to a given user (rather than opportunistically to the user with the best channel), a second scheme is developed that employs relaying from other users from whom the message is still kept secret. In this non-opportunistic case, a positive secrecy rate is again shown to be achievable in the limit of a large number of system users.

Optimization of frequency-shifted reference ultrawideband systems

Recently, there has been significant interest in developing ultrawideband (UWB) radio techniques. A transmitted reference (TR)-UWB technique is often considered for low-to-moderate data rate applications. However, the TR-UWB technique is limited by the difficulty in building the required highly accurate wideband analog delay line in the receiver. Frequency-shifted reference (FSR)-UWB avoids this analog delay by shifting the reference signal from the data signal in frequency, and FSR-UWB has been shown to outperform the standard TR-UWB scheme. The performance of the FSR-UWB system has been further improved in succeeding work by sending multiple data signals that share a common reference carrier, referred to as multi-differential (MD) FSR, but this comes with a significant increase in the peak-to-average power (PAPR) of the pulses. In this paper, three improvements are proposed for the FSR-UWB system: the front-end filter and the integration time at the receiver are optimized, and a PAPR minimization scheme for the MD-FSR scheme is introduced. After describing and characterizing these approaches, simulation results are provided to consider the varying degrees of improvement offered through the proposed techniques.

Everlasting secrecy in wireless communications: Challenges and approaches

A guarantee of everlasting secrecy is of great interest in modern communication systems. Information-theoretic secrecy is a promising method for providing such and has been widely considered. However, the adoption of information-theoretic security has been hampered by the difficulty of guaranteeing the necessary conditions for secrecy to be realized in a wireless communications environment, where the adversary might have a significant (and likely unknown) signal-to-noise ratio (SNR) advantage over the intended recipient. Thus, whereas the wireless communications environment provides opportunities to gain an advantage over the adversary, as has been considered widely in the literature, it is that same environment that challenges its ultimate utility. We review pertinent past and emerging work to address these challenges and provide perspectives on future directions in the field.

Peak Minimization for Reference-Based Ultra-Wideband (UWB) Radio

We introduce a peak mitigation technique for reference based systems that is similar to the tone reservation scheme employed in orthogonal frequency division multiplexing (OFDM) systems but without the cost in data rate. A comparison of reference-based systems under either peak or average power constraints is presented.