Onur Gungor

Joint Power and Secret Key Queue Management for Delay Limited Secure Communication

In recent years, the famous wiretap channel has been revisited by many researchers and information theoretic secrecy has become an active area of research in this setting. In this paper, we design a wireless communication system that achieves constant bit rate data transmission over a block fading channel, securely from an eavesdropper that listens to the transmitter over another independent block fading channel. It is well known that, the method of sending secure information using the binning techniques inspired by the wiretap channel fails to secure the information at times when the eavesdropper channel has favorable conditions over the main channel. This phenomenon is called secrecy outage. In our system, however, we exploit the times at which the main channel is favorable over the eavesdropper channel for us to be able to transmit some random secret key bits along with the data bits. These key bits are stored in a separate key queue at the transmitter as well as the receiver, and are utilized to secure data bits, whenever the channel conditions favor the eavesdropper. We show that, our system achieves a high performance at any given desired outage probability by jointly controlling the key queue and the transmit power. We show that the optimal power control involves a time sharing between secure waterfilling and channel inversion strategies and the key queue operates in the heavy traffic regime to achieve the maximum delay limited rate possible, under a small outage constraint. This work can be viewed as a first step in providing a framework that combines both information theory and queueing analysis for the study of information theoretic security.

Secret Key Generation Via Localization and Mobility

We consider secret key generation by a pair of mobile nodes utilizing observations of their relative locations in the presence of a mobile eavesdropper. In our proposed algorithm, the legitimate node pair makes noisy observations of the relative locations of each other. Based on these observations, the nodes generate secret key bits via information reconciliation, data compression, and privacy amplification. We characterize a theoretically achievable secret key bit rate in terms of the observation noise variance at the legitimate nodes and the eavesdropper and show that the performance of our algorithm is comparable to the theoretical bounds. We also test our algorithm in a vehicular setting based on observations made using wireless beacon exchange between the legitimate nodes. To achieve this, we used TelosB wireless radios mounted on the sides of the vehicles on local roads and freeways. Note that our approach relies solely on distance reciprocity, and thus, it is not restricted to the use of wireless radios and can be used with other localization systems (e.g., infrared and ultrasound systems) as well. Overall, this study proves, via both information theoretic and practical analysis, that localization information provides a significant additional resource for secret key generation in mobile networks.

On the Secrecy Capacity of Block Fading Channels With a Hybrid Adversary

We consider a block fading wiretap channel, where a transmitter attempts to send messages securely to a receiver in the presence of a hybrid half-duplex adversary, which arbitrarily decides to either jam or eavesdrop the transmitter-to-receiver channel. We provide bounds to the secrecy capacity for various possibilities on receiver feedback and show special cases where the bounds are tight. We show that, without any feedback from the receiver, the secrecy capacity is zero if the transmitter-to-adversary channel stochastically dominates the effective transmitter-to-receiver channel. However, the secrecy capacity is nonzero even when the receiver is allowed to feed back only one bit at the end of each block. Our novel achievable strategy improves the rates proposed in the literature for the nonhybrid adversarial model. We also analyze the effect of multiple adversaries and delay constraints on the secrecy capacity. We show that our novel time sharing approach leads to positive secrecy rates even under strict delay constraints.

Secrecy outage capacity of fading channels

This paper considers point to point secure communication over flat fading channels under an outage constraint. More specifically, we extend the definition of outage capacity to account for the secrecy constraint and obtain sharp characterizations of the corresponding fundamental limits under two different assumptions on the transmitter CSI (Channel state information). First, we find the outage secrecy capacity assuming that the transmitter has perfect knowledge of the legitimate and eavesdropper channel gains. In this scenario, the capacity achieving scheme relies on opportunistically exchanging private keys between the legitimate nodes. These keys are stored in a key buffer and later used to secure delay sensitive data using the Vernams one time pad technique. We then extend our results to the more practical scenario where the transmitter is assumed to know only the legitimate channel gain. Here, our achievability arguments rely on privacy amplification techniques to generate secret key bits. In the two cases, we also characterize the optimal power control policies which, interestingly, turn out to be a judicious combination of channel inversion and the optimal ergodic strategy. Finally, we analyze the effect of key buffer overflow on the overall outage probability.

Secret key generation from mobility

We consider secret key generation from relative localization information of a pair of nodes in a mobile wireless network in the presence of a mobile eavesdropper. Our scheme consists of two phases: in the first phase, legitimate node pair exchanges beacon signals to establish localization information based on noisy observations of these beacons; in the second phase, nodes generate secret key bits via a public discussion. Our problem can be categorized under the source models of information theoretic secrecy, where the distance between the legitimate nodes acts as the observed common randomness. We characterize the achievable secret key bit rate in terms of the observation noise variance at the legitimate nodes and the eavesdropper. This work provides a framework that combines information theoretic secrecy and wireless localization, and proves that the localization information provides a significant additional resource for secret key generation in mobile wireless networks.

An information theoretic approach to RF fingerprinting

RF fingerprinting exploits the variations in the RF chain of radios to uniquely identify transmitters, and distinguish adversarial transmissions from legitimate nodes. We provide a systematic approach rooted from information theory to understand basic performance limits of RF fingerprinting. We develop a novel channel model to cover RF fingerprinting systems, where the imperfections in the RF chain are modeled as a fingerprint channel, cascaded to the physical channel. We analyze authentication problem in the presence of an adversary, where both the legitimate transmitter and the adversary are equipped with unique fingerprint channels. We provide bounds for the error exponents of the legitimate nodes, and the success exponent of the adversary, as a function of their fingerprints. We illustrate that concepts analogous to Maurers simulatability are necessary to guarantee authentication via RF fingerprints.

On the Basic Limits of RF-Fingerprint-Based Authentication

RF fingerprinting exploits the variations in the RF chain of radios to uniquely identify transmitters, and distinguish adversarial transmissions from the transmissions of legitimate nodes. We provide a systematic approach rooted from the information theory to evaluate the basic performance limits of RF fingerprinting. We develop a novel channel model for RF fingerprinting, where the imperfections in the RF chain are modeled as a fingerprint channel, cascaded to the actual physical channel. We address the authentication problem in the presence of an adversary, where both the legitimate transmitter and the adversary are equipped with unique fingerprint channels, in addition to a possible secret key available at the legitimate nodes. We provide bounds for the error exponents for reliable communication of the legitimate nodes, and the success exponent for impersonation and substitution attacks of the adversary, as a function of certain parameters based on their RF-fingerprints, and the shared key rate. We illustrate that keyless authentication is possible via RF fingerprints when the legitimate channel is not simulatable. We also show that the probability of these attacks can be reduced significantly by employing additional dedicated authenticated nodes.

RF-fingerprint based authentication: Exponents and achievable rates

RF fingerprinting exploits the variations in the RF chain of radios to uniquely identify transmitters, and distinguish adversarial transmissions from transmissions of legitimate nodes. We provide a systematic approach rooted from information theory to evaluate basic performance limits of RF fingerprinting. We develop a novel channel model for RF fingerprinting, where the imperfections in the RF chain are modeled as a fingerprint channel, cascaded to the actual physical channel. We address the authentication problem in the presence of an adversary, where both the legitimate transmitter and the adversary are equipped with unique fingerprint channels, in addition to a possible secret key available at the legitimate nodes. We provide achievable bounds for the error exponents for reliable communication of the legitimate nodes, and the success exponent for impersonation and substitution attacks of the adversary, as a function of certain parameters based on their RF-fingerprints, and the shared key rate. We illustrate that keyless authentication is possible via RF fingerprints when the legitimate channel is not simulatable.

Proactive source coding

A coding problem, over a slotted system, is introduced where the sender has to transmit one out of several packets to the receiver, but learns the request only at the beginning of each slot with prior statistical information about which packet is needed at the receiver. There is an associated cost of sending bits at each slot, and the goal is to minimize the expected cost of the communication. A proactive coding scheme is proposed, where the source proactively communicates with the receiver before the receiver requests the message. This way, by designing a cost optimal side information at the receiver, the scheme is able to minimize the expected cost of the communication. Numerical results are provided demonstrating the gains obtained by proactive coding over the conventional coding technique.

On secrecy outage capacity of fading channels under relaxed delay constraints

We consider information theoretic secrecy over flat fading channels under relaxed delay constraints. More specifically, we extend the definition of outage secrecy capacity for single-input single-output single-eavesdropper case (SISOSE) to account for relaxed delay constraints, and study the fundamental limits under two different assumptions on the transmitter CSI (channel state information). First, we provide bounds on secrecy outage capacity with k+1 block delay constraint. We show that the bounds are tight for several special cases. We also provide a weaker lower bound that is easier to compute, and show that under low SNR, delay constraint has significant impact on secrecy outage capacity. The analysis serves as an important step towards complete characterization of information theoretic security with delay and outage constraints.