This dissertation studies how information-theoretically secure mechanisms for confidentiality and data-traffic privacy can be incorporated into existing and emerging wireless systems. The dissertation consists of three parts. In the first two parts, we study how certain properties of wireless channels can be employed to enhance confidentiality services that have traditionally been the responsibility of higher layers. We first explore the use of the wireless medium for the extraction of secret keys at the two ends of a wireless link, wherein the transceivers at either end are separated by a rich multipath scattering environment. We build a low complexity algorithm that allows two wireless devices to extract a common sequence of random secret bits, by repeatedly probing and estimating a time-varying channel between themselves. Further, we report on an implementation and evaluation of our algorithm on a modified 802.11a system. Next, we study the problem of securely pairing wireless devices in proximity of one another by establishing a shared secret key using a public source of RF transmissions. We employ measurement data to characterize the rate at which bits can be extracted and explore the simultaneous use of multiple transmitters to increase rate. Finally, we study the case when the public transmitter itself is under the arbitrary control of an adversary and we demonstrate a method that can allow successful key-extraction even with such an active adversary. In the final part of this dissertation, we introduce the problem of an unintended information-leakage channel in data traffic consisting of varying packet sizes. Packet sizes convey semantic information that can be related to their con- tent, which can be used as a fingerprint for classification. We formally study the packet-size side channel and explore obfuscation approaches to prevent infor- mation leakage, while considering padded dummy traffic and delay as bounded resources. We show that randomized algorithms for obfuscation can be studied as well known information-theoretic constructs, such as discrete channels with and without memory, and often lead to efficiently solvable constrained optimization problems.
This dissertation studies how information-theoretically secure mechanisms for confidentiality and data-traffic privacy can be incorporated into existing and emerging wireless systems. The dissertation consists of three parts. In the first two parts, we study how certain properties of wireless channels can be employed to enhance confidentiality services that have traditionally been the responsibility of higher layers. We first explore the use of the wireless medium for the extraction of secret keys at the two ends of a wireless link, wherein the transceivers at either end are separated by a rich multipath scattering environment. We build a low complexity algorithm that allows two wireless devices to extract a common sequence of random secret bits, by repeatedly probing and estimating a time-varying channel between themselves. Further, we report on an implementation and evaluation of our algorithm on a modified 802.11a system. Next, we study the problem of securely pairing wireless devices in proximity of one another by establishing a shared secret key using a public source of RF transmissions. We employ measurement data to characterize the rate at which bits can be extracted and explore the simultaneous use of multiple transmitters to increase rate. Finally, we study the case when the public transmitter itself is under the arbitrary control of an adversary and we demonstrate a method that can allow successful key-extraction even with such an active adversary. In the final part of this dissertation, we introduce the problem of an unintended information-leakage channel in data traffic consisting of varying packet sizes. Packet sizes convey semantic information that can be related to their con- tent, which can be used as a fingerprint for classification. We formally study the packet-size side channel and explore obfuscation approaches to prevent infor- mation leakage, while considering padded dummy traffic and delay as bounded resources. We show that randomized algorithms for obfuscation can be studied as well known information-theoretic constructs, such as discrete channels with and without memory, and often lead to efficiently solvable constrained optimization problems.
With the rapid deployment of new wireless devices and pervasive use of wireless data and voice services, the demand for reliable and secure communications is becoming more and more urgent. The focus of this thesis is on the fundamental trade-off among throughput, reliability, and security of various wireless networks. Our study adopts the notion of provable security from an information theoretic perspective. Using equivocation to measure the confidentiality of messages, we establish, for various communication models, the fundamental rate-equivocation trade-off. We first study capacity bounds for discrete memoryless broadcast channels with two confidential messages, which is a generalization of Csiszár and Körner's classical model. The outer bounds are proposed for the rate equivocation region of this channel model, which, together with a previously proposed inner bound, help establish the rate equivocation region of several classes of discrete memoryless broadcast channels. Furthermore, specializing to the general broadcast channel by removing the confidentiality constraint, the proposed outer bounds reduce to new capacity outer bounds for the discrete memoryless broadcast channel. Next, we consider another variation of Csiszár and Körner's model. The transmitter sends both a confidential message and a non-confidential message (public message) to the intended receiver. While the unintended receiver should be kept ignorant from the confidential message, we do not impose the requirement that the public message needs to be perfectly recovered by the unintended receiver. This more liberal treatment of the non-confidential message is perhaps a more reasonable model than Csiszár and Körner's model where the non-confidential message (common message) is required to be decoded by both receivers. A single-letter characterization of the achievable rate equivocation region of this model is given and the result is then extended to the case when an extra secret key is available to the transmitter and the intended receiver. Utilizing the developed framework of broadcast channels with confidential and public messages, we further study the problem of secure communication over a network in which each link may be noisy or noiseless. A single-source single-sink acyclic planar network is assumed, and the communication between the source and the sink is subject to non-cooperating eavesdropping on each link. Sufficient conditions, in terms of communication rates and network parameters, are found for provably secure communication. A constructive proof, which combines Shannon's key encryption, Wyner's random coding, and the Ford-Fulkerson algorithm, is provided which constitutes a readily implementable secure coding scheme for provably secure communications. The derived achievable rate equivocation region is tight when specializing to several special cases. In particular, when the communication network decouples into non-overlapping parallel paths, the proposed encoding scheme is optimal, i.e., it achieves the secure communication capacity for such networks.
Secure communication ensures the integrity and confidentiality of communication between connected devices. An information-theoretic approach to secure communication gives the strongest confidentiality guarantee by assuming that the attacker has unlimited computing power. The earliest formal model and definition of information-theoretic secure communication is by Shannon, who employed a secret key shared between communicating parties to provide confidentiality. An alternative elegant information-theoretic approach to secure communication views the natural characteristics of the environment (i.e., channel’s noise) as a resource to build security functionalities. This approach was first proposed by Wyner, and the corresponding secure communication model is called the wiretap channel model. These two approaches introduce two primary resources for providing information-theoretic secure communication: the shared secret key and physical properties of the communication medium. In this thesis, we study how to employ the above two resources for secure message transmission. We study this by using channel’s noise in the wiretap channel model. In this model, a sender is connected to the receiver and the adversary through two noisy channels. We propose a new wiretap encoding scheme with strong secrecy that provides perfect secrecy and reliability, asymptotically. The construction treats the noise in the adversary’s channel as a source of randomness that is extracted and used to hide the message from the adversary. We realize the wiretap channel model using cooperative jamming to evaluate the performance of wiretap codes in practice. We consider a model called keyed wiretap channel that unifies Wyner’s model with Shannon’s model of perfect secrecy for information systems, and propose a keyed encoding schemes with strong secrecy and other properties that are attractive in practice. We also study two-party information-theoretic secret key agreement when the two parties have access to samples of a common source of randomness and use a single message transmission to arrive at a shared random key. We propose a secret key agreement protocol in this setting, prove its security, and show its superior performance compared to other known protocols with the same properties. Finally, we propose an information-theoretic secret key agreement over a virtual wiretap channel created by cooperative jamming.
In this dissertation, we present novel physical (PHY) and cross-layer design guidelines and resource adaptation algorithms to improve the security and user experience in the future wireless networks. Physical and cross-layer wireless security measures can provide stronger overall security with high efficiency and can also provide better flexibility in response to the typically time-varying wireless environment. To better utilize limited wireless resources, we present approaches and techniques based on new performance criteria for wireless system optimization by thoroughly exploiting user information including its Quality of Service (QoS) requests and channel state information (CSI) to improve network throughput and security.
In recent years, there has been a growing research interest in wireless system security from PHY layer perspective. In a wiretap channel model introduced in the seminal work by Wyner, a sender “Alice” wishes to transmit a secret message to the intended receiver “Bob” in presence of a passive eavesdropper “Eve”. Existing works have characterized maximum achievable secrecy rate or secrecy capacity for single- and multiple-antenna systems by applying Gaussian signaling and secrecy code. Despite the impracticality of Gaussian input, its compact closed-form expression of mutual information motivated the wide use of Gaussian input assumption in theoretical analysis. In contrast to the Gaussian codebook, practical wiretap codes must consist of finite-alphabet symbols. Because of this constraint, the achievable secrecy rate for a finite-alphabet input scenario would differ from the secrecy rate achievable by a Gaussian codebook. In this thesis, we quantify the effect of finite discrete-constellation on instantaneous and ergodic secrecy rate of multiple-antenna wire-tap channels. Our results demonstrate substantial performance difference between systems involving finite-alphabet inputs and systems with Gaussian inputs. In addition, we investigate the secrecy performance of multi-terminal wiretap systems when a codebook based transmission beamforming is implemented. We characterize the secrecy outage probability of a communication link under eavesdropping. We consider a limited feedback scenario where the transmitter uses a pre-defined codebook known to both the transmitter and the receiver for beamforming, and analyze the secrecy outage probability of the link when it being eavesdropped.
Next, we consider the security of wireless multi-hop networks. Certainly, network security is an overarching issue that transcends all layers of communication protocol stack. Therefore, we tackle the problem of network security by proper user scheduling, routing, and resource management. Our approach of secrecy is complementary to the traditional cryptography based techniques. Together, they can provide stronger overall security and flexibility in selecting the desired solution and can reduce bandwidth and energy overhead as much as possible. Multi-hop wireless networks frequently employ the traditional routing and resource allocation strategies. Such strategies often fail to take into account the additional need for protection against security vulnerabilities due to the wireless medium. Thus, to address security concerns, we investigate the problem of eavesdropping control by proposing security-aware power allocation strategies and routing algorithms for multi-hop networks.
The future wireless infrastructure will consists of a plethora of heterogeneous collections of applications and services. A differentiated service structure would therefore be an essential part of the future wireless infrastructure. In addition to the security consideration of wireless network, we focus on the heterogeneous structure of the network. We study the problem of cross-layer resource allocation and admission control in a multiuser heterogeneous OFDMA network providing both QoS-constrained user servicesand best-effort services. Such heterogeneous networks provide service to both High-Priority (HP) users and Best-Effort (BE) users. By clustering subcarriers, efficient algorithms for cluster and power allocation have been proposed. Our strategy maximizes the total network utility of the BE users while satisfying the QoS request for maximum number of HP users. The feasibility of the resource allocation problem depends on the number of HP users in the network. Since a large number of demanding HP users would render the resource allocation problem infeasible, a joint admission control and resource allocation scheme could be an efficient way of tackling both problems with less overhead. By incorporating admission control, we propose an efficient and optimal joint admission control and resource allocation algorithm.
Inherent openness of the wireless medium imposes stronger challenges on the security of wireless communications. Information-theoretic security addresses these challenges at the physical layer by using tools from wireless communication theory, signal processing and information theory. In information-theoretic security, physical layer communication is intelligently designed to exploit the characteristics of the wireless medium, such as fading, interference, cooperation, and multi-dimensional signaling, in order to provide or improve security. In this dissertation, we study the security of several fundamental wireless network configurations from an information-theoretic perspective. First, we study the Gaussian multiple-input multiple-output (MIMO) wiretap channel. In this channel, the transmitter sends a common message to both the legitimate user and the eavesdropper. In addition to the common message, a private message is sent only to the legitimate user, which needs to be kept hidden as much as possible from the eavesdropper. We obtain the entire capacity-equivocation region for this channel model. In particular, we show the sufficiency of jointly Gaussian auxiliary random variables and channel input to evaluate the existing single-letter description of the capacity-equivocation region due to Csiszar-Korner. Next, we study the secure broadcasting problem, where a transmitter wants to have secure communication with multiple legitimate users in the presence of an external eavesdropper. We study several special cases of the secure broadcasting problem. First, we consider the degraded multi-receiver wiretap channel, and establish its secrecy capacity region. Second, we consider the parallel less noisy multi-receiver wiretap channel, and obtain its common message secrecy capacity and sum secrecy capacity. Third, we consider the parallel degraded multi-receiver wiretap channel for the two-user and two-sub-channel case, and obtain its entire secrecy capacity region. Finally, we consider a parallel channel model with two sub-channels, where the transmitter can use only one of the subchannels at any time, and characterize its secrecy capacity region. Then, we study the two-user Gaussian MIMO broadcast channel with common and confidential messages. In this channel model, the transmitter sends a common message to both users, and a confidential message to each user which needs to be kept perfectly secret from the other user. We obtain the entire capacity region of this channel. We also explore the connections between this channel model and its non-confidential counterpart, i.e., the Gaussian MIMO broadcast channel with common and private message. Next, we consider the Gaussian MIMO multi-receiver wiretap channel and obtain its secrecy capacity region for the most general case. We first show that even for the single-input single-output (SISO) case, existing converse techniques fall short of proving the secrecy capacity region, to emphasize the need for a new proof technique, which we develop by using the relationships between the Fisher information and the differential entropy. Using this new proof technique, we obtain the secrecy capacity region of the degraded MIMO channel. We then establish the secrecy capacity region of the general MIMO channel by using the channel enhancement technique in conjunction with the capacity result we obtained for the degraded MIMO channel. For the general MIMO channel, we show that dirty-paper coding (DPC) combined with stochastic encoding attains the entire secrecy capacity region. Then, we study the multi-receiver wiretap channel for a more general scenario, where, in addition to confidential messages, the transmitter sends public messages to the legitimate users, on which there are no secrecy constraints. First, we consider the degraded discrete memoryless channel, and obtain inner and outer bounds for the capacity region. These inner and outer bounds match for certain cases, providing the capacity region. Second, we obtain an inner bound for the general discrete memoryless channel by using Marton's inner bound. Third, we consider the degraded Gaussian MIMO channel, and show that jointly Gaussian auxiliary random variables and channel input are sufficient to exhaust the inner and outer bounds. Finally, we provide an inner bound for the capacity region of the general Gaussian MIMO channel. Next, we focus on the multiple access wiretap (MAC-WT) channel whose capacity region is unknown. We consider a special class of MAC-WT channels which we call the weak eavesdropper class, where each user's link to the legitimate receiver is stronger than its link to the eavesdropper. For this class of channels, we develop an outer bound for the secrecy capacity region, which partially matches the achievable region in an n-letter form. We evaluate a looser version of our outer bound for the Gaussian case, and show that our outer bound is within 0.5 bits/channel use of the achievable rates along the individual secrecy rates for all weak eavesdropper Gaussian MAC-WT. Then, we investigate the effects of user cooperation on the secrecy of broadcast channels by considering the cooperative relay broadcast channel (CRBC). We propose an achievable scheme that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward (CAF) scheme for relay channels. For the Gaussian CRBC, we show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation. We further investigate the effects of user cooperation on secrecy by considering the multiple access channel with generalized feedback (MAC-GF), which can be viewed as the MAC-dual of the CRBC. We propose a CAF-based achievable secrecy rate region for the MAC-GF. Specializing our results to a Gaussian MAC-GF, we present numerical results which demonstrate that cooperation can improve secrecy for the MAC-GF. Next, we study the two-user one-eavesdropper discrete memoryless compound wiretap channel, and provide the best known lower bound for the secrecy capacity of this compound channel. We evaluate this achievable secrecy rate for the Gaussian MIMO case by using DPC. We show that this achievable secrecy rate achieves at least half of the secrecy capacity of this Gaussian MIMO compound wiretap channel, and also attains the secrecy capacity when the eavesdropper is degraded with respect to one of the two users. Then, we study the degraded compound multi-receiver wiretap channel (DCMRWC), which, in addition to a group of eavesdroppers, has two groups of users, namely the stronger group and the weaker group. We study two different communication scenarios for this channel. In the first scenario, there is only one eavesdropper, and the transmitter sends a confidential message to each group of legitimate users while keeping both messages secret from the eavesdropper. In the second scenario, we study the DCMRWC with layered messages without any restriction on the number of eavesdroppers. For both scenarios, we obtain the secrecy capacity region for the discrete memoryless channel, the parallel channel, and the Gaussian parallel channel. For the Gaussian MIMO channel, we obtain the secrecy capacity region when there is only one user in the second group. Next, we study the two-user fading broadcast channel and obtain its ergodic secrecy capacity region. We show that, thanks to fading, both users can have simultaneous secure communication with the transmitter, although this is not possible in the scalar non-fading Gaussian broadcast channel where only one user can have secure communication. This simultaneous secrecy of both users is achieved by an opportunistic communication scheme, in which, at each time instant, the transmitter communicates with the user having a better channel gain. Then, we study the secure lossy transmission of a vector Gaussian source to a legitimate user in the presence of an eavesdropper, where both the legitimate user and the eavesdropper have vector Gaussian side information. We obtain an outer bound for the rate, equivocation and distortion region. Moreover, we obtain the maximum equivocation at the eavesdropper when there is no constraint on the transmission rate. By using this maximum equivocation result, we show two facts. First, for this problem, in general, Wyner-Ziv scheme is suboptimal, although, it is optimal in the absence of an eavesdropper. And, second, even when there is no transmission rate constraint, an uncoded transmission scheme is suboptimal; the presence of an eavesdropper necessitates the use of a coded scheme to attain the maximum equivocation. Finally, we revisit the secure lossy source coding problem. In all works on this problem, either the equivocation of the source at the eavesdropper or the equivocation of the legitimate user's reconstruction of the source at the eavesdropper is used to measure secrecy. We first propose the relative equivocation of the source at the eavesdropper with respect to the legitimate user as a new secrecy measure. We argue that this new secrecy measure is the one that corresponds to the natural generalization of the equivocation in a wiretap channel to the context of secure lossy source coding. Under this new secrecy measure, we provide a single-letter description of the rate, relative equivocation and distortion region, as well as its specializations to degraded and reversely degraded cases. We investigate the relationships between the optimal scheme that attains this region and the Wyner-Ziv scheme.
The broadcasting attribute of the wireless communication channel poses severe security vulnerabilities since any adversary node in the range of reception is able to listen as well as to disrupt the ongoing communication. This dissertation considers the problem of security from two different perspectives: The information-theoretic approach at the physical-layer to avert eavesdropping and centralized and decentralized detection algorithms to detect the presence of a jammer. In the first part, we apply the available tools in information-theoretic security to study the achievable secrecy rate region of the following channel models: the discrete memoryless broadcast channel with two confidential messages for each of the two users, the discrete memoryless interference channel with two confidential messages, and the discrete memoryless untrusted relay channel with decode-and-forward (DF) relays (DMURC). For the first two scenarios, we extend the existing results on the weak secrecy to the strong secrecy criterion. Owing to its definition, the strong secrecy has a practical interpretation, whereas weak secrecy provides asymptotic results. In the third scenario, we apply a precoder and a post-decoder in addition to the common channel coding required to provide secrecy, to deal with the dishonest nature of the relays. An achievable secrecy rate region for DMURC is established. We revisit the untrusted relay scenario with some different assumptions: with Gaussian channel and both DF and amplify-and-forward (AF) relaying. The objective, however, is to develop a secure scheme that allows for weaker secrecy than Shannon’s perfect secrecy but with a higher transmission rate. We propose a new measure for secrecy, referred to as a-secrecy (0 = a = 1), where a = 1 represents the perfect secrecy, whereas a = 0 denotes no secrecy at all. We suggest using a cross-layer design where at the physical layer the information-theoretic security is applied and some higher layer scheme, e.g. secure network coding, allows to obtain any point in the transmission rate and a-secrecy trade-off. The analysis of performance achievable by the scheme leads to a mixed-integer optimization problem. Moreover, we study the problem asymptotically to gain some insights. Active security attacks are the main focus of the second part of this thesis. We study the problem of joint jamming detection and spectrum sensing. The problem arises in the cognitive radio scenario, where the secondary users (SU)s sensing the channel to detect the primary users (PU)s. We model the problem as a multiple-hypothesis testing problem where the observations from the SUs experience spatial channel correlation. The optimal test statistic is derived according to Neyman-Pearson lemma. We derive the exact probability distribution of the test statistic in terms of Meijer’s G-functions. We consider a practical application where the SUs’ power efficiency is analyzed. This leads to a non-convex optimization problem that addresses the power efficiency and detection performance trade-off. The detection performance in the exact form is too complex to be handled analytically, therefore, we consider the asymptotic regime. Two heuristics are proposed to tackle the optimization problem; one based on the projected gradient method and the other one is motivated by the KKT conditions. All of the algorithms mentioned above are designed for a centralized system. Taking into account the susceptibility of centralized systems to the Byzantine attacks, we study the decentralization of eigenvalue-based detection algorithms in the last chapter of this thesis. We use generic consensus algorithm as a building block in decentralizing an eigenvalue estimator algorithm. The algorithm estimates all eigenvalues of a covariance matrix, which is available only partly to each node. We further modify the algorithm for two special practical scenarios, including the channel probing for admission control problem.
This dissertation studies the fundamental rate limits of wireless communication systems through which confidential messages can be transmitted without being intercepted by an adversary. It is assumed that the adversary is subject to noise and interference, as are the legitimate nodes in the system. The security guarantees offered are irrespective of the computational power of the adversary.
Three themes are developed in parallel throughout the dissertation. The first theme explores the relationship between cooperation and secrecy. In wireless networks, cooperation from intermediate relay nodes are essential for facilitating long range communications. This dissertation considers a series of network models where the information from a source to a destination needs to be kept confidential from intermediate relays. Four different network models with untrusted relay nodes are considered: the three-node relay channel, the two-hop link, the two-way relay channel and the multi-hop line network. These models are studied for relays that are honest but curious. That is, the relay nodes are legitimate nodes with lower security clearance, that willingly carry out the designated relay schemes. Secrecy capacity lower and upper bounds for each channel model are found and cases where secrecy capacity can be found are identified. Next the two-hop link is studied where the relay turns malicious in that it can manipulate the signals being relayed and carry out a Byzantine attack. In this case, two requirements must be satisfied simultaneously: The message must be kept confidential from the relay node, and, any modification carried out by the relay node must be detected reliably by the legitimate parties. The dissertation shows that both requirements can be fulfilled for this model with arbitrarily small overhead.
The next two themes developed in this dissertation serve as the technical foundation for the first theme. One establishes the foundation for providing secrecy using structured codes. Traditionally, the achievability proofs for information theoretic secrecy rely on Shannon’s random codebook generation argument in which the codebook does not exhibit structure. In this dissertation, it is shown that by imposing structure requirements in random codebook generation, higher rates for secure communication can be achieved than known results obtained using the conventional approach. This dissertation develops the necessary tool and general methodology for computing the secrecy rate when a nested lattice structure is used in codebook generation. Two results are presented that rely on this foundation. First, it is proved that, for a class of Gaussian channel models with interference and secrecy constraints, structured codes outperform Gaussian signaling at medium to high SNR. Second, this tool is used to develop a strong secrecy scheme for the two-hop link with an untrusted relay, which in turn is the centerpiece of the Byzantine detection scheme developed in this dissertation.
The third theme is methods of proving strong secrecy. Information theoretic secrecy results often rely on the weak secrecy constraint which provides only asymptotic guarantees with respect to the information rate leaked to the adversary. As this would likely be insufficient in a practical system, a non-asymptotic, strong secrecy guarantee must be sought. More often than not, weak secrecy rates can be strengthened to strong secrecy rates in a straightforward manner, leading back to the justification of proving weak secrecy only. This dissertation challenges this conventional wisdom and identifies two cases where this approach does not follow. In both cases, direct strong secrecy proofs are established. These are: the two-hop link with a Byzantine relay, and models with arbitrarily varying eavesdropper channel considered in the last chapter of this dissertation.
The latter model is motivated by the need to bring the theory of providing unconditional secrecy guarantees closer to practice. In particular, the dissertation addresses the case where the channel that the adversary (eavesdropper) experiences is completely unknown to the legitimate parties. To do so, a multiple input-multiple output (MIMO) wiretap channel is considered where the eavesdropper’s channel is arbitrarily varying. A coding theorem is derived for with this setting. The secrecy rate obtained from this coding theorem is tight in terms of secure degree of freedom, which is a high SNR characterization of the secrecy capacity. In addition, the secure degree of freedom regions for the MIMO multiple access (MAC) wiretap channel and the MIMO broadcast wiretap channel are derived when the legitimate nodes have the same number of antennas, identifying the high SNR characterization of the fundamental transmission limits for these models.