ples of how physical-layer security can be applied to the area of wireless .... School of Engineering and Applied Science ... University Park, PA 16802 USA.
IEEE TRANSACTIONS ON INFORMATION FORENSICS AND SECURITY, VOL. 6, NO. 3, SEPTEMBER 2011
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Guest Editorial Special Issue on Using the Physical Layer for Securing the Next Generation of Communication Systems
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IRELESS communication systems have undergone considerable evolution in the past decade, in large part due to significant advances in the underlying physical-layer technologies, leading to substantial performance leaps in data rates and reliability. These new communication advancements have made wireless devices the platform of choice for communicating, and have emphasized the importance of having a strong all-optical backbone interconnecting the mobile edge. However, as wireless devices become increasingly pervasive, they are more and more likely to serve both as targets for attack and as means for such attacks to be carried out successfully. Traditional higher layer computer and network security techniques can, and must, play an important role in combating such attacks and providing basic security services, such as authentication, integrity, and confidentiality. Accordingly, there have been numerous attempts to make various wireless platforms secure by migrating traditional network security strategies to the wireless domain. In spite of these efforts, the development of secure wireless protocols has proven to be an elusive goal—a fact that is supported by numerous papers revealing successful attacks on many wireless security protocols. One of the most fundamental reasons why wireless systems have been difficult to secure stems from the vulnerability resulting from the broadcast nature of the medium itself, which facilitates both eavesdropping and easier network intrusion. One of the goals of this special issue is to demonstrate that the physical property that causes this vulnerability may very well hold the key to overcome it. Research in communication and networking technologies is undergoing somewhat of a renaissance as there is a broad-based movement to explore new, clean slate approaches for building communication networks. Since future design efforts promise to bring new perspectives on how to design protocols that support our need for high bandwidth and access-anything from anywhere services, it becomes imperative to ensure that the communication systems of the future are secure by design. A thorough reexamination of how to build secure communication infrastructures is, therefore, fully justified. For example, traditional approaches to building and securing networks are tied tightly to the concept of protocol layer separation. As an example, in the security arena, medium access control (MAC)layer security solutions (e.g., WPA2 for 802.11 devices) are typically considered as point-solutions to address threats facing the link layer, while routing and transport layer security issues are dealt with in distinct, nonintegrated protocols like IPSEC and
Digital Object Identifier 10.1109/TIFS.2011.2160572
TLS, and rarely is there any notion of how the physical layer could be used to enhance the security of the network. This special issue seeks to provide a venue for ongoing research in physical-layer security across all types of communication media, ranging from wireless networks at the edge to optical backbones at the core of the network. We received an overwhelming submission of 51 manuscripts, out of which 31 high-quality papers were selected for publication in this special issue after rigorous peer reviews. In surveying the papers, the papers can be categorized at a high-level into: a collection of theoretically focused works with suitability to a wide range of communication media, a collection of papers that are focused primarily on the wireless medium; a survey paper that highlights the state-of-the-art in physical-layer security for optical networks; a collection of papers that explore security just slightly above the physical layer; and an assortment of papers that fill out the special issue by exploring alternative viewpoints on physical-layer security. We begin the special issue by focusing on papers whose contributions are largely theoretical. One fundamental problem in secret key formation is the role of the discussion channel for secretly resolving the shared bits—this is a classical problem that arises in physical-layer security, whether in the domain of quantum cryptography or in the wireless domain. Additionally, low density parity check (LDPC) codes have played a significant role in physical-layer security, especially as the basis of resolving errors or discrepancies that might exist between two parties during the key establishment process. We had many valuable contributions to these and other theoretical problems, and have chosen to lead the special issue off with a variety of papers that address these fundamental aspects of physical-layer security. The next category of papers can be largely classified as examples of how physical-layer security can be applied to the area of wireless security. In general, physical-layer security for wireless systems seeks to turn the nature of the wireless medium from a security disadvantage into a security advantage. In essence, rather than rely solely upon generic, higher layer cryptographic mechanisms, as has been the norm, researchers have shown that it is possible to achieve a lower layer approach that supports important security objectives, such as authentication and confidentiality. An enabling factor for physical-layer security in wireless networks is the fact that, in the rich multipath environment typical of wireless scenarios, the response of the medium along any transmit-receive path is frequency-selective in a way that is location-specific. In particular, channel characterizations (e.g., a set of complex gains at different frequencies, or the impulse response at different time delays) decorrelate from one transmit–receive path to another if the paths are separated by
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IEEE TRANSACTIONS ON INFORMATION FORENSICS AND SECURITY, VOL. 6, NO. 3, SEPTEMBER 2011
the order of an RF wavelength or more. These unique space, time, and frequency characteristics of the wireless physical layer can be used to augment traditional higher layer authentication and confidentiality methods. Two wireless entities can identify or authenticate each other’s transmitter by tracking each other’s ability to produce an appropriate received signal at the recipient. Similarly, the fact that pairwise radio propagation laws between two entities are unique and decorrelate quickly with distance can serve as the basis for establishing shared secrets, either by appropriately quantizing these shared secrets or by using fading phenomena as a means to secretly convey bits through proper encoding. These shared secrets may be used as encryption keys for higher layer applications or wireless system services that need confidentiality. In addition to confidentiality guarantees for small building blocks of wireless networks, network-wide connectivity for secure information transmission can be analyzed for large networks using secrecy graphs. We next place our sole contribution from the optical community. The paper “Optical Layer Security in Fiber-Optic Networks” represents a survey paper that describes the broad array of physical-layer security topics being investigated by the optical communications community. We have placed this paper immediately after our wireless-focused papers to serve point of reference for the reader so they can see the similarity and differences between these two different interpretations of physical-layer security. Perhaps the most notable difference is the fact that the optical physical layer is being used as a “computation” engine for actually performing encryption at the physical layer. Fok et al. show, for example, that it is possible to create an XOR with feedback building block using the physical (nonlinear) properties of light. Interestingly, whereas physicallayer confidentiality for wireless systems involves new forms of coding and communication strategies to extract whatever confidentiality the wireless system can provide, optical systems have the potential to implement conventional cipher algorithms in an ultrafast manner by using such basic XOR building blocks. Continuing in the comparison, we note that there has been extensive work recently in the optical community to stealthily embed messages inside of optical communication signals. This is a trend that we hope the broader IEEE TRANSACTIONS ON INFORMATION FORENSICS AND SECURITY audience will appreciate and further hope that this aspect of the survey paper will open up new opportunities for forensics and watermarking research in the optical domain. The next collection of papers that we have included we have classified as papers that pull the “network” aspect into physicallayer security. Many aspects of modern communication systems and network protocols have been built using error control coding and retransmissions, and it is natural to, therefore, explore whether such ARQ-like mechanisms can be used to enhance security. We have also included a paper that examines the role of timing in privacy. This paper specifically looks at the problem of extracting “private” information related to a communication flow by analyzing the timing and size of packets traversing a network. We have chosen to include this paper in the special issue since the techniques and results provided can be employed at any protocol layer to obfuscate traffic inference. Additionally, we have included two papers that examine security aspects associated with multiple access in networks. Finally, we have included a handful of papers that represent unique aspects of physical-layer security that we felt were im-
portant to include in the special issue. The first paper, “Spatial Models for Human Motion-Induced Signal Strength Variance on Static Links” deals with what the editors would like to refer to as “making physical-layer security really physical.” In particular, in this paper, the authors examine the problem of detecting disturbances in the physical environment due to individuals or objects traversing that environment. This is a unique take on the role of the physical layer in securing systems. Next we have included a paper on a physical-layer attack on RFID systems that prevents the reception of communication. Such an attack represents a compromise in the availability of a service, and thus illustrates another aspect of physical-layer security that is not covered in the earlier papers in this special issue. We also include two papers that formulate secrecy in a game-theoretic setting. These two papers go a long distance to illustrate the fact that security should be cast in a multiparty setting and, in particular, explicitly include the role of an adversary in the problem formulation. Finally, we conclude the special issue by including a paper on modulation recognition, which explores the problem of identifying parameters associated with a network, and raises the concern over whether adversaries can learn about the communication techniques being used by another network—a critical step that must be accomplished before any attack can be launched against such a network. In closing, we would like to thank all of the authors who submitted their manuscripts to this special issue and the reviewers who provided valuable reviews in a timely manner. We also would like to thank Rebecca Wollman for her professional assistance in the paper review and publication process. Although much has been accomplished in the last few years towards understanding the potential of the physical layer to increase the security of digital communication systems, the papers in this special issue offer many avenues for future research with obvious practical relevance. We are, therefore, confident that this nascent field is very likely to continue to flourish both in theory and in practice. WADE TRAPPE, Co-Lead Guest Editor Rutgers University, WINLAB North Brunswick, NJ 08902 USA VINCENT POOR, Co-Lead Guest Editor School of Engineering and Applied Science Princeton Unversity Princeton, NJ 08544 USA HISATO IWAI, Guest Editor Doshisha University Kyotanabe City, Kyoto 610-0321 Japan AYLIN YENER, Guest Editor Penn State University University Park, PA 16802 USA PAUL PRUCNAL, Guest Editor School of Engineering and Applied Science Princeton Unversity Princeton, NJ 08544 USA JOÃO BARROS, Guest Editor DEEC/FEUP University of Porto Porto, 4200-465 Portugal
