Traditionally, spread-spectrum systems have been employed to provide low probability-of-intercept (LPI) and low probability-of-detection (LPD) performances at the physical layer, but the messages transmitted over such a system are still encrypted with a powerful cipher to protect their secrecy. Our challenge is to find a solution to provide an additional level of security at the physical layer so that simple systems such as RFID tags with limited resources can be secure without using standard encryption. It has recently been suggested that the cryptographic security of the system can be enhanced by exploiting physical properties of UWB signals. With an eavesdropper observing the communications over multipath channels between two legitimate partners sharing a secret key of a limited length, we consider both coherent and reference-based UWB schemes to enhance security. The security of the legitimate nodes is achieved by signal attributes based on the secret key, conferring an advantage over the adversary. We propose UWB signaling schemes to improve physical layer security when the transmission is intended for coherent reception and TR reception. Among possible improvements, we consider removing the frame structure of the UWB coherent signaling scheme, resulting in pulses that can be located anywhere in the symbol period. Our proposed signaling schemes could potentially suggest a solution for applications relying on conventional cryptography, especially for low-data rate RFID systems.
Traditionally, spread-spectrum systems have been employed to provide low probability-of-intercept (LPI) and low probability-of-detection (LPD) performances at the physical layer, but the messages transmitted over such a system are still encrypted with a powerful cipher to protect their secrecy. Our challenge is to find a solution to provide an additional level of security at the physical layer so that simple systems such as RFID tags with limited resources can be secure without using standard encryption. It has recently been suggested that the cryptographic security of the system can be enhanced by exploiting physical properties of UWB signals. With an eavesdropper observing the communications over multipath channels between two legitimate partners sharing a secret key of a limited length, we consider both coherent and reference-based UWB schemes to enhance security. The security of the legitimate nodes is achieved by signal attributes based on the secret key, conferring an advantage over the adversary. We propose UWB signaling schemes to improve physical layer security when the transmission is intended for coherent reception and TR reception. Among possible improvements, we consider removing the frame structure of the UWB coherent signaling scheme, resulting in pulses that can be located anywhere in the symbol period. Our proposed signaling schemes could potentially suggest a solution for applications relying on conventional cryptography, especially for low-data rate RFID systems.
Traditionally, spread-spectrum systems have been employed to provide low probability-of-intercept (LPI), low probability-of-detection (LPD) performance at the physical layer, but the messages transmitted over such a system are still encrypted with a powerful cipher to protect their secrecy. However, it has recently been suggested that the cryptographic security of the system can be enhanced by exploiting physical properties of ultra-wideband (UWB) signals. With an eavesdropper observing the communications over multipath channels between two legitimate partners sharing a secret key of a limited length, we consider both coherent and reference-based UWB schemes to enhance security. The security of the legitimate nodes is achieved by signal attributes based on the secret key, giving them an advantage over the adversary. In particular, we propose transmission schemes and investigate the physical layer security performance of UWB systems intended for coherent reception and UWB transmitted-reference (TR) systems in IEEE 802.15.4a multipath environments. Critical to the TR scheme is employing true randomness to keep a sophisticated adversary from decoding the signal coherently. Numerical results for IEEE 802.15.4a channel models reveal not only that the proposed schemes provide promising support for higher-layer cryptographic protocols, but also, perhaps surprisingly, that the baseline UWB TR system can demonstrate better security tradeoffs than the baseline UWB system intended for coherent reception under the IEEE 802.15.4a channel model.
Backscatter wireless communication lies at the heart of many practical low-cost, low-power, distributed passive sensing systems. The inherent cost restrictions coupled with the modest computational and storage capabilities of passive sensors, such as RFID tags, render the adoption of classical security techniques challenging; which motivates the introduction of physical layer security approaches. Despite their promising potential, little has been done to study the prospective benefits of such physical layer techniques in backscatter systems. In this paper, the physical layer security of wireless backscatter systems is studied and analyzed. First, the secrecy rate of a basic single-reader, single-tag model is studied. Then, the unique features of the backscatter channel are exploited to maximize this secrecy rate. In particular, the proposed approach allows a backscatter system's reader to inject a noise-like signal, added to the conventional continuous wave signal, in order to interfere with an eavesdropper's reception of the tag's information signal. The benefits of this approach are studied for a variety of scenarios while assessing the impact of key factors, such as antenna gains and location of the eavesdropper, on the overall secrecy of the backscatter transmission. Numerical results corroborate our analytical insights and show that, if properly deployed, the injection of artificial noise yields significant performance gains in terms of improving the secrecy of backscatter wireless transmission.
Physical layer security (PLS) is a new paradigm aiming at securing communications between legitimate parties at the physical layer. Conventionally, achieving confidentiality in communication networks relies on cryptographic techniques such as public-key cryptography, secret-key distribution and symmetric encryption. Such techniques are deemed secure based on the assumption of limited computational abilities of a wiretapper. Given the relentless progress in computational capacities and the dynamic topology and proliferation of modern wireless networks, the relevance of the previous techniques in securing communications is more and more questionable and less and less reliable. In contrast to this paradigm, PLS does not assume a specific computational power at any eavesdropper, its premise to guarantee provable security via employing channel coding techniques at the physical layer exploiting the inherent randomness in most communication systems. In this dissertation, we investigate a particular aspect of PLS, which is secret-key agreement, also known as secret-sharing. In this setup, two legitimate parties try to distill a secret-key via the observation of correlated signals through a noisy wireless channel, in the presence of an eavesdropper who must be kept ignorant of the secret-key. Additionally, a noiseless public channel is made available to the legitimate parties to exchange public messages that are also accessible to the eavesdropper. Recall that key agreement is an important aspect toward realizing secure communications in the sense that the key can be used in a one-time pad scheme to send the confidential message. In the first part, our focus is on secret-sharing over Rayleigh fading quasi-static channels. We study the fundamental relationship relating the probability of error and a given target secret-key rate in the high power regime. This is characterized through the diversity multiplexing tradeoff (DMT) concept, that we define for our model and then characterize it. We show that the impact of the secrecy constraint is to reduce the effective number of transmit antennas by the number of antennas at the eavesdropper. Toward this characterization, we provide several schemes achieving the DMT and we highlight disparities between coding for the wiretap channel and coding for secret-key agreement. In the second part of the present work, we consider a fast-fading setting in which the wireless channels change during each channel use. We consider a correlated environment where transmit, legitimate receiver and eavesdropper antennas are correlated. We characterize the optimal strategy achieving the highest secret-key rate. We also identify the impact of correlation matrices and illustrate our analysis with various numerical results. Finally, we study the system from an energy-efficiency point of view and evaluate relevant metrics as the minimum energy required for sharing a secret-key bit and the wideband slope.
One benefit of 5G network deployment relates to the smaller size of cells allowing low latency and increased users capacity. The downside is that the potential eavesdroppers could be closer to the victim making the security more challenging. So it is clear the need for enhanced privacy protection in the next generation communications systems even for low power/low complexity IoT devices. The last few years there are numerous research efforts to enhance transmitted information security with complementary techniques that operate in physical layer. In this paper we propose a physical layer scheme that complements the security schemes implemented in the upper layers or/and in the physical layer. The proposed scheme takes advantage of the pilot subcarriers that are part of an OFDM transmission. Pilot subcarriers are essential for the pilot channel estimation process performed at the receiver. In conventional OFDM systems the positions of pilot subcarriers are known. In this paper we propose the positions of the subcarriers to change on every OFDM symbol following a probability distribution known only to the legitimate transmitter and legitimate receiver. This puts a passive eavesdropper at a relative disadvantage compared to a legitimate receiver, because it does not know the position of the pilot subcarriers. To evaluate the security performance of the proposed scheme the secrecy capacity and the outage probability are used as metrics.
Passive radio-frequency identification (RFID) communication raises new transmission secrecy protection challenges, since passive tags stored information lack effective information protection mechanisms. Due to constraints of passive tags, such as limited computation and storage capabilities, security solutions based on the physical-layer security (PLS) are promising candidates compared to those based on conventional lightweight cryptography. Unlike existing endeavors on PLS of RFID wireless communication, we consider an RFID system in the presence of a special proactive eavesdropper, which is able to both enhance the information wiretap and interfere with the information detection at the RFID reader simultaneously by broadcasting its own continuous-wave (CW) signal. To defend against proactive eavesdropping attacks, we propose a wiretap-channel-conscious artificial-noise (AN)-aided secure transmission scheme for the RFID reader, which first estimates both legitimate and wiretap channels and then superimposes an AN signal on the CW signal to confuse the proactive eavesdropper. The transmit power and power allocation between the AN signal and the CW signal are optimized to maximize the secrecy rate. Furthermore, we model the attack and defense process between the proactive eavesdropper and the RFID reader as a hierarchical security game and prove it can achieve the equilibrium. The simulation results show the superiority of our proposed scheme in terms of the secrecy rate and the interactions between the RFID reader and the proactive eavesdropper.
Wireless communications are naturally exposed to important security risks due to their broadcast nature. In contrast with wired communications, no physical medium is isolating the transmission surroundings to a specific path. Traditional methods to reach the goals of communications security rely on algorithms at protocol stack level, which are based on mathematically derived cryptographic protocols. With the tremendous proliferation of wireless devices, and the increase in computational power available for decryption, traditional cryptographic measures may be compromised. To protect communications at the lowest level, physical layer (PHY) security methods have been proposed recently. These techniques aim to ensure secure communications by exploiting the unique characteristics of the physical medium between communicating nodes, effectively reducing the decryption capabilities of an eavesdropper to intercept transmissions. This thesis explains within a general framework the most important PHY layer security schemes. First we explore approaches which utilize the knowledge of the unique channel between communicating nodes to distort a transmitted signal with various kinds of randomizations, in such a way that the intended receivers are unaffected by the distortion, but any other observer will be exposed to it. These techniques, however, introduce several trade-offs between transmission resources and the effectiveness in achieving secrecy. Of particular interest for this investigation is a class of methods which can be referred to as "security enhancing techniques", in which minimal priors about the potential eavesdroppers locations and capabilities are assumed. In this thesis, a wave-based approach to the communication secrecy problem is introduced. An interesting perspective is provided by studying the relation between electromagnetic theory and information theory, the so-called electromagnetic information theory. This allows to properly identify the unavoidable limitations imposed by the laws of electromagnetics to our ability to communicate securely. The proposed model can be extended to the analysis of different signal distortion methodologies and different channel conditions. The resulting approach is highly relevant to the study of PHY layer security methods since it is physically fundamental as well as architecture independent, which leads to a consolidating point of view of the secrecy problem. In addition, the thesis presents computer simulation results which shed insight on the security possibilities and limitations of three different security scenarios, which are termed MISO (multiple-input, single-output) (with single eavesdropper), MISOME (multiple-input, single-output, multiple eavesdropper), and MIMOME (multiple-input, multiple-output, multiple-eavesdropper).