In much recent discussion on wireless physical layer security there is a strong tendency to take for granted that the randomness provided by the wireless channel is adequate for cryptographic applications. One typically assumes that multipath fading, interferences and changes in the environment lead to sufficiently random channel measurements. We show that this does not hold for the majority of practical implementations where the underlying channel response is estimated based on the Received Signal Strength (RSS) values, and we identify the statistical defect that is primarily responsible for the statistical bias in the RSS values and discuss the source of this defect. Our observation implies that RSS-based physical layer key agreement schemes require a dedicated post-processing of the channel measurements together with an on-line statistical test suite. To address the latter, we define requirements for statistical testing in the context of physical layer key agreement. Based on these requirements, we select some well-known low complexity statistical tests to form a reduced test suite that requires less than 10,000 bit of input data. The selected tests have low complexity allowing to execute the test suite at the run-time even on severely resource-constrained devices. Our results show that it is possible to identify statistical defects with high reliability.
In much recent discussion on wireless physical layer security there is a strong tendency to take for granted that the randomness provided by the wireless channel is adequate for cryptographic applications. One typically assumes that multipath fading, interferences and changes in the environment lead to sufficiently random channel measurements. We show that this does not hold for the majority of practical implementations where the underlying channel response is estimated based on the Received Signal Strength (RSS) values, and we identify the statistical defect that is primarily responsible for the statistical bias in the RSS values and discuss the source of this defect. Our observation implies that RSS-based physical layer key agreement schemes require a dedicated post-processing of the channel measurements together with an on-line statistical test suite. To address the latter, we define requirements for statistical testing in the context of physical layer key agreement. Based on these requirements, we select some well-known low complexity statistical tests to form a reduced test suite that requires less than 10,000 bit of input data. The selected tests have low complexity allowing to execute the test suite at the run-time even on severely resource-constrained devices. Our results show that it is possible to identify statistical defects with high reliability.
The use of wireless networks for communication has grown significantly in recent times, and continues to develop further. The broadcast nature of wireless communications makes them susceptible to a wide variety of security attacks. Unlike traditional solutions, which usually handle security at the application layer, the primary concern of this dissertation is to analyse and develop solutions for secure communication using channel coding techniques at the physical-layer. \nThe topic of physical layer authenticated encryption using high rate key generation through shared randomness is investigated in this work. First, a physical layer secret key generation scheme is discussed exploiting channel reciprocity in wireless systems. In order to address the susceptibility of this family of schemes to active attacks, a novel physical layer authentication encryption protocol is presented along with its extension to multi-node networks in the presence of active adversaries. Unlike previous work in the area of generating secret keys through shared randomness, it is demonstrated that the proposed scheme is semantically secure with respect to chosen plaintext and chosen cipher text attacks. \nSecondly, in order to increase the rate in bits per seconds at which agreed cryptographic keys are been generated, a multi-level quantization algorithm with public feedback is discussed. It is demonstrated that the proposed scheme is superior to direct information distillation approaches and can substantially increase the key generation rates even at low and medium SNRs. Furthermore, the employment of this low-overhead feedback at the information distillation process can largely simplify the information reconciliation process. \nThe proposed secret key generation schemes are tested for randomness such as required for cryptographic keys. The validation test is perfomed with the aid of National Institute of Standards and Technology (NIST) statistical test suite. The P-values obtained in each of the test carried out indicates that the key sequence generated by our algorithm is random.
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.
Abstract Wireless communications between two devices can be protected by secret keys. However, existing key generation schemes suffer from the high bit disagreement rate and low bit generation rate. In this paper, we propose an efficient physical layer key generation scheme by exploring the Received Signal Strength (RSS) of signals. In order to reduce the high mismatch rate of the measurements and to increase the key generation rate, a pair of transmitter and receiver separately apply adaptive quantization algorithm for quantifying the measurements. Then, we implement a randomness extractor to further increase key generation rate and ensure randomness of generated of keys. Several real-world experiments are implemented to verify the effectiveness of the proposed scheme. The results show that compared with the other related schemes, our scheme performs better in bit generation rate, bit disagreement rate, and randomness.
Physical layer security (PLS) techniques hold promise for augmenting secure communications in the 5thgeneration of mobile wireless networks. Secret key generation (SKG) is a PLS technique which exploits the wireless propagation channel's randomness to generate symmetric key bits for information encryption and decryption. This work proposes symmetric key generation based on non-uniform quantization of the received signal strength (RSS) samples of a Nakagami-m fading channel. The proposed strategy for non-uniform quantization for SKG aims to set quantization thresholds for maximal key randomness and high values of key generation rate (KGR) and key agreement probability (KAP). Finally, a framework is proposed to use single node RSS measurements, readily available in the literature, to generate RSS samples at the other link end. This framework facilitates the testing of new SKG algorithms that require simultaneous RSS measurements by the legitimate nodes, which are not readily available in the open literature. The effectiveness of the proposed SKG scheme is validated through Monte Carlo methods and the National Institute of Standards and Technology (NIST) test suite for assessing the randomness of the generated key sequence.
In physical-layer key generation (PLKG), researchers often assess key security using randomness tests or secret key capacity analysis. However, randomness tests can be misleading, as low-entropy inputs may still pass when obscured by hash functions or extractors. While key capacity captures the theoretical upper limit of secret bits from raw channels, it overlooks the impact of practical preprocessing. Recent studies have explored min-entropy as a more practical security metric. However, commonly used min-entropy estimators, such as those in NIST SP 800-90B (90B), may yield inaccurate results for complex sources, even overestimating entropy and thereby undermining security. Their applicability to PLKG also remains unverified. To address the above research gap, this paper focuses on modeling the received signal envelope, a common feature extracted for key generation. Leveraging typical wireless fading scenarios considered in key capacity analysis, we adopt the more general Weibull distribution to characterize the envelope statistics, which generalizes the Rayleigh model and better captures diverse propagation conditions. We first derive an explicit expression for the min-entropy in the absence of preprocessing, and then systematically analyze the impact of typical preprocessing operations on the statistical properties of the observed channel. Building on this foundation, we further establish theoretical upper and lower bounds on the min-entropy under representative preprocessing strategies. We further evaluate all 90B entropy estimators using both simulations and real-world channel measurements, comparing their estimates with our analytical results. While some estimators work well on raw data, they often fail or overestimate entropy after preprocessing, compromising security. In contrast, our conservative lower bound remains robust across all tested scenarios and preprocessing conditions. It offers a reliable fallback when standard estimators break down, providing a stronger theoretical foundation for secure key extraction in PLKG systems.
We evaluate the effectiveness of secret key extraction, for private communication between two wireless devices, from the received signal strength (RSS) variations on the wireless channel between the two devices. We use real world measurements of RSS in a variety of environments and settings. Our experimental results show that (i) in certain environments, due to lack of variations in the wireless channel, the extracted bits have very low entropy making these bits unsuitable for a secret key, (ii) an adversary can cause predictable key generation in these static environments, and (iii) in dynamic scenarios where the two devices are mobile, and/or where there is a significant movement in the environment, high entropy bits are obtained fairly quickly. Building on the strengths of existing secret key extraction approaches, we develop an environment adaptive secret key generation scheme that uses an adaptive lossy quantizer in conjunction with Cascade-based information reconciliation [7] and privacy amplification [14]. Our measurements show that our scheme, in comparison to the existing ones that we evaluate, performs the best in terms of generating high entropy bits at a high bit rate. The secret key bit streams generated by our scheme also pass the randomness tests of the NIST test suite [21] that we conduct.