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IEEE Vehicular Technology Conference

Robust secret key generation for frequency-selective channels

Magnus Sandell

Published Jun 1, 2022• 1 citation
Publisher

Abstract

One effective way of generating and distributing secret keys for secure communication is to exploit the randomness of wireless channels. Since a potential eavesdropper has a different propagation channel than two legitimate users, it can be used to generate unique and secure keys. After reciprocal channel sounding, a frequency-selective channel can be quantised to produce the secret keys. Due to different sampling times at the two legitimate devices, the phase of the frequency response will not be equal and hence compromise the secret key generation. In this paper, we show how to overcome this problem and verify the results through both simulations and hardware measurements. We also compare scalar and vector quantisation, and show how the latter can be designed to be robust to synchronisation offsets.

DOI
10.1109/vtc2022-spring54318.2022.9860793

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One effective way of generating and distributing secret keys for secure communication is to exploit the randomness of wireless channels. Since a potential eavesdropper has a different propagation channel than two legitimate users, it can be used to generate unique and secure keys. After reciprocal channel sounding, a frequency-selective channel can be quantised to produce the secret keys. Due to different sampling times at the two legitimate devices, the phase of the frequency response will not be equal and hence compromise the secret key generation. In this paper, we show how to overcome this problem and verify the results through both simulations and hardware measurements. We also compare scalar and vector quantisation, and show how the latter can be designed to be robust to synchronisation offsets.
One effective way of generating and distributing secret keys for secure communication is to exploit the randomness of wireless channels. Since a potential eavesdropper has a different propagation channel than two legitimate users, it can be used to generate unique and secure keys. If the two devices are far apart, a relay can aid the process. This paper presents a novel precoding scheme for the relay which delivers the keys directly and with very little processing required at the end devices. We also compare scalar and vector quantisation and show that a tree-based vector quantisation scheme has better performance than the former but much lower complexity than the latter.
Wireless channel reciprocity can be successfully exploited as a common source of randomness for the generation of a secret key by two legitimate users willing to achieve confidential communications over a public channel. This paper presents an analytical framework to investigate the theoretical limits of secret-key generation when wireless multi-dimensional Gaussian channels are used as source of randomness. The intrinsic secrecy content of wide-sense stationary wireless channels in frequency, time and spatial domains is derived through asymptotic analysis as the number of observations in a given domain tends to infinity. Some significant case studies are presented where single and multiple antenna eavesdroppers are considered. In the numerical results, the role of signal-to-noise ratio, spatial correlation, frequency and time selectivity is investigated.
Abstract Wireless propagation channels can be used to obtain secret keys to enhance security. By exploiting the reciprocity of the channel between legitimate users and its independence of an eavesdropper's channel, it can be quantised to produce a string of bits. The quality of these bits can be enhanced by using guard bands when quantising, which means that some samples are not used due to their poor quality. Although this increases the probability that the legitimate users produce the same bits, it also reduces the number of generated bits. We derive the optimal trade‐off between these contrasting quantities, first for a single sample and then extended to a frequency‐selective channel. The results are presented for both software simulations and hardware evaluations.
A few years ago, physical layer based techniques have started to be considered as a way to improve security in wireless communications. A well known problem is the management of ciphering keys, both regarding the generation and distribution of these keys. A way to alleviate such difficulties is to use a common source of randomness for the legitimate terminals, not accessible to an eavesdropper. This is the case of the fading propagation channel, when exact or approximate reciprocity applies. Although this principle has been known for long, not so many works have evaluated the effect of radio channel properties in practical environments on the degree of randomness of the generated keys. To this end, we here investigate indoor radio channel measurements in different environments and settings at either 2.4625 GHz or 5.4 GHz band, of particular interest for WIFI related standards. Key bits are extracted by quantizing the complex channel coefficients and their randomness is evaluated using the NIST test suite. We then look at the impact of the carrier frequency, the channel variability in the space, time, and frequency degrees of freedom used to construct a long secret key, in relation to the nature of the radio environment such as the LOS/NLOS character.
Physical layer security key generation exploits unpredictable features from wireless channels to achieve high security, which requires high reciprocity in order to set up symmetric keys between two users. This paper investigates enhancing the channel reciprocity using a loop-back scheme with multiple frequency bands in time-division duplex (TDD) communication systems, in order to mitigate the effect of hardware fingerprint interference and synchronization offset. The scheme is evaluated to be robust to passive eavesdropping and active Man-in-the-Middle attack through both theoretical analyses and practical measurements. A secret key generation protocol is subsequently designed. The performance of the proposed secret key generation method is then evaluated through both numerical simulation and experiments. Results demonstrate that the proposed scheme can effectively mitigate non-reciprocity and outperforms the classical TDD scheme in both key disagreement rate and key generation rate.
The broadcast nature of wireless communications imposes the risk of information leakage to adversarial users or unauthorized receivers. Therefore, information security between intended users remains a challenging issue. Most of the current physical layer security techniques exploit channel randomness as a common source between two legitimate nodes to extract a secret key. In this paper, we propose a new simple technique to generate the secret key. Specifically, we exploit the estimated channel to generate a secondary random process (SRP) that is common between the two legitimate nodes. We compare the estimated channel gain and phase to a preset threshold. The moving differences between the locations at which the estimated channel gain and phase exceed the threshold are the realization of our SRP. We simulate an orthogonal frequency division multiplexing (OFDM) system and show that our proposed technique provides a drastic improvement in the key bit mismatch rate (BMR) between the legitimate nodes when compared to the techniques that exploit the estimated channel gain or phase directly. In addition to that, the secret key generated through our technique is longer than that generated by conventional techniques.