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Security and Privacy

On the trade‐off between rate and reliability for secret key generation in wireless channels

Magnus Sandell

Published Jun 30, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1002/spy2.439

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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.
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.
A secret key can be generated between two legitimate parties from observations of the common wireless channel. However, key disagreement usually occurs due to the estimation errors. Considering a guard band, the key disagreement rate (KDR) is firstly derived in a closed‐form expression. Although increasing the size of the guard band can decrease KDR; the length of the raw key bit sequence is also shortened. To evaluate such an effect, the raw key generation rate is defined and derived. The simulation results verify the theoretical analysis and are useful for the determination of pilot power and guard band size.
Providing extra security in wireless communication networks using channel impulse response as a source of common randomness is an appealing direction in prospective systems. While some theoretical and experimental work has demonstrated its potential, a number of issues remain unexplored. This paper is concerned with the reliability of secret key generation under practical estimation requirements. We provide an analytical expression which relates the channel SNRs and degree of reciprocity to probability of key mismatch. In the next step we show how this affects key reconciliation and secrecy leaking. Finally, we consider how quantization with a protective band can help to trade the secret key rate for increased reliability and security of the scheme.
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.
There is growing interest in wireless security methods that provide strong or even perfect secrecy by taking advantage of features of the physical propagation channel. In advantage-based methods, high channel quality in an average or opportunistic sense is exploited between two legitimate nodes, such that nonzero secrecy capacity can be achieved. Since such methods require bounds on the quality of the eavesdropper channel, they are somewhat impractical. Secret key generation based on tracking channel evolution in time division duplex systems is a more attractive option, where two nodes generate secret key bits based on a mutually known random channel. Since the eavesdropper channel is typically independent of the legitimate channel, the key can only be broken by brute force attacks, which are difficult when new keys are continuously generated. In this paper, the information theoretic limits of key generation schemes are investigated, based on the level of estimation error, temporal correlation, and dependence of the eavesdropper and legitimate channels. Three practical candidate key generation schemes are also considered: channel quantization and channel quantization with guardband.
Secure wireless communications is a challenging problem due to the shared nature of the wireless medium. Most existing security protocols apply cryptographic techniques for bit scrambling at the application layer by exploiting a shared secret key between pairs of communicating nodes. However, more recent research argues that multipath propagation - a salient feature of wireless channels - provides a physical resource for secure communications. In this context, we propose a protocol that exploits the inherent randomness in multipath wireless channels for generating secret keys through channel estimation and quantization. Our approach is particularly attractive in wideband channels which exhibit a large number of statistically independent degrees of freedom (DoF), thereby enabling the generation of large, more-secure, keys. We show that the resulting keys are distinct for distinct pairwise links with a probability that increases exponentially with the key-size/channel DoF. We also characterize the probability that the two users sharing a common link generate the same key. This characterization is used to analyze the energy consumption in successful acquisition of a secret key by the two users. For a given key size, our results show that there is an optimum transmit power, and an optimum quantization strategy, that minimizes the energy consumption. The proposed approach to secret key generation through channel quantization also obviates the problem of key pre-distribution inherent to many existing cryptographic approaches.