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Moscow Workshop on Electronic and Networking Technologies (MWENT)

Analisys of frequency-correlation properties of multipath channel for encyprion key generation using samples of differential phase

Сулимов Амир Ильдарович, Oleg N. Sherstyukov, A. V. Karpov

Published Mar 1, 2018• 1 citation
Publisher

Abstract

Wireless Key Generation exploits randomness of fast fading of a multipath radio channel to create identical copies of a shared encryption key at two communication nodes. A promising way for creating a highly secure key is use of samples of differential phase, which allows to overcome short-term instability of frequency standards and to make key generation devices smaller. This study examines frequency-correlation properties of the multipath channel to justify a feasibility of encryption keys generation with the differential phase method. By computer simulation, frequency autocorrelation functions of the envelope and carrier phase of a multipath radio signal are obtained, and estimates of the channel coherence bandwidth are made for a typical urban propagation environment. For random variations of the differential phase of a two-sine probe signal, a probability distribution is analyzed, its uniformity tests are done, and estimates of Shannon entropy at various frequency separations of the two probing tones are made. An effect of the line-of-sight wave and the number of multipaths on the channel frequency-correlation function and on probabilistic properties of the differential phase is considered.

DOI
10.1109/mwent.2018.8337300

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Wireless Key Generation exploits randomness of fast fading of a multipath radio channel to create identical copies of a shared encryption key at two communication nodes. A promising way for creating a highly secure key is use of samples of differential phase, which allows to overcome short-term instability of frequency standards and to make key generation devices smaller. This study examines frequency-correlation properties of the multipath channel to justify a feasibility of encryption keys generation with the differential phase method. By computer simulation, frequency autocorrelation functions of the envelope and carrier phase of a multipath radio signal are obtained, and estimates of the channel coherence bandwidth are made for a typical urban propagation environment. For random variations of the differential phase of a two-sine probe signal, a probability distribution is analyzed, its uniformity tests are done, and estimates of Shannon entropy at various frequency separations of the two probing tones are made. An effect of the line-of-sight wave and the number of multipaths on the channel frequency-correlation function and on probabilistic properties of the differential phase is considered.
Channel randomness in wireless communication systems can be employed for generating two copies of a key bits sequence in a given pair of nodes. Among others, the differential phase method that uses phase samples of two coherent frequency tones for generating high-entropy key bits sequence seems to be a promising technique. Unfortunately, implementation of the differential phase method relies on correct specification of the frequency separation of the probing tones, which requires additional study of the frequency-correlation properties of the propagation medium. This paper is devoted to experimental study of the channel frequency-correlation properties in a typical indoor environment. Frequency autocorrelation functions are experimentally determined for the signal envelope and carrier phase on four different indoor radio links. The estimates of the channel coherence bandwidth and multipath delay spread are performed, and the influence of the line-of-sight component (LOS) is considered. Our experiments showed that, depending on the LOS intensity, the coherence bandwidth on typical indoor radio links of 20-meters length varies from 8 MHz to 25 MHz (for the correlation level of 0.2), whereas for samples of the carrier phase, the coherence bandwidth is somewhat narrower and varies from 6 MHz to 19 MHz.
The paper addresses a problem of creating a system for generation and distribution of shared secret encryption key based on the physical properties of multipath radio propagation. A multipath radio channel with reciprocal properties is considered as a source of shared randomness, as the phase of the passed through multipath environment signal becomes unpredictable. The paper presents the results of experimental verification of feasibility of the generation and distribution of shared secret encryption key based on measurements of random phase of a multipath signal. The problem of checking the symmetry of instances of the generated shared encryption key through the exchange of cyclic redundancy codes is considered. The probability of a bit asymmetry of the generated from the experimental phase measurements instances of the shared encryption key is evaluated. The effectiveness of encryption key distribution process is estimated.
The paper addresses a problem of creating a system for generation and distribution of shared secret encryption key based on the physical properties of multipath radio propagation. A multipath radio channel with reciprocal properties is considered as a source of shared randomness, as the phase of the passed through multipath environment signal becomes unpredictable. The paper presents a general block diagram of encryption key generation process. A method for estimating the rate of common secrete encryption key generation is proposed. An influence of physical parameters of the multipath environment on the encryption key generation rate is considered.
The Wireless Key Distribution is one of the most promising and fast growing areas in modern applied cryptography. This area covers various techniques of secure secret key distribution between two legitimate users who share a common radio channel with unpredictable signal fading in a multipath environment. In essence, the pair of legitimate nodes uses their multipath radio channel as a source of common randomness to establish a shared encryption key. There are a number of studies have been presented in recent publications devoted to experimental implementation of the Wireless Key Distribution using random variations in the received power of fading signal. Despite a number of valuable benefits, there is a much fewer experimental verifications of phase method with all of them are limited to a key distribution within some indoor environments only. Apparently, this is due to the technical difficulties of precise synchronization of legitimate users' equipment to provide coherent carrier phase measurements in a microwave radio frequency range. In this regard, our experiments can be considered as the first experimental verification of secure Wireless Key Distribution by observing random variations in the carrier phase of multipath signal at moving a mobile user within a real outdoor environment. To perform this, we used wireless Internet transmission of concurrent service data to maintain a required level of synchronization of one stationary and one mobile legal nodes. Despite the humble key generation rates we have achieved in practice, our results show possibility of secure wireless key distribution between the base station and mobile subscriber in a cellular communications scenario.
The paper presents the results of experimental distribution of encryption keys based on random carrier phase of fading radio signal measured in a multipath environment. The random bits extraction scheme was proposed and tested in practice. The proposed scheme is universal and applicable to measurements digitizing of any observable random variable. Experimental study of spatial correlation of multipath signal phase in the case of transverse spatial diversity is carried out. Experimental estimation of the key generation rate and the probability of its passive interception at different distances between the legal user and potential eavesdropper are also performed. It is shown that the parameters of bit extraction procedure significantly affect on the performance and security of the key distribution process.
A less expensive and more flexible solution to the problem of sharing secret keys between wireless nodes is to use the inherent randomness in the wireless channel between them as the source for extracting bits of the secret key between these nodes. We propose a new secret key distribution approach that utilizes the uniformly distributed phase information of channel responses to extract shared cryptographic keys under narrowband multipath fading models. Compared to existing approaches which only support pair wise key distribution, our scheme is highly scalable and can support efficient group key distribution. Various key distribution schemes are appraised to determine the level of security and connectivity. Simulated results show that key distribution exploiting channel phase has improved resilience against node capture. them as the source for extracting bits of the secret key between these nodes. Shared secret key distribution from channel measurements is an application which benefits from the randomnessof the multipath channel. It would not, for example, work in a truly free-space environment (such as deep space radio links). Secret sharing benefits from: • Reciprocity of the wireless radio channel: The multipath properties of the radio channel (gains, phase shifts, and delays) at any point in time and on any given frequency channel are identical on both directions of the link. • Temporal variations in the radio channel: Over time, the multipath channel changes due to movement of either end of the link, and any motion of people and objects in the environment near the link. An application may specifically request a user to move or shake the wireless device in order to generate more temporal variation. • Spatial variations: The properties of the radio channel are unique to the locations of the two endpoints of the link. An eavesdropper at a third location more than a few wavelengths from either endpoint will measure a different, uncorrelated radio channel. The proposed scheme is more flexible and can be applied in both static and mobile environments. Our scheme introduces phase randomness to bit distribution and removes the reliance on node mobility to obtain high entropy bits. We evaluate the proposed schemes through both analytical and simulation studies. The parameters of the scheme can be selected such that a desired level of key distribution accuracy and reliability is achieved with high efficiency.