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IEEE Vehicular Technology Conference

Physical Layer Secret Key Capacity of Two Moving Terminals

Xu Wang, Mingliang Li, Liang Jin, Kaizhi Huang

Published Jun 1, 2017• 0 citations
Publisher

Abstract

When extracting secret key from shared wireless channel between two legitimate moving terminals, its capacity is affected by the time difference of channel sampling, terminal's mobility, and additive noise. In order to quantitatively analyze their effects, a closed-form solution to physical layer secret key capacity in the time domain is derived over the uniform scattering environment. Afterwards, simulations in the uniform and non-uniform scattering environment are carried out to demonstrate its validity and applicability, respectively. Simula-tion results show that the derivation can be utilized to provide theoretical reference for physical layer secret key extraction between two moving terminals located among dense scatters.

DOI
10.1109/vtcspring.2017.8108519

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When extracting secret key from shared wireless channel between two legitimate moving terminals, its capacity is affected by the time difference of channel sampling, terminal's mobility, and additive noise. In order to quantitatively analyze their effects, a closed-form solution to physical layer secret key capacity in the time domain is derived over the uniform scattering environment. Afterwards, simulations in the uniform and non-uniform scattering environment are carried out to demonstrate its validity and applicability, respectively. Simula-tion results show that the derivation can be utilized to provide theoretical reference for physical layer secret key extraction between two moving terminals located among dense scatters.
When extracting physical layer security secret key with multiple wireless channel samples, its capacity is affected by the time difference of channel sounding, terminal's moving speed, sampling period, the number of samples, and additive noise. In order to quantitatively analyze their effects and determine the constrains on the optimal sampling period, a closed-form solution to the physical layer secret key capacity in the time domain is derived over the uniform scattering environment. Simulation results indicate that the results of this paper can be applied to both the uniform and non-uniform scattering environment. Furthermore, the feasibility to extract physical layer secret key from the commercial TD-LTE and the promising co-frequency co- time full duplex communication systems is verified.
Physical layer secret key capacity is affected by such factors as additive noise, the time difference of channel sampling, terminal's moving speed, sampling period, and the number of samples, whose effects on the physical layer secret key capacity are analyzed quantitatively using the single-input single-output wireless channel over the uniform scattering environment. Specifically, a closed-form solution to the secret key capacity is derived to determine the constraints on the optimal sampling period. Analysis and simulation results reveal that the results can also be applied to the nonuniform scattering environment. Furthermore, the feasibility to utilize the physical layer secret key extraction techniques in the mobile communication systems is verified.
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.
As an alternative and appealing approach to providing information security in wireless communication systems, secret key generation at physical layer has demonstrated its potential in terms of efficiency and reliability over traditional cryptographic methods. Without the necessity of a management centre for key distribution or reliance on computational complexity, physical layer key generation protocols enable two wireless entities to extract identical and dynamic keys from the randomness of the wireless channels associated with them.\nIn this thesis, the reliability of secret key generation at the physical layer is examined in practical wireless channels with imperfect channel state information (CSI). Theoretical analyses are provided to relate key match rate with channel's signal-to-noise ratio (SNR), degrees of channel reciprocity, and iterations of information reconciliation.\nIn order to increase key match rate of physical layer secret key generation, improved schemes in the steps of channel estimation and sample quantization are proposed respectively. In the channel estimation step, multiple observations of the wireless channels are integrated with a linear processor to provide a synthesized and more accurate estimation of the wireless channel. In the sample quantization step, a magnitude based quantization method with two thresholds is proposed to quantize partial samples, where specific quantization areas are selected to reduce cross-over errors. Significant improvements in key match rate are proven for both schemes in theoretical analysis and numerical simulations. Key match rate can even achieve 100% in both schemes with the assistance of information reconciliation process.\nIn the end, a practical application of physical layer secret key generation is presented, where dynamic keys extracted from the wireless channels are utilized for securing secret data transmission and providing efficient access control.
Physical layer key generation (PLKG) has emerged as a promising strategy for securing wireless communication using shared channel knowledge between two legitimate users. Traditional PLKG systems rely on time-frequency domain channels using metrics such as Received Signal Strength Indicator (RSSI) or Channel State Information (CSI) to generate secure keys. As a result of the rapid variation in the time-frequency domain channels, these systems are prone to error and suffer from a high reconciliation overhead requirement. Conversely, the same channels have a sparse and static representation in the delay-Doppler domain, which is utilized by the recently introduced Orthogonal Time Frequency Space (OTFS) modulation scheme. Furthermore, the channel representation in the delay-Doppler domain is uniquely determined by the scattering environment between the transmitter-receiver pair. In this paper, we leverage the sparsity of the delay-Doppler domain channel as well as the uniqueness of the channel representation to introduce two OTFS based PLKG schemes. We show through extensive simulations that the introduced OTFS schemes can get < 10% bit disagreement ratio (BDR) in scenarios where time-frequency domain based RSSI methods break down.
Secret key generation (SKG) from the randomness provided by random channels is currently considered as one way to improve security in wireless communications at the physical layer level. However, the relation between the performance of SKG schemes and the characteristics of the radio channel has been moderately investigated. In this work, we evaluate the security performance on the basis of a simple channel model based on scatterers distributed around terminals, which enables going beyond the common assumption of spatial stationarity between the legitimate terminal and the eavesdropper. This performance is assessed both from information theory metrics and from a practical key extraction algorithm. Owing to the model flexibility, we evaluate the impact of several major channel characteristics such as the distribution and density of scatterers or the existence of a dominant LOS path.