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IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC)

Reciprocity-diversity trade-off in quantization for secret key generation

Iulia Tunaru, Benoît Denis, Bernard Uguen

Published Sep 1, 2014• 2 citations
Publisher

Abstract

Symmetric secret key generation from physical layer measurements has recently emerged as an alternative to computationally secure protocols or predistribution techniques for key sharing. This new approach takes advantage of the intrinsic properties of radio channels (e.g. reciprocity and spatial decorrelation) by quantizing measurable channel characteristics into a suite of codewords. However, noise or other transmission artifacts can degrade the link reciprocity and lead to bit errors between the users' codewords, which are strongly related to the quantization thresholds. In this paper, we present a framework for quantization in the context of key generation. In order to find a trade-off between reciprocity (an indicator of robustness) and codeword diversity (an indicator of randomness) under a fixed-length constraint, we investigate the non-uniform quantization and we show the interest of optimizing the quantization thresholds as a function of the signal-to-noise ratio.

DOI
10.1109/pimrc.2014.7136147

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Symmetric secret key generation from physical layer measurements has recently emerged as an alternative to computationally secure protocols or predistribution techniques for key sharing. This new approach takes advantage of the intrinsic properties of radio channels (e.g. reciprocity and spatial decorrelation) by quantizing measurable channel characteristics into a suite of codewords. However, noise or other transmission artifacts can degrade the link reciprocity and lead to bit errors between the users' codewords, which are strongly related to the quantization thresholds. In this paper, we present a framework for quantization in the context of key generation. In order to find a trade-off between reciprocity (an indicator of robustness) and codeword diversity (an indicator of randomness) under a fixed-length constraint, we investigate the non-uniform quantization and we show the interest of optimizing the quantization thresholds as a function of the signal-to-noise ratio.
New approaches that generate secret keys from radio channel characteristics have been recently proposed. The security of these schemes usually relies on the reciprocity principle which states that the channel fluctuations can be a source of common randomness for two communicating peers and these fluctuations can't be measured by any eavesdropper. A validation of these properties for indoor environments is presented in this work. The shared keys are created by measuring the reciprocal channel features and converting this information to binary vectors using a quantization algorithm. This paper addresses the problem of quantization. It identifies an important weakness of existing key generation algorithms and it shows that the secret bits extraction has a significant impact on the robustness and security of these algorithms. A new adaptive quantization algorithm for secret key generation is presented. This method has the advantages to create sufficient long secret keys with a high key agreement ratio between authorized users without revealing information to the attacker. The new scheme is experimentally validated using Ultra Wide Band technology.
Providing secure communication links between devices of low computational power has been increasingly investigated during recent years. The need for fast and easy-to-implement security for computationally weak wireless devices has lead to the development of physical layer based key generation approaches. The generation of symmetric cryptographic keys out of wireless channel properties turned out to be a promising approach comprising advantages of symmetric as well as asymmetric cryptography. Numerous quantization schemes have been proposed in previous works to increase the Key Generation Rate (KGR). Also by increasing the sampling rate, the input data can be generated faster. However, due to fast sampling rates, redundancy of subsequent bits will increase. This lowers the quality, that is, the randomness of the generated secret key and makes the system more vulnerable to brute-force attacks. We present and analyze different techniques for transforming a temporally correlated sequence into a compressed sequence of decorrelated bits and, therefore, assure non-redundant key sequences.
Wireless network security is crucial for modern communication systems, requiring robust security mechanisms to safeguard transmitted data. Physical layer security, leveraging channel characteristics for key generation, has emerged as a promising approach. In this paper, we study the process of generating and sharing keys among authorized users without compromising secrecy. However, existing methods face challenges, particularly with a high rate of errors in the generated keys and the risk of using the same key multiple times, which could compromise security. We have explored generating a unique key for each session. Our study examines the bit generation rate and verify the consistency of the secure key. We found the trade-off between the number of symbols and the quantization levels for quadrature amplitude modulation as well as binary phase shift keying. Further, the discussion delves into the impact of channel non-reciprocity on the process of secure key generation. Moreover, we achieved almost zero bit error rate at high signal-to-noise ratio.
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 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-mmm 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.
The physical layer secret key generation exploiting wireless channel reciprocity has attracted considerable attention in the past two decades. On-going research have demonstrated its viability in various radio frequency (RF) systems. Most of existing work rely on quantization technique to convert channel measurements into digital binaries that are suitable for secret key generation. However, non-simultaneous packet exchanges in time division duplex systems and noise effects in practice usually create random channel measurements between two users, leading to inconsistent quantization results and mismatched secret bits. While significant efforts were spent in recent research to mitigate such non-reciprocity, no efficient method has been found yet. Unlike existing quantization-based approaches, we take a different viewpoint and perform the secret key agreement by solving a bipartite graph matching problem. Specifically, an efficient dual-permutation secret key generation method, DP-SKG, is developed to match the randomly permuted channel measurements between a pair of users by minimizing their discrepancy holistically. DP-SKG allows two users to generate the same secret key based on the permutation order of channel measurements despite the non-reciprocity over wireless channels. Extensive experimental results show that DP-SKG could achieve error-free key agreement on received signal strength (RSS) with a low cost under various scenarios.