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Analyse et modélisation du canal radio pour la génération de clés secrètes

Taghrid Mazloum

Published Feb 12, 2016• 0 citations
Publisher

Abstract

Nowadays, the security of ubiquitous wireless communications becomes more and more a crucial requirement. Even though data is widely protected via symmetric ciphering keys, a well-known difficulty is the generation and distribution of such keys. In the recent years therefore, a set of works have addressed the exploitation of inherent characteristics of the fading propagation channel for security. In particular, secret keys could be generated from the wireless channel, considered as a shared source of randomness, available merely to a pair of communicating entities. ln the present dissertation, we are interested in the approach of secret key generation (SKG) from wireless channels, especially in relating the radio channel properties to the generated keys quality. We first develop a stochastic channel model, focusing on the security with respect to the eavesdropper side, which shows a residual channel memory weil beyond a few wavelengths distance (spatially nonstationary scenarios). Then, we analyze the channel degrees of freedom (DoF) and their impact on the SKG performance in different channel conditions, especially by considering more realistic channels in both outdoor and indoor environments (respectively through simulated ray tracing data and through measurements). The results show that, even for moderately wide band (such as standardized in IEEE 802.11), the sole frequency DOF or its association with the spatial DOF is often enough for generating long keys, provided an efficient quantization method of the complex channel coefficients is used.

DOI
10.70675/5eb01df3zbc5bz4671z9a4fz1351b615dfb0

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Nowadays, the security of ubiquitous wireless communications becomes more and more a crucial requirement. Even though data is widely protected via symmetric ciphering keys, a well-known difficulty is the generation and distribution of such keys. In the recent years therefore, a set of works have addressed the exploitation of inherent characteristics of the fading propagation channel for security. In particular, secret keys could be generated from the wireless channel, considered as a shared source of randomness, available merely to a pair of communicating entities. ln the present dissertation, we are interested in the approach of secret key generation (SKG) from wireless channels, especially in relating the radio channel properties to the generated keys quality. We first develop a stochastic channel model, focusing on the security with respect to the eavesdropper side, which shows a residual channel memory weil beyond a few wavelengths distance (spatially nonstationary scenarios). Then, we analyze the channel degrees of freedom (DoF) and their impact on the SKG performance in different channel conditions, especially by considering more realistic channels in both outdoor and indoor environments (respectively through simulated ray tracing data and through measurements). The results show that, even for moderately wide band (such as standardized in IEEE 802.11), the sole frequency DOF or its association with the spatial DOF is often enough for generating long keys, provided an efficient quantization method of the complex channel coefficients is used.
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.
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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.
In the recent years, a set of works have addressed inherent characteristics of the fading propagation channel in an information-theoretic framework oriented towards security. In particular, for cryptographic purposes, secure key bits can be extracted from reciprocal radio channels, seen as a shared source of randomness between any two entities. In this paper we intend to assess the impact of true radio channel features on the security performance, by quantizing the complex channel coefficients into discrete key bits. To this end, indoor channel measurements were performed and investigated in different scenarios, e.g. LOS/NLOS condition, narrowband (NB) as well as wide band (WB) targeting OFDM systems. Secret key robustness is firstly studied for the legitimate terminals in terms of key bit disagreement ratio and key randomness, and secondly by considering an eavesdropper in a wide set of positions around these terminals and by evaluating the difference between the generated keys. A simplified channel model is also analyzed in comparison with the measured channels, as regards the characteristics of generated key bits.
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.
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.