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IEEE International Symposium on Information Theory (ISIT)

A secret key agreement scheme with multi-level quantization and parity check using fluctuation of radio channel property

Shimpei Yasukawa, Hisato Iwai, Hideichi Sasaoka

Published Jul 1, 2008• 27 citations
Publisher

Abstract

As a countermeasure for eavesdroppers in wireless communication, a secret key agreement scheme using a variable directional antenna named ESPAR antenna was developed. In this paper, we describe an improved key generation method using multi-level quantization and parity check in the secret key agreement scheme. It enhances key agreement probability in legitimate parties efficiently without reducing the security. There was a trade-off between the key agreement probability and the security. Simulation result shows that the proposed method realizes higher security than the conventional one in the key agreement system using ESPAR antenna.

DOI
10.1109/isit.2008.4595083

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As a countermeasure for eavesdroppers in wireless communication, a secret key agreement scheme using a variable directional antenna named ESPAR antenna was developed. In this paper, we describe an improved key generation method using multi-level quantization and parity check in the secret key agreement scheme. It enhances key agreement probability in legitimate parties efficiently without reducing the security. There was a trade-off between the key agreement probability and the security. Simulation result shows that the proposed method realizes higher security than the conventional one in the key agreement system using ESPAR antenna.
As a way to ensure security on wireless communication, a secret key agreement scheme exploiting the unique properties of the wireless medium, the reciprocity of radio propagation and the irregularity of multipath fading, is being proposed [1-3]. The method enables two legitimate users to share a secret key without distributing the actual key, by using the radio propagation channel between the two as a common and exclusive source of randomness upon which a secret key could be derived. As an application of this scheme to environments where the radio propagation characteristics are static, a method employing a variable directivity antenna called the ESPAR antenna [4] has been known to be effective. In this method, secret keys are generated from received signal strength indicator (RSSI) sequences of legitimate users. One known problem of this method is that in environments with simple radio propagation characteristics such as a rectangular room of concrete with no fixtures, the strength of the direct wave becomes the dominant factor affecting the fluctuation of the RSSI, thus increasing the chance for eavesdroppers located near the path of such waves to estimate the secret key. Since all previous considerations of the aforementioned scheme has been done with environments where the legitimate users are in line-of-sight, this paper aims to illustrate how the location of eavesdroppers affect the secrecy of keys generated with the scheme, in a radio propagation environment with no clearly dominant wave. The key capacity in respect to the position of the eavesdropper in such environment is shown using computer simulations.
We describe a secure communication scheme that uses the random fluctuation of the natural environment of communication channels. Only the transmitter and the receiver share the communication channel characteristics. From reciprocity between a transmitter and a receiver, it is possible for them to share one-time information of their fluctuating channel. This can provide a secret key agreement scheme without key management and key distribution processes. In this paper, we propose a new secret key generation and agreement scheme that uses the fluctuation of channel characteristics with an electronically steerable parasitic array radiator (ESPAR) antenna. This antenna, which has been proposed and prototyped, is a smart antenna designed for consumers. Using the beam-forming technique of the ESPAR antenna, we can increase the fluctuation of the channel characteristics. From experimental results, we conclude that the proposed scheme has the ability to generate secret keys from the received signal strength indicator (RSSI) profile with sufficient independence.
We consider secret key agreement based on radio propagation characteristics in two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a relay. In this system, Alice and Bob share secret keys generated from their radio propagation characteristics by the help of the relay. In this paper, we present two secret key agreement schemes: an amplify-and-forward (AF) scheme and a multiple-access amplify-and-forward (MA-AF) scheme. In both of the schemes, the basic idea is to share the effective fading coefficient between Alice and Bob and use it as the source of secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, while the MA-AF scheme utilizes the inherent combining of signals provided by simultaneous transmissions over a multiple-access channel in order to share the secret keys more securely efficiently. We analyze eavesdropping strategy in terms of eavesdropper's location and show that if the eavesdropper is located near the relay and can receive signals from the relay without multipath fading and noise, the AF scheme is not secure. Simulation results show that the MA-AF scheme is more secure and efficient than the AF scheme.
Generating a secret key between two parties by extracting the shared randomness in the wireless fading channel is an emerging area of research. Previous works focus mainly on single-antenna systems. Multiple-antenna devices have the potential to provide more randomness for key generation than single-antenna ones. However, the performance of key generation using multiple-antenna devices in a real environment remains unknown. Different from the previous theoretical work on multiple-antenna key generation, we propose and implement a shared secret key generation protocol, Multiple-Antenna KEy generator (MAKE) using off-the-shelf 802.11n multiple-antenna devices. We also conduct extensive experiments and analysis in real indoor and outdoor mobile environments. Using the shared randomness extracted from measured Received Signal Strength Indicator (RSSI) to generate keys, our experimental results show that using laptops with three antennas, MAKE can increase the bit generation rate by more than four times over single-antenna systems. Our experiments validate the effectiveness of using multi-level quantization when there is enough mutual information in the channel. Our results also show the trade-off between bit generation rate and bit agreement ratio when using multi-level quantization. We further find that even if an eavesdropper has multiple antennas, she cannot gain much more information about the legitimate channel.
Wireless communication is riskier than wired communication due to eavesdrop by unauthorized stations. To deal with this problem, public key cryptosystem and common key cryptosystem are used in general. In contrast, key agreement schemes based on information-theoretically security attract attention. One example is a secret key agreement scheme based on radio wave propagation characteristics. This technique uses channel reciprocity and location dependency of radio wave propagation. Most of the secret key agreement schemes generate keys by scalar quantization. However, secret keys generated by scalar quantization occur key disagreement in higher probability than these by vector quantization. A general quantizer selection scheme selects a quantizer with minimum key disagreement between two authorized stations. An eavesdropper is able to generate keys with this quantizer more easily. This paper assumes correlated MIMO channel, and proposed the quantizer utilizing correlation between authorized stations as key information and the quantizer selection scheme taking key entropies leak to eavesdroppers into consideration. This scheme that reduces information leaks to eavesdropper maintaining key disagreement probability between authorized stations.
We consider secret key agreement based on radio propagation characteristics in a two-way relaying system where two legitimate parties named Alice and Bob communicate with each other via a trusted relay. In this system, Alice and Bob share secret keys generated from measured radio propagation characteristics with the help of the relay in the presence of an eavesdropper. We present four secret key agreement schemes: an amplify-and-forward (AF) scheme, a signal-combining amplify-and-forward (SC-AF) scheme, a multiple-access amplify-and-forward (MA-AF) scheme, and an amplify-and-forward with artificial noise (AF with AN) scheme. In these schemes, the basic idea is to share the effective fading coefficients between Alice and Bob and use them as the source of the secret keys. The AF scheme is based on a conventional amplify-and-forward two-way relaying method, whereas in the SC-AF scheme and the MA-AF scheme, we apply the idea of physical-layer network coding to the secret key agreement. In the AF with AN scheme, the relay transmits artificially generated noise, as well as channel information signal, in order to conceal the latter. Simulation results show that the MA-AF scheme outperforms the other schemes in Rayleigh fading channels, whereas the AF with AN scheme is suitable for Rician fading channels.