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European Conference on Antennas and Propagation

Experimental assessment of secret key generation using parasitic reconfigurable aperture antennas

Rashid Mehmood, Jon W. Wallace

Published Mar 1, 2012• 10 citations
Publisher

Abstract

The potential of automatically generating secret keys by randomly changing the state of a reconfigurable antenna is investigated through direct measurement. Measurements are performed with a 5×5 parasitic reconfigurable aperture (RECAP) antenna at 2.54 GHz in an indoor laboratory environment, illustrating that keys can be generated with physical-layer reciprocal channel key generation (RCKG) even in static or line-of-sight (LOS) conditions. Details of the parasitic array, varactor-diode reconfigurable elements, and network-analyzer based measurement setup are presented. Analysis of the data shows that secure key bits is lower when an eavesdropper is in the LOS path between legitimate nodes as opposed to a direction perpendicular to that path. It is also illustrated that the statistics of fading induced with the reconfigurable array are not strictly Gaussian for LOS channels. Encouragingly, measurements show that around 80% of generated key bits are secure even in the worst case conditions.

DOI
10.1109/eucap.2012.6206173

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The potential of automatically generating secret keys by randomly changing the state of a reconfigurable antenna is investigated through direct measurement. Measurements are performed with a 5×5 parasitic reconfigurable aperture (RECAP) antenna at 2.54 GHz in an indoor laboratory environment, illustrating that keys can be generated with physical-layer reciprocal channel key generation (RCKG) even in static or line-of-sight (LOS) conditions. Details of the parasitic array, varactor-diode reconfigurable elements, and network-analyzer based measurement setup are presented. Analysis of the data shows that secure key bits is lower when an eavesdropper is in the LOS path between legitimate nodes as opposed to a direction perpendicular to that path. It is also illustrated that the statistics of fading induced with the reconfigurable array are not strictly Gaussian for LOS channels. Encouragingly, measurements show that around 80% of generated key bits are secure even in the worst case conditions.
The ability of parasitic reconfigurable aperture (RECAP) antennas to generate secret keys is investigated, which allows physical-layer reciprocal channel key generation (RCKG) methods to be employed even in the case of static and line-of-sight channels. Since the artificial channel fluctuations created by RECAP structures are not necessarily Gaussian, a numerical procedure for computing available and secure key bits is developed that is applicable to channels with arbitrary fading. It is identified that for limited RECAP complexity, a reduced-complexity brute-force attack is possible, and a lower bound on the required RECAP complexity to avoid this possibility is developed. Numerical examples of a 9×9 parasitic RECAP with varying levels of complexity illustrate the importance of controlling the reflection coefficient to ensure Gaussian statistics, the need for sufficient complexity to attain maximum secure key bits, and the importance of placing the RECAP on the node near the eavesdropper.
When establishing secret encryption keys using estimates of the reciprocal wireless channel, the number of key bits that can be generated for a static or slowly varying propagation environment can be enhanced by randomly changing the radiation properties of a reconfigurable antenna. However, prior studies of this approach have been limited and have not considered a high degree of antenna reconfigurability. This paper uses simulations and experimental measurements to characterize the impact of reconfigurable antenna complexity on the performance of key establishment in different static propagation environments and in the presence of a multi-antenna eavesdropper. The results demonstrate that reconfigurable antennas can significantly enhance the security, even when the eavesdropper antennas are adjacent to or surround one of the legitimate nodes. The results further demonstrate that increasing the number of reconfigurable parasitic elements notably increases the achieved performance.
Secure transmission is one of the most important aspects of wireless communications systems. In contrast to traditional cryptographic methods, physical layer security exploits the propagation channel to ensure security. It has been proven that a time-varying reciprocal channel can be used to generate secret keys which are secure even when an eavesdropper is located near one of the communicating nodes [1]. For line-of-sight (LOS) and slow-fading channels, reconfigurable aperture (RECAP) antennas have demonstrated their utility for physical layer security [2].
Information theoretic limits for random key generation in multiple-input multiple-output (MIMO) wireless systems exhibiting a reciprocal channel response are investigated experimentally with a new three-node MIMO measurement campaign. As background, simple expressions are presented for the number of available key bits, as well as the number of bits that are secure from a close eavesdropper. Two methods for generating secret keys are analyzed in the context of MIMO channels and their mismatch rate and efficiency are derived. A new wideband indoor MIMO measurement campaign in the 2.51- to 2.59-GHz band is presented, whose purpose is to study the number of available key bits in both line-of-sight and nonline-of-sight environments. Application of the key generation methods to measured propagation channels indicates key generation rates that can be obtained in practice for four-element arrays.
There is growing interest in physical-layer security methods that exploit the random nature of the physical propagation channel to strengthen existing crypto-systems. In reciprocal channel key generation (RCKG), legitimate nodes (Alice and Bob) observe a common fluctuating channel to generate keys that are safe from the eavesdropper (Eve). Previous work [1, 2] derived expressions for the available key bits and those safe from an eavesdropper for MIMO systems with correlated complex Gaussian statistics. This paper applies these expressions to new MIMO measurements in indoor LOS and NLOS environments, indicating how propagation effects limit secure key generation and what key generation rates can be expected in practice.
Generation of secret encryption keys from the reciprocal electromagnetic channel represents an emerging topic within the broad field of propagation channel research. This paper uses multi-antenna channel measurements to assess the potential performance of such techniques, with the emphasis on using a reconfigurable antenna element controlled by the channel sounder to create time-variation in the wireless channel that enables establishment of long encryption keys. The results demonstrate that reconfigurable antennas can significantly enhance the security, even when the eavesdropper antennas are adjacent to or surround one of the legitimate nodes.