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European Conference on Antennas and Propagation

Experimental characterization of channel-based key establishment using reconfigurable antennas

Rashid Mehmood, Jon W. Wallace, Michael A. Jensen

Published Apr 1, 2014• 1 citation
Publisher

Abstract

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.

DOI
10.1109/eucap.2014.6901788

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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.
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.
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.
While secret encryption keys are commonly-used for securing wireless transmissions, establishment of such keys has traditionally been accomplished using computational techniques at the higher layers of the wireless communication protocol stack. However, the multipath electromagnetic propagation typical of many environments can also be an excellent source of random variables useful for encryption key generation. Furthermore, antennas play an important role in how the transmitter and receiver excite or sense the channel and therefore become a critical component in the performance of techniques that exploit the multipath channel for key establishment. This paper focuses on how using multiple antennas can dramatically improve the performance of such key establishment methods.
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].
An emerging area of research in wireless communications is the generation of secret encryption keys based on the shared (or common) randomness of the wireless channel between two legitimate nodes in a communication network. However, to date, little work has appeared on methods to use the increased randomness available when the network nodes have multiple antennas. This paper provides theoretical performance bounds associated with using multi-antenna communications and proposes two practical methods for generating secret keys exploiting the increased randomness. Performance simulations reveal the efficiency of the methods.