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IEEE Transactions on Wireless Communications

Key Establishment Employing Reconfigurable Antennas: Impact of Antenna Complexity

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

Published Jun 19, 2014• 35 citations
Publisher

Abstract

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.

DOI
10.1109/twc.2014.2330835

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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.
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.
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.
Propagation-based key establishment is a physical layer method for generating encryption keys based on two radios observing a reciprocal propagation channel. This work explores the impact of mutual coupling when communicating nodes are equipped with multiple antennas, multipath richness in the propagation environment, and practical limitations caused by the nonreciprocal nature of RF circuits on key establishment. First, network theory is used to formulate a model of a realistic communication system which incorporates transmit sources and receive loads, impedance matching networks, low-noise amplifiers (LNAs), mutually coupled antenna arrays, and a passive eavesdropper. Afterwards, a detailed analysis is performed to quantify the impact of coupling, type of impedance matching network, and proximity of a multi-antenna eavesdropper on key rate metrics. Next, the degradation on key establishment caused by the radiocircuitry non-reciprocal contributions to the propagation channel is analyzed. A calibration technique based on total least square algorithm is used to overcome the non-reciprocity. Results demonstrate that the method is highly effective in removing the impact of non-reciprocal circuit contributions over a range of operational parameters. Lastly, for key establishment, the propagation conditions can cause the available key rate to be significantly different from the secure key rate which takes into account the presence of a passive eavesdropper. To study this in detail, a realistic multiple-input multiple-output (MIMO) propagation environment is modeled for two communicating radios and an eavesdropper. Afterwards different propagation conditions are assumed and results demonstrate that secure key rate converges to available key rate when K-factor is small and the eavesdropper is not located very close (< 2.5 wavelength) to one of the nodes.
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.
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.