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Digest - IEEE Antennas and Propagation Society. International Symposium

Secure key establishment in the presence of a passive eavesdropper: Impact of antenna coupling

Attiya Mahmood, Michael A. Jensen

Published Jul 1, 2015• 0 citations
Publisher

Abstract

While the establishment of encryption keys based on reciprocal electromagnetic propagation has received recent attention, work has so far ignored the impact of mutual coupling. This paper formulates the rate at which key bits can be generated incorporating the effects of antenna mutual coupling and a passive eavesdropper that is close to one of the radios.

DOI
10.1109/aps.2015.7304446

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While the establishment of encryption keys based on reciprocal electromagnetic propagation has received recent attention, work has so far ignored the impact of mutual coupling. This paper formulates the rate at which key bits can be generated incorporating the effects of antenna mutual coupling and a passive eavesdropper that is close to one of the radios.
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.
When generating encryption keys using the reciprocal propagation channel, the secure key rate bounding the number of bits that can be generated securely differs from the available key rate that ignores the presence of an eavesdropper. To explore the impact of propagation on this difference, this work formulates a model for computing the secure and available key rates for a multiuser scenario involving two legitimate nodes and a passive eavesdropper. Application of the model to a variety of different propagation conditions demonstrates that the secure and available key rates differ appreciably when the K-factor is large or when the separation between the eavesdropper and one of the legitimate nodes is small.
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.
This work integrates electromagnetic analysis with circuit theory to explore the impact of antenna mutual coupling and radio frequency circuit design on the achievable performance of methods that establish secret encryption keys using reciprocal propagation channels. The analysis includes the impact of the impedance match between the coupled antennas and the transmit sources or receiver loads, demonstrating that an optimally-designed matching network can significantly improve the achieved performance. The model is then used to account for the fact that transceiver circuit paths differ during transmission and reception, therefore destroying the channel reciprocity required for key establishment. A total least squares method for removing the impact of the non-reciprocal contributions to the channel is then applied, with the results showing that the algorithm is able to achieve key rates similar to those obtained when the non-reciprocal contributors are known perfectly.
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.