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Security and eavesdropping in terahertz wireless links

Abstract

Resiliency against eavesdropping and other security threats has become one of the key design considerations for communication systems. As wireless systems become ubiquitous, there is an increasing need for security protocols at all levels, including software (such as encryption), hardware (such as trusted platform modules) and the physical layer (such as wave-front engineering)1,2,3,4,5. With the inevitable shift to higher carrier frequencies, especially in the terahertz range (above 100 gigahertz), an important consideration is the decreased angular divergence (that is, the increased directionality) of transmitted signals, owing to the reduced effects of diffraction on waves with shorter wavelengths. In recent years, research on wireless devices6,9,8 and systems9,12,11 that operate at terahertz frequencies has ramped up markedly. These high-frequency, narrow-angle broadcasts present a more challenging environment for eavesdroppers compared to the wide-area broadcasts used at lower frequencies12,13. However, despite the widespread assumption of improved security for high-frequency wireless data links14,15,16, the possibility of terahertz eavesdropping has not yet been characterized. A few recent studies have considered the issue at lower frequencies5,6,,12,13,17,18, but generally with the idea that the eavesdropper’s antenna must be located within the broadcast sector of the transmitting antenna, leading to the conclusion that eavesdropping becomes essentially impossible when the transmitted signal has sufficiently high directionality15. Here we demonstrate that, contrary to this expectation, an eavesdropper can intercept signals in line-of-sight transmissions, even when they are transmitted at high frequencies with narrow beams. The eavesdropper’s techniques are different from those for lower-frequency transmissions, as they involve placing an object in the path of the transmission to scatter radiation towards the eavesdropper. We also discuss one counter-measure for this eavesdropping technique, which involves characterizing the backscatter of the channel. We show that this counter-measure can be used to detect some, although not all, eavesdroppers. Our work highlights the importance of physical-layer security in terahertz wireless networks and the need for transceiver designs that incorporate new counter-measures.

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Fig. 1: Schematic of a line-of-sight transmission channel with an eavesdropper.
Fig. 2: Measured blockage and secrecy capacity for eavesdropping attacks using metal cylinders.
Fig. 3: Angular distribution of power received by Eve, using metal cylinders.
Fig. 4: Measured blockage and secrecy capacity for eavesdropping attacks using flat objects.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

This work was funded in part by the US National Science Foundation, the US Army Research Office and the W. M. Keck Foundation.

Reviewer information

Nature thanks K.-Y. Lam and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

D.M.M. and E.K. conceived the experiments. J.M., R.S. and J.A. performed the measurements. Z.H. and J.M.J. performed the computations shown as dotted lines in Fig. 3. D.M.M., E.K. and C.-Y.Y. analysed and interpreted the data. All authors contributed to the writing of the manuscript.

Corresponding author

Correspondence to Daniel M. Mittleman.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Table 1 Specifications for the terahertz wireless communication system

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Ma, J., Shrestha, R., Adelberg, J. et al. Security and eavesdropping in terahertz wireless links. Nature 563, 89–93 (2018). https://doi.org/10.1038/s41586-018-0609-x

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Keywords

  • Terahertz Wireless
  • Trusted Platform Module
  • Directional Horn Antenna
  • Average Secrecy Capacity
  • Beam Splitter Attack

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