diagram of setup

Schematic of a line-of-sight terahertz transmission channel (from “Alice” to “Bob”) with an eavesdropper (“Eve”). The orange cylinder represents a small object that passively scatters radiation toward the eavesdropper. The inset depicts the measured (blue) and computed (red) angular distribution of 200-GHz radiation for the equipment used in the experiments. [Image: Mittleman Lab, Brown University]

To boost bandwidth in the next generation of wireless communications networks, developers may turn to terahertz carrier frequencies. With decreased beam divergence at these higher frequencies (above 100 GHz), telecom companies are looking forward to an era of steerable antennas and narrow-beam receivers.

But now, researchers at three U.S. universities have found a potential vulnerability in these plans (Nature, doi:10.1038/s41586-018-0609-x). For certain geometries, a small object inserted into a narrow terahertz beam could divert some of the signal to an eavesdropper without either of the main communicators noticing it.

It’s all in the geometry

Current microwave cellular communications networks send out wide-angle broadcasts that are picked up by receivers that are relatively insensitive to the direction of the signal. This geometry makes it relatively easy for would-be signal interceptors to scoop up some of the beam without anyone noticing. Plans for next-generation networks have assumed that narrow-beam terahertz transmissions would block such eavesdropping, as any equipment large enough to collect the signal would thereby noticeably interrupt the beam.

The research team, however, found a geometry in which the eavesdropping would go undetected. The group inserted a small passive object—a metal cylinder—into an open-air terahertz channel between a transmitter (“Alice”) and a receiver (“Bob”). The group conducted tests at three different frequencies: 100, 200 and 400 GHz. In each case, the metal cylinder scattered the terahertz waves over a wide angle, so that the eavesdropper (“Eve”) could find some position to siphon off the signal, while Bob still received at least half of the total transmission.

Countermeasure: Characterize the backscatter

The results suggest a potential countermeasure. This same cylinder would scatter at least some of the terahertz waves 180 degrees—right back to Alice. Eve's passive device could also block some of the signal reflected from Bob in Alice's direction, which Alice could notice if she used a transceiver, not just a transmitter. “Thus, to incorporate security into a directional wireless link,” conclude the authors, “systems will require new physical-layer components and new protocols for channel estimation.”

Engineering researchers at Brown University (Rhode Island), Rice University (Texas) and the University at Buffalo (New York) participated in the experiments.