Experiment Shows How to Eavesdrop on Terahertz Frequencies

If you position yourself between a stationary transmitter and receiver, it’s possible to deflect signals and steal information

Two black stands with an antenna and receiver are positioned to face each other in an open room.
Photo: Mittleman Group/Brown University
In a series of experiments, researchers at Brown University showed that it’s possible to use a metal plate to divert part of a wireless signal broadcast in the terahertz range.
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Society is poised to enjoy a major communications upgrade with the widespread launch of commercial 5G networks. Meanwhile, a handful of researchers have already started to think ahead to 6G—the next (next) generation of wireless and cellular technology.

Just as 5G networks will transmit data on higher frequencies than previous generations, preliminary research suggests that the trend may well continue with 6G networks. Harnessing these waves brings a wealth of benefits. Along with offering more bandwidth for data transfers, higher-frequency wavelengths are expected to be more difficult to intercept—and lead to more secure communications.

Today’s wireless networks rely on low-frequency, omnidirectional waves; these wide-area signals can easily be picked up by a third party using a transceiver without much concern for where the transceiver is placed. 5G networks will add narrower, higher-frequency millimeter waves, which technically span from about 30 gigahertz to 300 GHz, though frequencies between 25 GHz and 35 GHz have proven most popular with carriers. 6G networks could rely on even higher frequencies and narrower beams, possibly in the terahertz range.

In theory, narrower beams that are highly directional could make it much more difficult for any eavesdroppers to access the broadcast. This is partly because if an eavesdropper placed a bulky transceiver in the path of a narrow terahertz broadcast, it would block the signal enough to alert the two communicating parties to the interception.

But no method of wireless transmission is entirely secure, and the same is true for terahertz frequencies. Researchers have now demonstrated, in a study published in Nature, how someone could intercept and eavesdrop on a private conversation broadcast with terahertz waves (terahertz includes frequencies of 0.1 terahertz—100 GHz—and higher).

“People have been saying for years that wireless links would become more secure as the frequency increases, because of the decreasing width of the transmitted beam,” explains Daniel Mittleman of Brown University, lead author of the study. “We thought it would be interesting to show that this conventional wisdom was, if not wrong, at least a bit oversimplified.”

Mittleman and his colleagues showed that a small object can be placed partially within the path of a terahertz transmission beam. The object could divert some of the signal, which could then be picked up by a third party, without detection in some cases.

In a series of 42 experiments with different cylindrical objects and broadcast frequencies, the researchers successfully eavesdropped in 10 scenarios. How easily they were able to intercept these beams was related to the angular width of the beam. In general, higher-frequency beams have narrower angular widths, meaning they would be theoretically more difficult to intercept without being detected—a trend that the researchers observed in their data.

In the study, they explored frequencies between 100 and 400 GHz—frequencies much higher than what most 5G networks are generally expected to use, although some may hit the lower end of this range. This technique for hacking narrow, directional beams is therefore more applicable to future networks that will likely utilize higher frequencies.

The researchers also suggest that this tactic could be modified depending on the location of the eavesdropper and the desired strength of a stolen signal.

For example, if an eavesdropper, named Eve, wanted to intercept a message that Alice is transmitting to Bob, and Eve was uncertain of where she would be positioned at the time of interception, she would simply use a cylindrical object to siphon off part of the signal. Cylindrical objects work best under such a scenario because they scatter radiation at many angles—so Eve could pick up the diverted signal from a number of positions. The downside to this approach is that the diffused signal would be fairly weak.

The authors note that a square planar object would be more ideal if Eve knew where she would be positioned at the time of interception. In that case, the angled object could concentrate the diverted part of the signal in her direction.

Mittleman says, “The most surprising aspect of this study is that it really is possible for Eve to win, if she is clever and sufficiently well informed about the nature of the link. It was not obvious, at the outset, that this would be true.”

Admittedly, the approach taken by Mittleman and his colleagues requires specific conditions, but he describes some scenarios where it’s feasible.

For instance, remote data centers have stationary transmitters and receivers within close proximity to one another, without many witnesses around to see an attacker place an intercepting object. However, if Alice or Bob were using smartphones and moving around during their conversation—it would be much more difficult for Eve to place the intercepting object and pick up the diverted signal.

In a fun twist of events, Mittleman and his collaborators describe a way that Alice, in some cases, could pick up on Eve’s interception. If the object used for the interception reflects some of the signal back toward Alice, it could alert her to the intrusion. Alice would have to be able to distinguish this signal from background noise, though.

In the endless push for secure wireless communication, will Alice and Bob ever manage to engage in a private conversation without the risk of Eve listening in? Perhaps not—even in a world with 6G.

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