How to Turn a Coke Can Into an Eavesdropping Device

BLACK HAT ASIA — A soda can, a smartphone stand, or any shiny, lightweight desk decoration could pose a threat of eavesdropping, even in a soundproof room, if an attacker can see the object, according to a team of researchers from Ben-Gurion University of the Negev.

At the Black Hat Asia security conference on Thursday, and aiming to expand on previous research into optical speech eavesdropping, the research team showed that audio conversations at the volume of a typical meeting or conference call could be captured from up to 35 meters, or about 114 feet, away. The researchers used a telescope to collect the light reflected from an object near the speaker and a light sensor — a photodiode — to sample the changes in the light as the object vibrated.

A lightweight object with a shiny surface reflects the signal with enough fidelity to recover the audio, said Ben Nassi, an information security researcher at the university.

“Many shiny, lightweight objects can serve as optical implants that can be exploited to recover speech,” he said. “In some cases, they are completely innocent objects, such as a smartphone stand or an empty beverage can, but all of these devices — because they share the same two characteristics, they are lightweight and shiny — can be used to eavesdrop when there is enough light.”

The eavesdropping experiment is not the first time that researchers have attempted side-channel attacks that pick up audio from surrounding objects.

Improving on Past Optical Eavesdropping
In 2016, for example, researchers demonstrated ways to reconfigure the audio-out jack on a computer to an audio-in jack and thereby use speakers as microphones. In 2014, a group of MIT researchers found a way to use a potato chip bag to capture sound waves. And in 2008, a group of researchers created a process to capture the keys typed on a keyboard by their sounds and the time between keystrokes.

The MIT research is similar to the technique pursued by the Ben-Gurion University researchers, except that exploitation required more restrictive placement of the reflective object and required substantial processing power to recover the audio, said Raz Swissa, a researcher with Ben-Gurion…