It has often been repeated that knowledge is power, so it is no surprise then that everyone, from individuals to world powers, go to great lengths to obtain it. Sometimes this knowledge is freely available to anyone that takes the time to seek it, but in other cases, certain knowledge is highly secretive and well protected. The Great Seal bug is a classic example of an effort to obtain highly secretive information. This was a carved wooden plaque of the Great Seal of the United States, given as a gift by the Soviet Union to the United States Ambassador to the Soviet Union in 1945. Hidden within was a passive listening device that was essentially a predecessor to modern RFID technology. The clever design of this device kept it from being discovered for a full seven years, and even then, only by accident.
Technology, and our creative ways of exploiting it to eavesdrop on private conversations, has continued to evolve tremendously since that time. Recently, researchers from Ben-Gurion University of the Negev have taken inspiration from the Great Seal bug to design a new type of secretive, passive listening device that they call the little seal bug. The method they describe makes use of lightweight, reflective objects, such as drink cans or souvenirs, that are commonly found in office environments to reconstruct private conversations.
Shhh! I think the bird is listening (📷: B. Nassi et al.)
When someone speaks, it causes fluctuations in air pressure. These air pressure variations cause lightweight objects to vibrate in ways that represent the specific changes in pressure. If those objects also happen to be shiny, they reflect light that is modulated by their vibratory patterns. The idea behind the little seal bug is to detect these light modulations and decode them to recover the speech that caused them to occur.
To decode the way in which sound waves influence lightweight objects, the team conducted an experiment in which a shiny weight was hung in the air by a thin wire. A gyroscope was attached to the bottom of the weight, and samples from the gyroscope were captured with a Raspberry Pi 3 computer. An audio file of a 200 to 1500 Hz frequency scan was played on a nearby speaker while the gyroscope recordings were captured. It was found that the weight vibrates based on the sound played, so the researchers next ran a similar experiment, but with a telescope and a photodiode pointed at the weight. They found that the vibration of the weight can be visually spotted, and that the response of the weight decreases as a function of the audio frequency.
Capturing audio-induced vibratory patterns (📷: B. Nassi et al.)
The experimental findings gave the team the information they needed to design and develop an optical-acoustic transformation that is capable of translating measurements from a photodiode pointed at a shiny object into recovered audio. Their techniques are adaptive and can be applied to a variety of reflective objects, as different objects will behave in different ways. The little seal bug was tested to work with objects that are up to 20 inches away from the speakers, which is reasonable, but does limit the use cases. The remote telescope was tested at distances up to 115 feet away.
This work demonstrates how far passive listening technologies have advanced in recent years. It is, however, simple to defeat this attack because it requires a visual line-of-sight to be open for operation. The team has pivoted to investigate how they can improve speech recovery by integrating advanced speech processing algorithms into their approach.
