The Little Seal Bug: Optical Sound Recovery from Lightweight Reflective Objects

In recent years, various studies have demonstrated methods to recover sound/speech with an optical sensor. Fortunately, each of these methods possess drawbacks limiting their utility (e.g., limited to recovering sounds at high volumes, utilize a sensor indicating their use, rely on objects not commonly found in offices, require preliminary data collection, etc.). One unaddressed method of recovering speech optically is via observing lightweight reflective objects (e.g., iced coffee can, smartphone stand, desk ornament) with a photodiode, an optical sensor used to convert photons to electricity. In this paper, we present the ‘little seal bug’ attack, an optical side-channel attack which exploits fluctuations in air pressure on the surface of a shiny object occurring in response to sound, to recover speech optically and passively using a photodiode. These air pressure fluctuations cause the shiny object to vibrate and reflect light modulated by the nearby sound; as a result, these objects can be used by eavesdroppers (e.g., private investigator, surveilling spouse) to recover the content of a victim's conversation when the victim is near such objects. We show how to determine the sensitivity specifications of the optical equipment (photodiode, ADC, etc.) needed to recover the minuscule vibrations of lightweight shiny objects caused by the surrounding sound waves. Given the optical measurements obtained from light reflected off shiny objects, we design and utilize an algorithm to isolate the speech contents from the optical measurements. In our evaluation of the ‘little seal bug’ attack, we compare its performance to that of related methods. We find eavesdroppers can exploit various lightweight shiny objects to optically recover the content of conversations at equal/higher quality than prior methods (fair-excellent intelligibility) while doing so from greater distances (up to 35 meters) and lower speech volumes (75 dB). We conclude that lightweight shiny objects are a potent attack vector for recovering speech optically, and can be harmful to victims being targeted for sensitive information conveyed in a spoken conversation (e.g., in cases of corporate espionage or intimate partner violence/surveillance) when seated at a desk near a lightweight reflective object.