NUSGuard: Smart Device Anti-Eavesdropping Protection Based on Near-Ultrasonic Interference

Voice assistants (VAs) have become ubiquitous in smart devices, and are highly valued for their ability to perform a variety of tasks through voice interaction, offering users hands-free convenience. However, the always-on microphones of VAs have raised significant privacy concerns in recent years. In this paper, we propose and implement NUSGuard, a novel and practical anti-eavesdropping system. To our knowledge, it is the first system to utilize the built-in speakers of commercial off-the-shelf (COTS) devices for anti-eavesdropping, thereby eliminating the need for dedicated ultrasonic transmitters. Specifically, it exploits human ears' insensitivity to near-ultrasonic signals and the inherent non-linearity of mic to inject jamming noises into the microphones of unauthorized smart devices. Furthermore, we propose a robust mixed-noise scheme and a lexical-level automatic jammer control strategy, effectively disrupting unauthorized recordings while maintaining seamless voice interaction with authorized VA devices. Extensive digital and real-world experiments have demonstrated NUSGuard's superior performance in terms of jamming effectiveness and security.