Ultrannel: Ultrasound Based Covert Communication Channel.

The covert channel is a popular method of secure data transmissions due to its stealthiness. In this paper, we propose a new type of covert channel model, Ultrannel. It allows two adjacent but air-gapped smart devices stealthily transfer data via ultrasonic waves. Unlike the former air-gap covert channels, Ultrannel modulates sensitive data on ultrasonic carriers to achieve inaudibility and does not require ultrasonic transducers or speakers.We first investigate the inverse piezoelectric effect of the capacitor and the nonlinearity of the microphone. Then we analyze how a capacitor can generate an acoustic signal. By leveraging the physical phenomenon of the capacitor and microphone, we establish a covert communication channel, with the capacitor as the signal sender and the microphone as the receiver. By transmitting a string of numbers, we validate this covert channel on common smart devices, a smart lamp, and a smartphone. The modulated sensitive data can be successfully demodulated and recovered. We evaluate the quality of the covert channel with different metrics. The experiment results demonstrate that Ultrannel can covertly transmit data between two air-gapped smart devices with 95% accuracy at a transmission speed of 1Kbps.