I. Introduction
Smartwatches are becoming ubiquitous in people's daily lives. There are various sensors embedded in the smartwatch, e.g., the optical sensor, accelerometer, and microphone, making it possible to provide useful applications for evaluating users' status, such as heart rate monitoring, sports logging, and fall detection. However, these sensors also carry sensitive information that could be potentially unsafe, especially when the devices are compact and non-intrusive, users could easily neglect them and relax their vigilance. The attacker can collect sensor data from the target users through the malicious application installed on their smartwatch and transmit the data to the attacker's machine so that the attacker can infer certain information about the targets, posing a threat to the privacy issues of smartwatch users.