Anti-Eavesdropping Technology Based On Electromagnetic Interference Against Microphones

The miniature of microphones has make them popular, and a wide variety of devices that have built into our daily fabric have build-in microphones. Such a trend brought great convenience to our daily lives. For instance, people may use various recording equipment to record their daily activities or work procedures. However, these devices can cause privacy issues. Because the small size of recording devices, attackers can easily hide them while attending confidential meetings and record private conversation secretly. In addition, it has been reported that hackers can utilize a camera’s microphone remotely to monitor people’s daily conversation. All these recording behaviors can violate individual, company, or even national security. Therefore, it is meaningful to design an anti-recording equipment to protect people’s privacy despite the usage of recording devices.In this paper, we propose a novel approach that utilizes EMI signal injection attacks to prevent unauthorized audio recording using recording pens. The basic idea is to let electromagnetic interference be coupled into the conducting wires inside recording pens and consequently change the recorded signals (in the form of voltage). In this these, we first analyze the underlying principle of EMI injection attacks against microphones and employ a vector signal generator (Agilent N5172B) to identify the vulnerable frequency of the devices. Then, we measure the recorded signals to obtain the resonant frequencies of the devices. After this step, we perform EMI injection attacks based on devices’ resonant frequencies and find that the injected signals can completely overwrite the recorded audios that are purely stimulated by the sounds in the environment. Afterwards, we evaluate the effectiveness of EMI injection attacks by estimating the understandability of the recorded Chinese speech. Based on the estimation, we propose an EMI injection scheme that can achieve anti-recording, and quantify the number of EMI injectors in various sounding environments. At the same time, we quantitatively determine the polarization angle range between the recording device and the anti-recording device’s antenna. Finally, we evaluate the EMI injection scheme against several recording devices and validate the effectiveness of the scheme.We use vector signal generator and USRP with WBX daughter board as the Anti-Recording devices, and test a few cording devices, i.e., Lenovo B680, Olympus recording pens and M189 webcam. We analyze their resonant frequencies and the effective range of interference. We found that emitting between 700MHz to 1GHz, the anti-recording device can disturb these recording devices will well.