Anti-Phase Signal Approach to Echo TEMPEST Self-Interference Suppression Retaining a Two-Antenna Setup

The threat of Echo TEMPEST, which causes information leakage by irradiating electromagnetic (EM) waves at a specific frequency, has been reported. In this threat, reflected waves (Echo) with different amplitudes are generated depending on the output state of the IC's output buffer, and the IC's output signal leaks as amplitude-modulated waves. However, a problem in conducting Echo TEMPEST is the selfinterference caused by EM coupling between the transmitting (TX) and receiving (RX) antennas. In previous research, the self-interference effects were reduced by generating signals with a different frequency from the TX signal within the target device. However, this method requires two TX antennas and one RX antenna, making it necessary to explore antenna placements and evaluation parameters while considering the transfer function between antennas and the target device, and the coupling between antennas. This paper proposes a method for suppressing self-interference during Echo TEMPEST without adding extra antennas. Specifically, we generate an anti-phase signal that is superimposed on the Echo alongside the TX signal, and combine them to reduce the self-interference effects. Evaluation on speakerphones confirmed mitigation of selfinterference and improved information-acquisition accuracy while retaining the original two-antenna configuration.