MagHop: Magnetic Spectrum Hopping for Securing Voltage and Current Magnetic Sensors

Abstract

Voltage and current magnetic sensors (VCMSs) are pervasive in safety-critical systems. They use a magnetic field as a transduction medium to sense the input signal. Therefore, if an attacker manipulates the magnetic transduction medium of this sensor by using an intentional EMI or external magnetic fields, no amount of security mechanism after the fact can help. Fortunately, our work provides a defense against this form of physical attack.The core idea of our defense is to shift the frequency spectrum of the magnetic field, which is used as the transduction medium of the sensor, to another spectrum unknown to an attacker. In addition, the frequency spectrum which carries the magnetic field in the transduction medium, is varied in a pseudo-random fashion so that the attacker will not be able to track it to inject any EMI into it. Even a sweeping attacker, who can vary the EMI’s frequency, cannot bypass our defense because of the check and select approach of our defense. As the magnetic field’s spectrum in the transduction medium of the sensor hops in a different spectrum, the defense is named as Magnetic Spectrum Hopping (MagHop). While prior works fail to prevent an EMI, which has the same frequency as the input signal, MagHop is equipped to handle this limitation of the prior works. Moreover, a low-power, real-time coherent prototype of MagHop is designed that is evaluated with a realworld application: a grid-tied inverter. Finally, we thoroughly evaluate MagHop on ten different sensors from six different manufacturers to prove its robustness against the EMI or external magnetic field injection attack on VCMSs.