Amplifying Electromagnetic Leakage by Hardware Trojans Through Cable Geometry Manipulation

Hardware Trojans (HTs) pose a major threat even to devices previously considered outside the TEMPEST scope due to their low electromagnetic (EM) emission intensity. Earlier research has demonstrated that an HT constructed of a MOSFET and wiring, activated through external irradiation with EM waves, can forcibly leak information from such devices. However, because these attacks rely on additional electronic components and external triggers, some studies have shown that monitoring the device's internal EM environment can effectively detect such HTs. Nevertheless, if an HT requires only minimal circuit modifications and no external triggers, it can circumvent these conventional approaches. This work investigates an HT realized solely by manipulating cable geometry-without installing new components or employing external irradiation. Specifically, we increase twist pitch or untwist the twisted-pairs in HDMI cables, thereby locally modifying impedance and creating discontinuities that significantly amplify EM radiation. Our experimental results show a roughly twentyfold increase in the feasible TEMPEST attack distance compared to standard cables. We also suggest that adopting established electromagnetic compatibility (EMC) measures effectively mitigate the risks posed by these HTs.