Comb Frequency Division Multiplexing: A Non-Binary Modulation for AirGap Covert Channel Transmission

Isolated networks ensure the confidentiality of sensitive data on a system by eliminating all physical connections to public networks or external devices, making the system air-gapped. However, previous work has shown that Electromagnetic (EM) emanations when correlated with secret data, can lead to side or covert channels. Specifically, EM emissions caused by clocks can modulate high-frequency signals, enabling unauthorized data transmission to cross the air-gap. This work focuses on covert channels where a software or hardware Trojan inserted in the victim system induces side channel emissions that the attacker can recover through the covert channel, producing an intentional transmission and leakage of sensitive information. This paper introduces a novel encoding method for covert channels called Comb Frequency Division Multiplexing (CFDM). CFDM leverages modulated signals emitted by the victim system, which are evenly spaced across the frequency spectrum, creating a comb-like pattern. Moreover, the uncontrolled nature of the side channel modulation can make each subcarrier carry different information. Unlike traditional methods such as Frequency Shift Keying (FSK) and Amplitude Shift Keying (ASK), CFDM encodes information in both the frequency and amplitude dimensions of the covert channel harmonic sub-carriers.