A Timing Channel Spyware for the CSMA/CA Protocol

This paper presents the design and implementation of spyware communication circuits built into the widely used carrier sense multiple access with collision avoidance (CSMA/CA) protocol. The spyware components are embedded within the sequential and combinational communication circuit structure during synthesis, rendering the distinction or dissociation of the spyware from the original circuit impossible. We take advantage of the timing channel resulting from transmission of packets to implement a new practical coding scheme that covertly transfers the spied data. Our codes are robust against the CSMA/CA's random retransmission time for collision avoidance and in fact take advantage of it to disguise the covert communication. The data snooping may be sporadically triggered, either externally or internally. The occasional trigger and the real-time traffic's variability make the spyware timing covert channel detection a challenge. The spyware is implemented and tested on a widely used open-source wireless CSMA/CA radio platform. We identify the following performance metrics and evaluate them on our architecture: 1) efficiency of implementation of the encoder; 2) robustness of the communication scheme to heterogeneous CSMA/CA effects; and 3) difficulty of covert channel detection. We evaluate criterion 1) completely theoretically. Criterion 2) is evaluated by simulating a wireless CSMA/CA architecture and testing the robustness of the decoder in different heterogeneous wireless conditions. Criterion 3) is confirmed experimentally using the state-of-the-art covert timing channel detection methods.