Covert Communications through Imperfect Cancellation

We propose a method for covert communications using an IEEE 802.11 OFDM/QAM packet as a carrier. We show how to hide the covert message so that the transmitted signal does not violate the spectral mask specified by the standard, and we determine its impact on the OFDM packet error rate (PER). We show conditions under which the hidden signal is not usable and those under which it can be retrieved with a usable bit error rate (BER). The hidden signal is extracted by cancellation of the OFDM signal in the covert receiver. We explore the effects of the hidden signal on OFDM parameter estimation and the covert signal BER. We test the detectability of the covert signal with and without cancellation. We conclude with an experiment where we inject the hidden signal into Over-The-Air (OTA) recordings of 802.11 packets and demonstrate the effectiveness of the technique using that real-world OTA data.