A robust packet-dropout covert channel over wireless networks

Wireless communication is the most important carrier for covert channels because increasing security threats have made covert channels an alternative to transmitting confidential information in untrusted wireless communications. As a solution for next-generation mobile communication, voice over LTE (VoLTE) is developed using a high-speed packet-switched core network to provide high-quality end-to-end communication services. Unlike traditional circuit-switched communication technologies, both inter-packet delays and packet orders in VoLTE traffic are limited by specific rules. Even minor modifications to the overt traffic can be detected, so the existing solutions do not apply to VoLTE. To solve this problem, we build a robust packet-dropout covert timing channel through parity casecade coding. Hash-based inter-codewords verification, cyclic redundancy check based codeword self-verification, and an adaptive mapping matrix are designed to ensure robustness and undetectability. In our scheme, the covert message is modulated into the sequence numbers of the actively dropped packets, which can be retrieved by the receiver. With the help of the verification section, the actual codewords combination can be identified to retrieve the embedded covert message. Several experiments are performed to evaluate the performance of the scheme, and the parameters of the covert timing channel (CTC) are adjusted to estimate robustness and throughput. Finally, the CTC scheme is proved to be feasible since both average bit error rate and throughput are acceptable while the covert message is stealthily transmitted.