ZM-CTC: Covert timing channel construction method based on zigzag matrix

VoLTE (Voice over Long-Term Evolution) is a widely used, high-performance, and end-to-end transmission solution in 4G and 5G. However, it also provides a new foundation for the CTC (covert timing channel), which transmits data in a hidden way. Because the packet type, transmission sequence, and transmission jitter in VoLTE have the strong regularity, it is challenging to build CTC. In this paper, a CTC construction method based on zigzag matrix named ZM-CTC is proposed to establish a connection between the codewords and dropped packets in a nonlinear rule. Moreover, a random salt and a random field in VoLTE are introduced to enhance the undetectability of ZM-CTC. The experimental results show that the ZM-CTC with 9 bits codeword could keep a balance among the undetectability, the robustness, the throughput, and the construction cost. The throughput of CTC is 0.88 bps, and the bit error rate of CTC is less than 1.5%.