Building packet length covert channel over mobile VoIP traffics

Mobile VoIP (Voice over Internet Protocol) has shown its widely prevalence and laid the feasibility for concealed communication. VoIP contains continuous and huge amount of data transmission during communication which provides attractive chance for constructing high throughput covert channels. Moreover, the redundancy and the specialties of mobile VoIP traffics enable the construction of stealthy and robust covert channel. In this paper, we propose constructing covert channel via partitioning the length distribution of legitimate packet traffics and mapping packet length partitions to data symbols. The reliable data transmission of covert channel is guaranteed by precoding the hidden data into a HDLC-like frame structure and the channel performance can be raised by selective parameter settings. A testbed is setup to simulate covert communication over the natural mobile VoIP traffics. Undetectability, reliability and throughput of the scheme are evaluated and experiment results show the potential of the proposed approach. Moreover, the proposed scheme can also be applied to other communication scenarios without or with little modifications.