Building a Covert Timing Channel over VoIP via Packet Length

With the rapid development of the Mobile Internet, the application of instant messaging technology has become increasingly abundant, among which Voice over Internet Protocol (VoIP) has played an important role. However, in the VoIP-based instant messaging scene, there is a risk of illegal eavesdropping by a third party. For this problem, we consider building a covert timing channel (CTC) to hide user messages and improve the security of instant messaging. At present, the existing methods of building covert timing channels (CTCs) are mainly applied to traditional Ethernet, and there is a lack of relevant methods for mobile networks, especially VoIP networks. In order to improve the transmission capacity of the CTC and improve the transmission security of user information, the research of this article focus on the method of building covert timing channel over VoIP according to the characteristics of packet length. And a prototype is established based on Linphone, which is an open-source VoIP application. Through reality transmission tests, covert messages are delivered effectively, which proves that the CTC based on packet length are feasible.