Liquid: A detection-resistant covert timing channel based on IPD shaping

Covert timing channels provide a way to surreptitiously leak information from an entity in a higher-security level to an entity in a lower level. The difficulty of detecting or eliminating such channels makes them a desirable choice for adversaries that value stealth over throughput. When one considers the possibility of such channels transmitting information across network boundaries, the threat becomes even more acute. A promising technique for detecting covert timing channels focuses on using entropy-based tests. This method is able to reliably detect known covert timing channels by using a combination of entropy and conditional entropy to detect anomalies in shape and regularity, respectively. This dual approach is intended to make entropy-based detection robust against both current and future channels. In this work, we show that entropy-based detection can be defeated by a channel that intelligently and adaptively manipulates the metrics used for detection. Specifically, we propose a new passive covert channel that uses a portion of the inter-packet delays in a compromised stream to smooth out the shape distortions detected by the entropy test. As a passive channel, it is not as prone to regularity-based detection as previously proposed active channels. We introduce a model for analyzing the effect of our techniques on the entropy of the channel and empirically investigate the accuracy of the model. In network experiments and simulation, we validate this model and demonstrate that the proposed channel successfully evades entropy-based detection and other known tests while maintaining reasonable throughput.