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the regularity the inter-arrival time of packets in the trace. The second, -Similarity, measures the similarity of pairs of sorted inter-arrival times.

We then empirically evaluated the performance of these methods in three different scenarios: a simple, unobfuscated timing channel; a channel in which the timing interval var- ied during transmission; and a timing channel that paused periodically for transmission of noise of a form that would mimic the protocol used for cover. Both detection meth- ods could reliably differentiate the covert traffic in the sim- ple case. In the second case, with varying timing intervals, the -Similarity measure succeeded in identifying the timing channel after the regularity measure failed. In the third sce- nario, as the amount of noise and available covert bandwidth increased, the success of our methods decreased.

This work was an initial exploration into the creation and detection of network covert timing channels and there are many avenues for future work. In the short term we will add error-correction and better synchronization techniques to in- crease the bandwidth of the covert channel. In the longer term we will investigate other detection methods designed to be robust in the face of attempts to hide its regularity.


Carla Brodley’s research was supported by AFRL grant num- ber F30602-02-2-0217 and by a grant from the National Sci- ence Foundation grant number 0335574. The authors would like to thank Miguel Rui Forte for his participation in dis- cussions about this research.



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