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Multimedia protocol tunneling enables the creation of covert channels by modulating data into the input of popular multimedia applications such as Skype. To be effective, protocol tunneling must be unobservable, ie, an adversary should not be able to distinguish the streams that carry a covert channel from those that do not. However, existing multimedia protocol tunneling systems have been evaluated using ad hoc methods, which casts doubts on whether such systems are indeed secure, for instance, for censorship-resistant communication.