The Parrot Is Dead: Observing Unobservable Network Communications

In response to the growing popularity of Tor and other censorship circumvention systems, censors in non-democratic countries have increased their technical capabilities and can now recognize and block network traffic generated by these systems on a nationwide scale. New censorship-resistant communication systems such as Skype Morph, Stego Torus, and Censor Spoofer aim to evade censors\´ observations by imitating common protocols like Skype and HTTP. We demonstrate that these systems completely fail to achieve unobservability. Even a very weak, local censor can easily distinguish their traffic from the imitated protocols. We show dozens of passive and active methods that recognize even a single imitated session, without any need to correlate multiple network flows or perform sophisticated traffic analysis. We enumerate the requirements that a censorship-resistant system must satisfy to successfully mimic another protocol and conclude that "unobservability by imitation" is a fundamentally flawed approach. We then present our recommendations for the design of unobservable communication systems.