Spying in the dark: TCP and Tor traffic analysis

We show how to exploit side-channels to identify clients without eavesdropping on the communication to the server, and without relying on known, distinguishable traffic patterns. We present different attacks, utilizing different side-channels, for two scenarios: a fully off-path attack detecting TCP connections, and an attack detecting Tor connections by eavesdropping only on the clients. Our attacks exploit three types of side channels: globally-incrementing IP identifiers, used by some operating systems, e.g., in Windows; packet processing delays, which depend on TCP state; and bogus-congestion events, causing impact on TCP's throughput (via TCP's congestion control mechanism). Our attacks can (optionally) also benefit from sequential port allocation, e.g., deployed in Windows and Linux. The attacks are practical - we present results of experiments for all attacks in different network environments and scenarios. We also present countermeasures for these attacks.