During the establishment of a circuit, computers on the Tor network have to pass a lot of data back and forth. The researchers showed that simply by looking for patterns in the number of packets passing in each direction through a guard, machine-learning algorithms could, with 99 percent accuracy, determine whether the circuit was an ordinary Web-browsing circuit, an introduction-point circuit, or a rendezvous-point circuit. Breaking Tor’s encryption wasn’t necessary.
Furthermore, by using a Tor-enabled computer to connect to a range of different hidden services, they showed that a similar analysis of traffic patterns could identify those services with 88 percent accuracy. That means that an adversary who lucked into the position of guard for a computer hosting a hidden service, could, with 88 percent certainty, identify it as the service’s host.