CRYPTO '11: Invited talk - Tor and Circumvention

Roger Dingledine's invited talk on "Tor and Circumvention" was a refreshing break from the technical program, providing a fascinating insight into a real-world application which is particularly relevant to recent world events.

Tor is an anonymity network which aims to provide safe, anonymous web browsing to anyone concerned with protecting themselves from Internet surveillance. The community of users is very diverse, ranging from citizens who simply value their privacy, businesses wanting to protect trade secrets, law enforcement agencies who wish to go undetected in their investigations, Internet users who are subject to harsh government firewalls, and journalists and activists trying to stay safe.

Of course, 'bad guys' can also use Tor to access material and engage in covert suspect/criminal activity. An interesting theme of the talk was the importance of separating the potential (mis/ab)uses of the technology from the role of Tor as service provider. This was nicely illustrated in the observation that organisations such as the Internet Watch Foundation themselves use Tor to track down 'bad guys' without being detected.

A different but related point was made to the effect that it is not for Tor to decide how governments should 'behave'; rather to facilitate actions by citizens. Much of the talk was devoted to the 'arms race' between Tor and regimes intent on restricting internet access. Dingledine presented some fascinating graphs on the responsiveness of Tor usage to government blocking and social/political events in places such as China, Iran, Tunisia, Libya, Egypt. Unfortunately these also show the drops in usage as governments respond by blocking Tor (e.g. by taking down all relay IP addresses as found in the centralised directory, or by simply blocking the Tor website so that people give up trying to use it). Tor's countermeasures include bridges - hidden relays by which blocked users can access the network. These are distributed in small lists (to make it harder for the adversary to disable the entire network) via email, social networking, or by private individuals. The challenge is have enough bridges and to change them sufficiently frequently to outpace the blocking rate.

Dingledine was keen to stress that Tor does not protect against every possible tracking method. At the application level, Javascript, cookies, history and flash all cause undesirable identity leakage and so are incompatible with (effective) Tor usage. Tor is also powerless against spyware and compromised systems, and such extreme measures as screen-directed cameras in internet cafes (as is mandatory in some countries).

The talk ended with a discussion of the extent to which Tor uptake and blockage act as early warning signals for political and social change or unrest. The observation of recent 'mysterious' peaks and crashes in (e.g.) Ghana, Venezeula and Chile invite attention.