Anonymous Communications and Censorship Resistance Using Tor to defeat Internet surveillance

When you use the Internet, you are being tracked. The websites which you visit know who you are; marketers track your behaviour and your preferences. Even criminals exploit the traces people leave online. Anonymous communication systems help defeat such monitoring. One such system, Tor, is used by over 500,000 people daily including law enforcement, human rights workers, military personnel, and ordinary citizens worldwide. While originally designed for enhancing privacy and safety, Tor is increasingly used for allowing its users to circumvent censorship, and access commonly blocked websites including social networking, reference and news. Anonymous communication systems introduce some unique challenges, but many of the problems faced by the Tor network mirror those which are found on the wider Internet. For this reason, the study of Tor and similar systems will be informative in general, and allow the testing of hypotheses which would be difficult to evaluate on larger systems. What is Tor? Encryption, as sometimes used with web browsing (SSL) and email (e.g. PGP), only hides message content, and not the traffic data: source, destination, size and timing. Traffic analysis is the study of such data to discover the behaviour and interests of groups and individuals. It is widely used to track people, for marketing, law-enforcement and by criminals. Anonymity systems, such as Tor, protect the privacy of Internet users from traffic analysis. Tor is primarily used for anonymous web browsing, and is built from a network of around 1,500 servers (nodes) run by volunteers throughout the world. Messages are encrypted then sent through a randomly chosen path of 3 servers, and the traces they leave are erased. This makes it difficult for an attacker to follow a message between source and destination. In addition to anonymity, Tor is used to circumvent national censorship systems. By hiding what websites a user is accessing, Tor makes it more difficult for countries to block access to certain websites. To make it difficult to block access to the Tor network, some Tor nodes (the “bridges”) are not listed publicly, but their addresses are given out gradually. If the user wants access to another site, Alice's Tor client selects a second random path. Again, green links are encrypted, red links are in the clear.