[SecureWV 2019] Dissect Tor Bridge and Pluggable Transport

This talk was presented at SecureWV 2019.

Tor is renowned for its ability of enabling anonymous Internet communication to protect privacy. It directs Internet traffic through an overlay network to conceal the user’s location and Internet activities. The overlay network consists of thousands of relay nodes run by volunteers worldwide.

As we all know, some countries censor Internet traffic. So Tor traffic is blocked there because its anonymity conflicts with the Internet censorship. Normal Tor traffic can be blocked by identifying its protocol packets and entry nodes which are listed in the main Tor directory. To circumvent Internet censorship, Tor introduces bridge relay nodes and pluggable transports. Bridge relay nodes are not listed in the main Tor directory so that nobody can get all the bridge relay nodes to block them. Pluggable transports manipulate all Tor traffic between Tor client and Tor entry node so that it’s not identifiable. Obfs4 is the latest pluggable transport of Tor, which can obfuscate the Tor traffic and have it look random.

In this talk, we first introduce Tor and Tor network. Then we dissect the mechanism of Tor bridge mode in combination with the pluggable transport obfs4.