Internet users in China have long used the Tor anonymity network as a method to get around the country's notorious firewall, but a few Swedish researchers have discovered that China is making it more difficult to surf anonymously by increasing the sophistication of its Tor-blocking tools. Tor essentially works by encrypting and randomly routing traffic through a series of relay servers around the globe. Additionally, Tor uses unlisted "bridge relays" for users to gain an additional level of anonymity when browsing — thanks to these unlisted bridges, ISPs won't be able to block them even if they are blocking other Tor links.
However, researchers Philipp Winter and Stefan Lindskog from Karlstad University in Sweden noticed that users were reporting that unpublished bridges would stop working quickly after being accessed by users in China and set out to figure out what had changed. It turns out that China is now blocking Tor bridges through the same technique it blocks publicly listed Tor servers. Winter told V3 that China's firewall searches all internet traffic that identifies itself as as Tor host and then scans it; the scan tries to communicate using Tor language and — if the host responds, the bridge is blocked. Winter noted that "the scanners are mostly random IP addresses originating from address pools of ISPs. Therefore it is very hard for a bridge to differentiate between a legitimate user from China and a scanner."
The researchers also noted that there's already another way to get around this block. A tool called "obfsproxy" tries to convert the Tor traffic between a user and a bridge to something not blocked by the firewall, like Skype, though Winter noted that most public obfsproxy bridges are already being blocked. There are still private obfsproxy bridges available, though they're harder to find. While users are continuing to find ways to stay one step ahead of the Great FIrewall of China, the country certainly isn't making it easy on them.