The Chinese government has used its unrivalled net censorship apparatus to attack parts of the Tor network ahead of the 60th anniversary of communist rule, according to activists.
The Tor Project Inc, the non-profit that oversees development of the network, said China's Great Firewall began blocking IP addresses on Friday. The news follows an update last week to blocking technology used by ISPs, known as "Blue Shield".
"We knew this day would come," the Tor Project said.
Tor - originally developed for the US Navy - is used by many Chinese dissidents to conceal their identity and protect their privacy. It allows them to access banned material such as websites about Tibet, the Tiananmen Square Massacre and the recent Uigur uprising.
The Tor network was also recently widely used in Iran to disseminate information about violent clashes following the presidential election.
About 80 per cent of public relays are now blocked inside China, the Tor Project said. Public relays are most users' gateway to Tor, which beats tracking and blocking by encrypting and bouncing traffic around a random global labyrinth of proxy servers run by members of the network.
China's Great Firewall is implemented by the country's ISPs so it is able to block this first hop onto Tor using the directory of public relays. However, Chinese users are cannot be completely denied access to Tor. It is also possible to hop on to the network via a "bridge" relay.
Bridge relays were added to the network as a counter-measure to blocking. No list of their IP addresses is publicly available.
The Tor Project called on members of the network to volunteer to run more bridge relays to help Chinese users. There are more details here. ®