Telex to help defeat web censors Published duration 19 July 2011

image caption China operates a very sophisticated web filtering system.

Data smuggling software could help citizens in countries operating strict net filters visit any site they want.

Developed by US computer scientists the software, called Telex, hides data from banned websites inside traffic from sites deemed safe.

The software draws on well-known encryption techniques to conceal data making it hard to decipher.

So far, Telex is only a prototype but in tests it has been able to defeat Chinese web filters.

Outside in

Telex was developed to get around the problem that stops other anti-censorship technologies being more effective, said Dr Alex Halderman, one of the four-strong team that has worked on Telex since early 2010.

Many existing anti-censorship systems involve connecting to a server or network outside the country in which a user lives.

This approach relies on spreading information about these servers and networks widely enough that citizens hear about them but not so much that censors can find out and block them.

Telex turns this approach on its head, said Dr Halderman.

"Instead of having some server outside the network that's participating we are doing it in the core of the network," he said.

Telex exploits the fact that few net-censoring nations block all access and most are happy to let citizens visit a select number of sites regarded as safe.

When a user wants to visit a banned site they initially point their web browser at a safe site. As they connect, Telex software installed on their PC puts a tag or marker on the datastream being sent to that safe destination.

Net routers outside the country recognise that the datastream has been marked and re-direct a request to a banned site. Data from censored webpages is piped back to the user in a datastream disguised to resemble that from safe sites.

Rights fight

The datastream is subtly altered using a well-known encryption technique called public key cryptography. This allows anyone with a public key to lock content but only allows the owner of the corresponding private key to unlock it.

This cryptographic technique helps secure Telex against interference, said Dr Halderman.

"You cannot see this marker unless you have a corresponding private key," he said.

The Telex-spotting routers know the key so they can unlock the content and discover the website a user is really interested in seeing. If Telex is deployed, ISPs would be encouraged to add marker-spotting software to the routers in their networks.

Although Telex was "not ready" for real users, Dr Halderman said the development team had been using it for their own web browsing for months. In addition, he said, the team had carried out some small scale tests against sophisticated filtering systems.

"We've also tried it from within China bouncing it off computers there," he said. "So far, we've had no problems with the censorship there."

image caption Protestors in Iran have used the web to co-ordinate their action

Telex allowed the team to view banned content such as high definition YouTube videos and sites deemed "subversive" by the Chinese authorities.

One stumbling block for Telex was getting the basic software to users without it being compromised by net censors who could add spyware or key loggers to it, said Dr Halderman.

There were other issues to be resolved as development continues.

"The most difficult part is making sure the connections the user is making to an uncensored website that we use to disguise the censored content are convincing enough," he said.

"But," he said "that's the parameter we would adjust as the censor becomes more sophisticated."

The developers are planning to give a more formal launch to Telex at the upcoming Usenix security conference. That conference will host an annual workshop for the growing numbers of people developing anti-censorship code, he said.