Computer scientists have released a tool that disguises communications sent through the Tor anonymity service as Skype video calls, a cloak that's intended to prevent repressive governments from blocking the anonymous traffic.

SkypeMorph, as the application is called, is designed to remedy a fundamental limitation of Tor: While the communications are cryptographically secured, unique characteristics of their individual data packets make them easy to identify as they travel over the networks. In the past, for example, the cryptographic key exchange was different in Tor transactions and the certificates used were typically valid for only a matter of hours, compared with as long as a year or two for certificates used by most Web servers. These fingerprints made it possible for government censors in Iran, China, and elsewhere to block data traveling over Tor while leaving the rest of the country's communications intact.

Tor developers have remedied those shortcomings, but other unique signatures still exist. The idea behind SkypeMorph is to camouflage Tor communications so they blend in as traffic that government censors are reluctant to restrict.

"The goal is to make the traffic look like some other protocol that they are not willing to block," Ian Goldberg, a professor at the Cheriton School of Computer Science at the University of Waterloo, told Ars. "They could just shut off the Internet, of course, like Egypt did for a few days a year or so ago, but that, of course, would be extremely unpopular to their own people that are wondering why can't see pictures of cute cats."

A censorship arms race

The release of SkypeMorph comes a few months after a separate research team in Sweden uncovered changes the Chinese government made to its "Great Firewall" censorship infrastructure to make it harder for citizens to use Tor. Although their research paper (PDF) was only recently published, the findings have been public for a few months, said Goldberg, who sits on the Tor Project's board of directors. As censors in China and elsewhere devise increasingly sophisticated measures of detecting and blocking the anonymity service, it falls on Tor volunteers to find new ways to thwart them.

"The whole point of SkypeMorph is exactly because the Great Firewall is so complex," said Goldberg, who refers to the jockeying between privacy advocates and governments as a censorship arms race. "You have to very convincingly pretend your traffic is something else, like Skype."

SkypeMorph relies on the Microsoft-owned VoIP service to establish a cryptographically secured connection between an end user and unlisted entry points, known as bridges, to the Tor network. By sending a few short Skype messages to one of the bridges, a Tor user performs a Diffie Hellman key exchange to make sure the connection can be trusted. Once the handoff is completed, SkypeMorph initiates a Skype video call to the bridge and quickly drops it. The bridge and the end user then use the key to securely communicate using normal Tor protocols.

To prevent the Tor traffic from being recognized by anyone analyzing the network flow, SkypeMorph uses what's known as traffic shaping to convert Tor packets into User Datagram Protocol packets, as used by Skype. The traffic shaping also mimics the sizes and timings of packets produced by normal Skype video conversations. As a result, outsiders observing the traffic between the end user and the bridge see data that looks identical to a Skype video conversation.

The SkypeMorph developers chose Skype because the software is widely used throughout the world, making it hard for governments to block it without arousing widespread criticism. The developers picked the VoIP client's video functions because its flow of packets more closely resembles Tor traffic. Voice communications, by contrast, show long pauses in transmissions, as one party speaks and the other listens.

"It's not enough just to send encrypted packets to a particular port, Goldberg explained. "You want to send them in patterns and sequences and sizes and distributions that look as realistic as possible. What our system does is go a step beyond traffic morphing and not only matches the packet size distributions but also matches the timing distributions."

To prevent the Skype network from being overburdened, SkypeMorph sends data directly over the Internet once the VoIP client has been used to establish a secured connection.

Modular obfuscation plugins

The application makes use of programming interfaces built into Tor that allow the program to work with obfuscation extensions called pluggable transports. Such add-ons appear as SOCKS proxies to the Tor client and allow data delivered to bridges to be sent in obfuscated ways. Developers can design pluggable transports for Tor in much the way people write add-ons for the Firefox or Chrome browsers.

So far, the only pluggable transport available for Tor is known as obfsproxy. It passes traffic between end users and bridges through a stream cipher. SkypeMorph is designed to extend the benefit of this plugin "to address its limitation of not outputting innocuous-looking traffic," Goldberg's research paper (PDF) describing the software said.

The SkypeMorph paper was co-authored by Hooman Mohajeri Moghaddam, Baiyu Li, and Mohammad Derakhshani, all of whom were students enrolled in a class taught by Goldberg titled Hot Topics in Privacy Enhancing Technologies.