A pair of researchers behind a system for avoiding internet censorship wants to deliver banned websites inside of cat videos. Their system uses media from popular, innocuous websites the way a high schooler might use the dust jacket of a textbook to hide the fact that he’s reading a comic book in class. To the overseeing authority—in the classroom, the teacher; on the internet, a government censor—the content being consumed appears acceptable, even when it’s illicit.
The researchers, who work at the University of Waterloo’s cryptography lab, named Slitheen after a race of aliens from Doctor Who who wear the skins of their human victims to blend in. The system uses a technique called decoy routing, which allows users to view blocked sites—like a social-networking site or a news site—while generating a browsing trail that looks exactly as if they were just browsing for shoes or watching silly videos on YouTube.
Slitheen’s web browser starts the process by sending off a normal request for a harmless, “overt” site—but in it, it embeds a secret tag: an encrypted, second request for the user’s true target, a sensitive “covert” website. Website requests pass through relay stations built into the internet’s infrastructure on their way from a browser to a web server. If a relay station were to install Slitheen software, when the request passes through it, the station would detect the secret request and decode it using its secret key. (A non-Slitheen relay station that doesn’t have the right secret key wouldn’t even be able to tell that there is a secret request bundled inside of the traffic, let alone decrypt it.)
Once the relay has decrypted the secret request, it begins the process of assembling the unique package that defines the Slitheen technique. It downloads the overt site, which will act as the camouflage for the sensitive data, and simultaneously downloads the target covert site—the payload that the user really wants to see.
It then strips out all the images and videos from the overt site, and replaces them with the entire contents of the covert site. If the whole covert site is smaller than the video and image data from the overt site, the relay station will add junk data until the sizes are identical. If the covert site is bigger, it can be split across multiple overt requests to different safe websites.
Finally, once the covert site has been hidden inside the overt site, the relay sends the whole bundle back to the user that requested it. When it arrives, the user’s Slitheen client sifts through the data, loading the overt site in the background, and displaying the covert site in a browser.
Slitheen’s defining feature is that the complex traffic it generates is indistinguishable from a normal request. That is, two computers sitting next to one another, downloading data from Amazon.com’s…