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 homepage—one that does so normally and another with the contents of this Atlantic story instead of Amazon’s images and videos—would create identical traffic patterns. The more complex Slitheen request would take slightly longer to come back, but its defining characteristics, from packet size to timing, would be the same.

To avoid being caught, it’s important to conform decoy traffic to normal traffic as closely as possible. Decoy routing was first proposed in 2011, but it was quickly discovered to be vulnerable to fingerprinting attacks, which involve analyzing patterns as encrypted data moves across the internet. Since the data is encrypted, censors can’t peer directly into the data stream to see what’s being downloaded. But if they know what traffic to a banned site looks like—how fast it downloads, and in what order different-sized chunks of data arrive—it can identify when that site’s being accessed just based on the data streams that pass through the networks it surveils. Last month, I wrote about a technique that uses fingerprinting to guess what YouTube video is being watched.

Slitheen is designed to work even if a censor knows exactly where the relay stations are, how they work, and which sites are being used as overt sites. “They still should not be able to distinguish a Slitheen connection from a normal connection,” said Ian Goldberg, a computer-science professor at Waterloo and one of Slitheen’s developers. Only if the censor obtained Slitheen’s private key—a randomly generated 256-bit number—could it decode the secret requests embedded within the overt ones. But that secret is easily protected, and can be swapped for a new one at a moment’s notice.