How Signal Is Evading Censorship

Signal, the encrypted messaging app I prefer, is being blocked in both Egypt and the UAE. Recently, the Signal team developed a workaround: domain fronting.

Signal's new anti-censorship feature uses a trick called "domain fronting," Marlinspike explains. A country like Egypt, with only a few small internet service providers tightly controlled by the government, can block any direct request to a service on its blacklist. But clever services can circumvent that censorship by hiding their traffic inside of encrypted connections to a major internet service, like the content delivery networks (CDNs) that host content closer to users to speed up their online experience -- or in Signal's case, Google's App Engine platform, designed to host apps on Google's servers.

"Now when people in Egypt or the United Arab Emirates send a Signal message, it'll look identical to something like a Google search," Marlinspike says. "The idea is that using Signal will look like using Google; if you want to block Signal you'll have to block Google."

The trick works because Google's App Engine allows developers to redirect traffic from Google.com to their own domain. Google's use of TLS encryption means that contents of the traffic, including that redirect request, are hidden, and the internet service provider can see only that someone has connected to Google.com. That essentially turns Google into a proxy for Signal, bouncing its traffic and fooling the censors.

This isn't a new trick (Tor uses it too, for example), but it does work.

from How Signal Is Evading Censorship