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Encrypted messaging app Signal uses Google to bypass censorship

Developers of the popular Signal secure messaging app have started to use Google’s domain as a front to hide traffic to their service and to sidestep blocking attempts.

Bypassing online censorship in countries where internet access is controlled by the government can be very hard for users. It typically requires the use of virtual private networking (VPN) services or complex solutions like Tor, which can be banned too.

Open Whisper Systems, the company that develops Signal—a free, open-source app—faced this problem recently when access to its service started being censored in Egypt and the United Arab Emirates. Some users reported that VPNs, Apple’s FaceTime and other voice-over-IP apps were also being blocked.

The solution from Signal’s developers was to implement a censorship circumvention technique known as domain fronting that was described in a 2015 paper by researchers from University of California, Berkeley, the Brave New Software project and Psiphon.

The technique involves sending requests to a “front domain” and using the HTTP Host header to trigger a redirect to a different domain. If done over HTTPS, such redirection would be invisible to someone monitoring the traffic, because the HTTP Host header is sent after the HTTPS connection is negotiated and is therefore part of the encrypted traffic.

“In an HTTPS request, the destination domain name appears in three relevant places: in the DNS query, in the TLS Server Name Indication (SNI) extension and in the HTTP Host header,” the researchers said in their paper. “Ordinarily, the same domain name appears in all three places. In a domain-fronted request, however, the DNS query and SNI carry one name (the “front domain”), while the HTTP Host header, hidden from the censor by HTTPS encryption, carries another (the covert, forbidden destination).”

Their research revealed that many cloud service providers and content delivery networks allow HTTP host header redirection, including Google, Amazon Cloudfront, Amazon S3, Azure, CloudFlare, Fastly and Akamai. However, most of them only allow it for domains that belong to their customers, so one must become a customer in order to use this technique.

Google, for example, allows redirection through the HTTP host header from google.com to appspot.com. This domain is used by Google App Engine, a service that allows users to create and host web applications on Google’s cloud platform.


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