Telex (anti-censorship system)

Telex is an internet censorship circumvention system developed and deployed primarily in Iran. It is designed to circumvent keyword and protocol-based blocking techniques used by internet service providers (ISPs) to censor online content. Unlike other circumvention tools like VPNs or Tor which rely on creating encrypted tunnels, Telex operates by subtly embedding censorship circumvention instructions within ordinary HTTPS traffic.

The system works by leveraging the large amount of legitimate encrypted traffic that already traverses the internet. A user wishing to bypass censorship installs a piece of client software, called a Telex client. When the user attempts to access a blocked website, the Telex client encodes a special signal, a "Telex signal," within the initial HTTPS request made to a non-blocked website. This signal is designed to be indistinguishable from legitimate HTTPS traffic to a casual observer.

ISPs equipped with Telex "interceptors" are able to detect this subtle signal. Upon detection, the interceptor reroutes the traffic towards a Telex server. The Telex server then retrieves the requested censored content and forwards it back to the user via the originally established HTTPS connection. Because the communication between the user and the Telex server is encrypted using standard HTTPS protocols, it appears to the ISP as ordinary encrypted traffic, thus bypassing censorship.

A key advantage of Telex is its low profile. Because it integrates circumvention instructions within ordinary HTTPS traffic, it's more difficult for censors to detect and block without disrupting a large portion of legitimate internet traffic. However, the deployment of Telex requires cooperation from ISPs, as they need to install Telex interceptors on their networks. This requirement presents a significant challenge for widespread adoption, particularly in countries with strict internet control where ISPs are directly controlled by the government. The effectiveness of Telex relies on keeping the Telex signal and interceptor technology secret; if either is compromised, the censoring authority can easily block the system.