Snowflake, a censorship circumvention system using temporary WebRTC proxies

Snowflake is a system for circumventing Internet censorship. Its blocking resistance comes from the use of numerous, ultralight, temporary proxies ("snowflakes"), which accept traffic from censored clients using peer-to-peer WebRTC protocols and forward it to a centralized bridge. The temporary proxies are simple enough to be implemented in JavaScript, in a web page or browser extension, making them much cheaper to run than a traditional proxy or VPN server. The large and constantly changing pool of proxy addresses resists enumeration and blocking by a censor. The system is built on the assumption that proxies may appear or disappear at any time: clients discover live proxies dynamically using a secure rendezvous protocol; when an in-use proxy goes offline, its client switches to another on the fly, invisibly to upper network layers. Snowflake has been deployed with success in Tor Browser and Orbot for several years. It has been a significant circumvention tool during high-profile network disruptions, including in Russia in 2021 and Iran in 2022. In this paper, we explain the composition of Snowflake's many parts, give a history of deployment and blocking attempts, and reflect on implications for circumvention generally.