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One of the cornerstones in censorship circumvention is fully encrypted protocols, which encrypt every byte of the payload in an attempt to “look like nothing”. In early November 2021, the Great Firewall of China (GFW) deployed a new censorship technique that passively detects—and subsequently blocks—fully encrypted traffic in real time. The GFW’s new censorship capability affects a large set of popular censorship circumvention protocols, including but not limited to Shadowsocks, VMess, and Obfs4. Although China had long actively probed such protocols, this was the first report of purely passive detection, leading the anti-censorship community to ask how detection was possible.

The paper discloses findings and suggestions to the developers of different anti-censorship tools, helping millions of users successfully evade this new form of blocking.