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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 circum- vention 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 de- tection, leading the anti-censorship community to ask how detection was possible. In this paper, we measure and characterize the GFW’s new system for censoring fully encrypted traffic. We find that, in- stead of directly defining what fully encrypted traffic is, the censor applies crude but efficient heuristics to exempt traffic that is unlikely to be fully encrypted traffic; it then blocks the remaining non-exempted traffic. These heuristics are based on the fingerprints of common protocols, the fraction of set bits, and the number, fraction, and position of printable ASCII characters. Our Internet scans reveal what traffic and which IP addresses the GFW inspects. We simulate the inferred GFW’s detection algorithm on live traffic at a university network tap to evaluate its comprehensiveness and false positives. We show evidence that the rules we inferred have good coverage of what the GFW actually uses. We estimate that, if applied broadly, it could potentially block about 0.6% of normal In- ternet traffic as collateral damage. Our understanding of the GFW’s new censorship mecha- nism helps us derive several practical circumvention strategies. We responsibly disclosed our findings and suggestions to the developers of different anti-censorship tools, helping millions of users successfully evade this new form of blocking
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This content will become publicly available on August 13, 2026
Exposing and Circumventing SNI-based QUIC Censorship of the Great Firewall of China
Despite QUIC handshake packets being encrypted, the Great Firewall of China (GFW) has begun blocking QUIC connections to specific domains since April 7, 2024. In this work, we measure and characterize the GFW’s censorship of QUIC to understand how and what it blocks. Our measurements reveal that the GFW decrypts QUIC Initial packets at scale, applies heuristic filtering rules, and uses a blocklist distinct from its other censorship mechanisms. We expose a critical flaw in this new system: the computational overhead of decryption reduces its effectiveness under moderate traffic loads. We also demonstrate that this censorship mechanism can be weaponized to block UDP traffic between arbitrary hosts in China and the rest of the world. We collaborate with various open-source communities to integrate circumvention strategies into a leading web browser, the quic-go library, and all major QUIC-based circumvention tools.
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- Award ID(s):
- 2319080
- PAR ID:
- 10656500
- Publisher / Repository:
- USENIX Security Symposium
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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