How the Great Firewall of China Detects and Blocks Fully Encrypted Traffic
Alex Zhang, GFW Report
February 6, 2024 1:00pm, in DC 1304 and Zoom
The Great Firewall of China (GFW) is one of the most sophisticated censorship apparatus in the world, which has not only been blocking tens of thousands of websites, but also actively attempting to block any attempts to bypass its censorship. In this talk, I will introduce two major censorship upgrades of the Great Firewall of China, which was able to accurately detect and block almost all major censorship circumvention tools used by tens of millions of users in China on a daily basis.
The first censorship incidents started from May 2019, when the GFW employed a combination of passive traffic analysis and active probing to detect and block Shadowsocks, a censorship circumvention protocol used by millions of users in China on a daily basis. (ref: https://censorbib.nymity.ch/pdf/Alice2020a.pdf, IMC'20 Best Paper Runner-Up). The second censorship incident happened in November 2021, when the Great Firewall of China deployed a purely traffic analysis-based technique to detect and block fully encrypted traffic in real time. This new censorship capability affected a large set of popular censorship circumvention protocols, including but not limited to Shadowsocks, VMess, and Obfs4. (ref: https://gfw.report/publications/usenixsecurity23/en/, Best Practical Paper Award from FOCI community, First Place in CSAW 2023 Applied Research Competition, IETF/IRTF Applied Networking Research Prize 2024).
I will show how censorship measurement experiments can help characterize, understand, and bypass these emerging censorship techniques and systems. In close collaboration with the anti-censorship tool developers and other members in the Internet freedom community, our work has been helping tens of millions of users in China and Iran to evade these emerging censorship incidents during politically sensitive periods of time throughout the years.
Alex Zhang is strongly passionate about research and activism in the areas of censorship measurement and circumvention. Over the past five years, the work he led has helped millions of users in China and Iran to bypass various emerging censorship challenges during politically sensitive periods of time. His work has thus received media coverage and several awards: the IMC 2020 Best Paper Runner-up, the 2023 Best Practical Paper Award from the FOCI community, First Place in the CSAW 2023 Applied Research Competition, and the IETF/IRTF Applied Networking Research Prize in 2024. Additionally, Zhang has been contributing to the GFW Report (website: https://gfw.report, twitter: https://twitter.com/gfw_report), an English and Chinese website focused on studying and understanding censorship incidents in China.