[{"abstract":[{"lang":"eng","text":"Censors have long censored Transport Layer Security (TLS) traffic by inspecting the domain name in the unencrypted Server Name Indication (SNI) extension. By encrypting the SNI extension, the Encrypted ClientHello (ECH) prevents censors from blocking TLS traffic to certain domains. Despite this promising outlook, ECH’s current capability to contest TLS censorship is unclear; for instance, Russia has started censoring ECH connections successfully. This paper clarifies ECH’s current role for TLS censorship. To this end, we evaluate servers’ support for ECH and its analysis and subsequent blocking by censors. We determine Cloudflare as the only major provider supporting ECH. Additionally, we affirm previously known ECH censorship in Russia and uncover indirect censorship of ECH through encrypted DNS censorship in China and Iran. Our findings suggest that ECH’s contribution to censorship circumvention is currently limited: we consider ECH’s dependence on encrypted DNS especially challenging for ECH’s capability to circumvent censorship. We stress the importance of censorship-resistant ECH to solve the long-known problem of SNI-based TLS censorship."}],"type":"conference","keyword":["censorship","circumvention","ECH","TLS"],"file":[{"relation":"main_file","date_updated":"2026-08-17T07:40:46Z","file_name":"foci-2025-0016.pdf","access_level":"local","file_size":755171,"file_id":"60505","content_type":"application/pdf","creator":"nniklas","date_created":"2025-07-03T07:11:14Z"}],"date_created":"2025-07-03T07:14:00Z","date_updated":"2026-08-17T07:40:46Z","title":"Encrypted Client Hello (ECH) in Censorship Circumvention","year":"2025","author":[{"full_name":"Niere, Niklas","first_name":"Niklas","last_name":"Niere","id":"63563"},{"full_name":"Lange, Felix","first_name":"Felix","last_name":"Lange","id":"67893"},{"full_name":"Heitmann, Nico","first_name":"Nico","last_name":"Heitmann","orcid":"0009-0003-7687-7044","id":"74619"},{"id":"83504","first_name":"Juraj","last_name":"Somorovsky","orcid":"0000-0002-3593-7720","full_name":"Somorovsky, Juraj"}],"main_file_link":[{"open_access":"1","url":"https://www.petsymposium.org/foci/2025/foci-2025-0016.pdf"}],"language":[{"iso":"eng"}],"file_date_updated":"2026-08-17T07:40:46Z","citation":{"mla":"Niere, Niklas, et al. <i>Encrypted Client Hello (ECH) in Censorship Circumvention</i>. 2025.","ama":"Niere N, Lange F, Heitmann N, Somorovsky J. Encrypted Client Hello (ECH) in Censorship Circumvention. In: ; 2025.","bibtex":"@inproceedings{Niere_Lange_Heitmann_Somorovsky_2025, title={Encrypted Client Hello (ECH) in Censorship Circumvention}, author={Niere, Niklas and Lange, Felix and Heitmann, Nico and Somorovsky, Juraj}, year={2025} }","apa":"Niere, N., Lange, F., Heitmann, N., &#38; Somorovsky, J. (2025). <i>Encrypted Client Hello (ECH) in Censorship Circumvention</i>. Free and Open Communications on the Internet, Washington, D.C.","ieee":"N. Niere, F. Lange, N. Heitmann, and J. Somorovsky, “Encrypted Client Hello (ECH) in Censorship Circumvention,” presented at the Free and Open Communications on the Internet, Washington, D.C., 2025.","short":"N. Niere, F. Lange, N. Heitmann, J. Somorovsky, in: 2025.","chicago":"Niere, Niklas, Felix Lange, Nico Heitmann, and Juraj Somorovsky. “Encrypted Client Hello (ECH) in Censorship Circumvention,” 2025."},"oa":"1","has_accepted_license":"1","status":"public","conference":{"start_date":"2025-07-14","name":"Free and Open Communications on the Internet","location":"Washington, D.C.","end_date":"2025-07-14"},"user_id":"63563","ddc":["006"],"_id":"60503"},{"quality_controlled":"1","file_date_updated":"2026-08-17T07:42:08Z","citation":{"short":"F. Bäumer, M. Brinkmann, N. Erinola, S.N. Hebrok, N. Heitmann, F. Lange, M. Maehren, R. Merget, N. Niere, M.M. Radoy, C. Schmidt, J. Schwenk, J. Somorovsky, in: Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24), 2024.","chicago":"Bäumer, Fabian, Marcus Brinkmann, Nurullah Erinola, Sven Niclas Hebrok, Nico Heitmann, Felix Lange, Marcel Maehren, et al. “TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations.” In <i>Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)</i>, 2024.","apa":"Bäumer, F., Brinkmann, M., Erinola, N., Hebrok, S. N., Heitmann, N., Lange, F., Maehren, M., Merget, R., Niere, N., Radoy, M. M., Schmidt, C., Schwenk, J., &#38; Somorovsky, J. (2024). TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations. <i>Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)</i>. Annual Computer Security Applications Conference, Hawaii.","ieee":"F. Bäumer <i>et al.</i>, “TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations,” presented at the Annual Computer Security Applications Conference, Hawaii, 2024.","ama":"Bäumer F, Brinkmann M, Erinola N, et al. TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations. In: <i>Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)</i>. ; 2024.","bibtex":"@inproceedings{Bäumer_Brinkmann_Erinola_Hebrok_Heitmann_Lange_Maehren_Merget_Niere_Radoy_et al._2024, title={TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations}, booktitle={Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)}, author={Bäumer, Fabian and Brinkmann, Marcus and Erinola, Nurullah and Hebrok, Sven Niclas and Heitmann, Nico and Lange, Felix and Maehren, Marcel and Merget, Robert and Niere, Niklas and Radoy, Maximilian Manfred and et al.}, year={2024} }","mla":"Bäumer, Fabian, et al. “TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations.” <i>Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)</i>, 2024."},"oa":"1","has_accepted_license":"1","status":"public","conference":{"end_date":"2024-12-13","name":"Annual Computer Security Applications Conference","start_date":"2024-12-09","location":"Hawaii"},"ddc":["006"],"user_id":"63563","_id":"57816","abstract":[{"text":"TLS-Attacker is an open-source framework for analyzing Transport\r\nLayer Security (TLS) implementations. The framework allows users\r\nto specify custom protocol flows and provides modification hooks to\r\nmanipulate message contents. Since its initial publication in 2016 by\r\nJuraj Somorovsky, TLS-Attacker has been used in numerous studies\r\npublished at well-established conferences and helped to identify\r\nvulnerabilities in well-known open-source TLS libraries. To enable\r\nautomated analyses, TLS-Attacker has grown into a suite of projects,\r\neach designed as a building block that can be applied to facilitate\r\nvarious analysis methodologies. The framework still undergoes\r\ncontinuous improvements with feature extensions, such as DTLS\r\n1.3 or the addition of new dialects such as QUIC, to continue its\r\neffectiveness and relevancy as a security analysis framework.","lang":"eng"}],"publication":"Proceedings of Cybersecurity Artifacts Competition and Impact Award (ACSAC ’24)","type":"conference","keyword":["SSL","TLS","DTLS","Protocol State Fuzzing","Planning Based"],"department":[{"_id":"632"}],"file":[{"content_type":"application/pdf","file_id":"66728","date_updated":"2026-08-17T07:42:08Z","relation":"main_file","file_size":530865,"access_level":"local","file_name":"comp-acsac24-final11.pdf","date_created":"2026-08-17T07:42:08Z","creator":"nniklas"}],"date_created":"2024-12-17T11:25:14Z","date_updated":"2026-08-17T07:42:41Z","year":"2024","title":"TLS-Attacker: A Dynamic Framework for Analyzing TLS Implementations","author":[{"last_name":"Bäumer","first_name":"Fabian","full_name":"Bäumer, Fabian"},{"last_name":"Brinkmann","first_name":"Marcus","full_name":"Brinkmann, Marcus"},{"last_name":"Erinola","first_name":"Nurullah","full_name":"Erinola, Nurullah"},{"first_name":"Sven Niclas","orcid":"0009-0006-1172-1665","last_name":"Hebrok","full_name":"Hebrok, Sven Niclas","id":"55616"},{"id":"74619","full_name":"Heitmann, Nico","last_name":"Heitmann","first_name":"Nico","orcid":"0009-0003-7687-7044"},{"id":"67893","first_name":"Felix","last_name":"Lange","full_name":"Lange, Felix"},{"full_name":"Maehren, Marcel","last_name":"Maehren","first_name":"Marcel"},{"full_name":"Merget, Robert","first_name":"Robert","last_name":"Merget"},{"id":"63563","first_name":"Niklas","last_name":"Niere","full_name":"Niere, Niklas"},{"full_name":"Radoy, Maximilian Manfred","orcid":"0009-0005-3059-6823","last_name":"Radoy","first_name":"Maximilian Manfred","id":"68826"},{"full_name":"Schmidt, Conrad","last_name":"Schmidt","first_name":"Conrad"},{"first_name":"Jörg","last_name":"Schwenk","full_name":"Schwenk, Jörg"},{"first_name":"Juraj","orcid":"0000-0002-3593-7720","last_name":"Somorovsky","full_name":"Somorovsky, Juraj","id":"83504"}],"main_file_link":[{"open_access":"1","url":"https://www.acsac.org/2024/program/artifacts_competition/comp-acsac24-final11.pdf"}],"language":[{"iso":"eng"}]}]