Encrypted ClientHello / ESNI

A June 2024 MESA Lab internal survey ("针对审查系统的科学研究及探测技术调研报告") explicitly states its purpose is to catalog academic/public censorship-measurement and circumvention research (OONI, Augur, Satellite, GFWatch, Citizen Lab, CensorBib, FOCI/IMC/NDSS/CCS/USENIX Security papers) in order to find and patch GFW/censorship-system vulnerabilities before outside researchers exploit them. It systematically covers circumvention protocols/tools (Shadowsocks, VMess, Trojan, decoy routing, Parrot-style mimicry, CovertCast, Slitheen++, ESNI/ECH, uTLS) and notes GFW blocked ESNI (not ECH) since July 2020, plus historical TLS-fingerprint blocking of meek by a Cyberoam firewall (2016).

The same SSL plugin added explicit detection of the Encrypted Client Hello extension (type 0xFE0D) with test cases in June 2023, and separately implements JA3/JA3S (2020) and JA4/JA4S (2024) TLS client/server fingerprint calculation as first-class features -- so TSG-class DPI can flag 'ECH is in use' even without decrypting it, and independently fingerprints TLS stacks via JA3/JA4.

A DPI session-plugin ("stellar" framework) SSL/TLS decoder fully parses live ClientHello/ServerHello handshakes, computing JA3/JA3S fingerprints (MD5 over version+cipher-suites+extensions with GREASE values stripped per the standard JA3 spec), extracting plaintext SNI, explicitly flagging whether a ClientHello carries ESNI (extension type 0xFFCE) or ECH (extension type 0xFE0D), and decoding full X.509 certificate chains (issuer/subject RDN sequences, SAN, validity window, serial, pubkey, signature algorithm) in real time. It also recognizes TLCP 1.0 (China's national "国密"/GmSSL TLS variant) as a distinct protocol version alongside SSLv2 through TLS1.3.

chongming/traffic_replay is a Flask/tcpreplay-based QA harness that replays curated pcaps against a TSG instance to exercise its decoders, including SSL fixtures specifically named ssl_ech_false.pcap, ssl_no_sni.pcap and ssl_tls13.pcap alongside esni.pcap, openvpn.pcap, and major-platform captures (facebook/google/twitter/ youtube). A companion update_tsg_ca.py/tsg_ca.txt installs a TSG-issued CA cert on the test client, confirming the QA environment exercises TLS interception as part of routine detection testing.

TSG's security-policy verification engine (tango/verify-policy) supports 'ssl.no_sni', 'ssl.ech' (Encrypted ClientHello), and 'ssl.esni' (Encrypted SNI) as first-class Boolean match conditions in its policy language (TSG-18943), meaning TSG can write and evaluate blocking/monitoring policies that specifically trigger on TLS connections carrying no SNI, ECH, or ESNI.