China’s Great Firewall “is now blocking HTTPS connections with TLS 1.3 and which use ESNI (Encrypted Server Name Indication)
The reason for the ban is obvious for experts. HTTPS connections negotiated via TLS 1.3 and ESNI prevent third-party observers from detecting what website a user is attempting to access. This effectively blinds the Chinese government’s Great Firewall surveillance tool from seeing what users are doing online.
There is a myth surrounding HTTPS connections that network observers (such as internet service providers) cannot see what users are doing. This is technically incorrect. While HTTPS connections are encrypted and prevent network observers from viewing/reading the contents of an HTTPS connection, there is a short period before HTTPS connections are established when third-parties can detect to what server the user is connecting. This is done by looking at the HTTPS connection’s SNI (Server Name Indication) field.
HTTPS connections negotiated via older versions of the TLS protocol (such as TLS 1.1 and TLS 1.2), the SNI field is visible in plaintext.