China now blocking ESNI-enabled TLS 1.3 connections, say Great-Firewall-watchers

And needs a very blunt instrument to do the job, because the protocol works as planned

China is now blocking encrypted HTTPS traffic that uses TLS 1.3 with ESNI enabled, according to observers at the Great Firewall Report (GFR).

TLS is the foundation of secure online communication and hides content users wish to access or have generated so it can pass over the internet without being observed by unrelated parties.

While TLS hides the content of a user's communication, it cannot always hide the server they are communicating with because its handshake optionally contains a Server Name Indication (SNI) field designed to explain where traffic is going. China and other nations have used this info to block their users from accessing particular websites.

To address that privacy gap, TLS introduced Encrypted SNI (ESNI). ENSI encrypts the SNI so that intermediaries cannot view it and thus, in theory at least, prevent overzealous censors from sniffing and blocking traffic headed to and from places they don't like.

But according to the GFR, China has found one way around this: outright block all TLS 1.3 connections with ESNI enabled.

The banhammer is implemented by simply dropping offending packets, which is blunter than China's previous content-control strategies. These relied on sending forged TCP reset commands to both server and client to close a connection.

The GFR team also found that any time an ESNI connection was black-holed, any connection with the same tuple triple of source IP, destination, and destination port would continue to be blocked for up to 180 seconds. The block also happens on all ports, not just port 443 as is usually the case.

The GFR found that it could circumvent the new blocks using Geneva, a genetic algorithm developed by the University of Maryland in America that manipulates packet streams without impacting the original connection. Using Geneva, the GFR team discovering six strategies that work from the client side and four that work from the server side with 100 per cent reliability. ®