GFW blocks VPN and proxies, and they are extremely good at it.
The wall CAN detect and block ProtonVPN, even in its stealth mode.
ProtonVPN and Tor Webtunnel did not consider advanced encrypted traffic analysis in their design, but the Chinese GFW has been performing ETA to detect proxies for a decade. There is another TLS-based proxy protocol called Trojan that did not mitigate this, and it was detected and blocked in China ever since 2022.
See https://www.usenix.org/conference/usenixsecurity24/presentation/xue-fingerprinting for a basic idea of how this is possible. To put it simple, the inner TLS handshake reflects in a fingerprint even with the encryption of the outer TLS, because you cannot hide the very stable signal of packet size, direction and timing with encryption.
And there are a million other fingerprints that make much more sense than this TLS-over-TLS fingerprint. For example, the libraries that mimic the browser TLS fingerprint (like JA4) are doing a terrible job. They are just barely enough to fool the GFW so far.
There are quite some protocols that mitigate TLS-over-TLS: the ones that work in China. You can arguably say they are the hardest to detect (though far from perfect). For example, vless-xtls-vision and vless-xhttp. They use padding and mutiplex, which are explained in the paper above.
They are both implemented in:
The docs are on
These stuff are extremely complex, and to achieve the highest level of anonymity (in terms of hard to detect) you need professional knowledge. You can find paid services that deploy these kind of proxies, Chinese students and programmers use them everyday. But from a security and privacy point of view you definitely don’t want to buy them.
Xray project does offer a beginners’ guide of deploying vless-xtls-vision on your own server. It is not their best protocol but still works for now.