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Free tool · runs in your browser
Post-quantum TLS check: is your connection quantum-safe?
This page asks the server which key exchange your browser negotiated a moment ago. If it says X25519MLKEM768, the connection used hybrid post-quantum key exchange: a recording of it should stay unreadable even to a future quantum computer.
Built by Kyeson Blake Utley for the free Quantum Defense course.
What the result means
Every HTTPS connection starts with a key exchange, where your browser and the server agree on the keys that encrypt everything after. For years that has been X25519, an elliptic-curve exchange that a large enough quantum computer running Shor’s algorithm could break. Anyone who records today’s traffic could decrypt it later. That is harvest now, decrypt later.
X25519MLKEM768 is the hybrid that replaced it on the web. It runs X25519 and ML-KEM-768 (NIST’s FIPS 203 standard) side by side and feeds both secrets into the session keys, so an attacker has to break both. Your browser’s key share grows from 32 bytes to 1,216, and the server’s reply from 32 to 1,120.
Why you might see “classical only”
- An older browser. Chrome switched to X25519MLKEM768 in version 131 (November 2024) and Firefox uses it by default from version 132 on desktop; Edge and other Chromium browsers followed Chrome.
- Something in the middle. Corporate TLS-inspection proxies and some antivirus products terminate your connection and open a new one, often without post-quantum support.
- A setting or policy that turned the hybrid off, usually to work around old network equipment that couldn’t handle the larger first message.
How the check works
This site runs behind Cloudflare, which reports the negotiated key exchange at /cdn-cgi/trace on the same domain. The page reads only the tls, http and kex lines, in your browser. Nothing is logged or sent anywhere. It tests your connection to this site; another site’s server may not offer the hybrid at all.
This tool comes from Lesson 8: Hybrid key exchange: post-quantum TLS today, part of a free 18-lesson post-quantum cryptography course. Every figure there links to its source.
Questions
What is X25519MLKEM768?
A hybrid TLS 1.3 key exchange (code point 0x11EC) that combines classical X25519 with ML-KEM-768 from NIST FIPS 203. Both shared secrets feed the session keys, so breaking the connection requires breaking both.
Does post-quantum key exchange make the whole connection quantum-safe?
It protects the traffic against later decryption, which is the urgent part. The server's certificate and signature are still classical (RSA or ECDSA), but those only matter at the moment of connection and can't be forged retroactively for traffic that was already recorded.
Which browsers support post-quantum TLS?
Current Chrome, Edge, Firefox and Safari 26 offer X25519MLKEM768 by default. Whether it's used also depends on the server and anything in between, such as a corporate proxy.
Is my data sent anywhere by this check?
No. The page reads Cloudflare's trace for this same site in your browser and shows the result. There are no trackers or analytics on this site.