Enable X25519MLKEM768 on nginx

nginx gets post-quantum key exchange from the OpenSSL it's built with. With OpenSSL 3.5 or later, X25519MLKEM768 is on by default; you only need to make sure your configuration doesn't switch it off.

Before you start

• nginx built with OpenSSL 3.5 or later. Check with nginx -V 2>&1 | grep -i openssl. The official nginx:1.29-alpine image uses OpenSSL 3.5.
• TLS 1.3 enabled: post-quantum key exchange needs it.

Option 1: use OpenSSL's defaults (simplest)

1. Find any ssl_ecdh_curve line in your configuration: grep -rn ssl_ecdh_curve /etc/nginx/.
2. Remove it, or set it to auto. OpenSSL 3.5's default list puts X25519MLKEM768 first.
3. Make sure TLS 1.3 is allowed:

ssl_protocols TLSv1.2 TLSv1.3;

4. Test and reload: nginx -t && nginx -s reload.

Option 2: list the groups yourself

Use this if you need a fixed list, for example to keep a classical fallback in a known order.

ssl_protocols TLSv1.2 TLSv1.3;
ssl_ecdh_curve X25519MLKEM768:X25519:prime256v1;

Keep X25519MLKEM768 first: if X25519 comes first, browsers that offer both get classical key exchange (our checker shows this as "supported, not preferred").

Test and reload: nginx -t && nginx -s reload.

Check from the command line

With OpenSSL 3.5 on your machine:

openssl s_client -connect yourdomain.com:443 -servername yourdomain.com -groups X25519MLKEM768 </dev/null 2>/dev/null | grep -i "group"

Then scan your domain below to check every host at once.