Enable post-quantum key exchange with OpenSSL 3.5

OpenSSL 3.5 is the first release with ML-KEM built in. Servers that use its default group list offer and prefer X25519MLKEM768 with no change.

Before you start

• Check the version your server links: openssl version shows the command-line tool; your web server or application may link a different copy (for nginx: nginx -V).
• Earlier OpenSSL releases need the separate OQS provider; upgrading to 3.5 is simpler.

Option 1: keep the defaults

OpenSSL 3.5's default list puts X25519MLKEM768 first, with X25519 and the NIST curves as fallbacks.

1. Look for a group list your server sets: grep -rni "groups\|curve" /etc/ssl/openssl.cnf and your server's own TLS settings.
2. Remove any list that leaves out X25519MLKEM768, then restart the server.

Option 2: set the groups system-wide

1. Find the openssl.cnf your server reads: openssl version -d shows the directory.
2. Under the section your system default points to, set:

[system_default_sect]
Groups = X25519MLKEM768:X25519:P-256

3. Restart the server so it reads the file again.

Option 3: set the groups in your own code

For an application that sets up TLS itself:

SSL_CTX_set1_groups_list(ctx, "X25519MLKEM768:X25519:P-256");

Keep X25519MLKEM768 first in every list, or clients that offer both get classical key exchange.

Check

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.