Let's Encrypt is a certificate authority that launched on April 12, 2016 that provides free X.509 certificates for Transport Layer Security (TLS) encryption via an automated process designed to eliminate the current complex process of manual creation, validation, signing, installation, and renewal of certificates for secure websites.

By default, certbot generates 2048 bit certificates, but 4096 bit should be preferred. This can be changed in the configuration:

# Use a 4096 bit RSA key instead of 2048
rsa-key-size = 4096

Then renew your certificate by requesting a new one for all your (sub)domains:

sudo certbot --nginx certonly -d -d -d

And select 2: Renew & replace the cert (limit ~5 per 7 days).

Diffie-Hellman Ephemeral Parameters

Diffie–Hellman key exchange (D–H) is a method of securely exchanging cryptographic keys over a public channel and was one of the first public-key protocols as originally conceptualized by Ralph Merkle and named after Whitfield Diffie and Martin Hellman.

Generate dhparams.pem:

openssl dhparam -out dhparams.pem 4096
sudo mv dhparams.pem /etc/ssl/
sudo chown root:ssl-cert /etc/ssl/dhparams.pem
sudo chmod 640 /etc/ssl/dhparams.pem

SSL cipher suite

Edit the global configuration file:

# SSL Settings
ssl_protocols TLSv1.3 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_dhparam /etc/ssl/dhparams.pem;
ssl_ecdh_curve secp521r1:secp384r1;
ssl_session_timeout 10m;
ssl_session_cache shared:SSL:10m;
ssl_session_tickets off;
ssl_stapling on;

HTTP headers

Create a configuration file:

add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload";
add_header X-Content-Type-Options nosniff;
add_header X-Frame-Options DENY;

sudo systemctl restart nginx.service