What makes a good certificate

How it works

The certificate confirms to the browser that the server's key belongs to your domain name. A certificate authority issued it; the server sends it along with one or more intermediate certificates that show the way to an authority the browser trusts.

Every public certificate is entered in Certificate Transparency logs, public directories of all certificates issued. The proofs of that (SCTs) sit in the certificate; Chrome and Safari require them. That way it shows when someone gets a certificate for your domain that you did not order.

Browsers learn whether a certificate was revoked early from revocation lists or through OCSP. If the certificate names an OCSP server, your server can send its answer right along (OCSP stapling). Let’s Encrypt has issued certificates without OCSP since 2025; then there is nothing to staple.

A CAA record in DNS says which certificate authorities may issue certificates for your domain at all. All others must refuse.

What matters

• Renew automatically. Since March 2026 certificates are valid for 200 days at most, from 2027 only 100, from 2029 only 47. That no longer works well by hand; an ACME client such as Certbot or your host's control panel renews in time.
• A key long enough. RSA with at least 2048 bits or ECDSA with P-256. ECDSA is shorter and faster.
• The complete chain in the right order. First your certificate, then the intermediates. With Let’s Encrypt that is the file fullchain.pem.
• No SHA-1 signatures. Browsers reject them; today's certificates use SHA-256 or more.
• A CAA record for your authority. It keeps another authority from issuing on a forged request. If you change authorities, update the record first, or the renewal fails.
• OCSP stapling where there is OCSP. If your certificate names an OCSP server, switch stapling on. Put “Must-Staple” in the certificate only if stapling runs reliably; otherwise Firefox refuses the connection.

How to set it up

The CAA record goes into the DNS zone of the domain; it also covers all subdomains without a record of their own. The second line names an address where certificate authorities report refused requests.

example.com.  CAA 0 issue "letsencrypt.org"
example.com.  CAA 0 iodef "mailto:security@example.com"

In the web server, the complete chain and, if the certificate names an OCSP server, stapling go into the configuration.

ssl_certificate     /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
ssl_stapling        on;
ssl_stapling_verify on;

SSLCertificateFile    /etc/letsencrypt/live/example.com/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/example.com/privkey.pem
SSLUseStapling        on

With Apache, stapling also needs SSLStaplingCache "shmcb:logs/ssl_stapling(32768)" outside the VirtualHost. If the certificate names no OCSP server, leave the stapling lines out.