SSL & TLS Certificate Checker

1. Certificate Validity & Expiry

We verify the certificate is issued by a publicly trusted Certificate Authority, the domain matches (no CN mismatch), and the expiration date is more than 14 days away. Certificates expiring within 30 days trigger a warning.

An expired certificate triggers the full “Your connection is not private” interstitial in Chrome, Firefox, and Safari. Studies consistently show 97%+ of users abandon sites that show this warning. Use Let's Encrypt with certbot renew --pre-hook and a cron job to auto-renew 30 days before expiry.

2. TLS Version Support

We test which TLS versions your server will accept and flag deprecated versions.

ssl_protocols TLSv1.2 TLSv1.3;

3. Cipher Suites

Cipher suites define the cryptographic algorithms used during a TLS session. Weak cipher suites allow attackers to decrypt traffic if they have enough computation or if the algorithm is broken.

Cipher suites to disable: RC4 (broken), DES and 3DES (SWEET32 attack), EXPORT ciphers (intentionally weak, used in FREAK attack), anonymous DH (no authentication), NULL encryption.

Perfect Forward Secrecy (PFS) is guaranteed by cipher suites using ECDHE or DHE key exchange. PFS means a future compromise of your server's private key cannot be used to decrypt recorded past sessions — each session key is ephemeral and never stored.

ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305;
ssl_prefer_server_ciphers off;

4. HSTS (Strict-Transport-Security)

A valid SSL certificate ensures the connection is encrypted. HSTS ensures the browser always initiates that encrypted connection — without an HTTP first step that could be intercepted.

Without HSTS, even with a valid certificate, a user on a coffee shop Wi-Fi can have their first HTTP request intercepted before the server redirects to HTTPS. The attacker strips TLS and reads the connection in plaintext — the user's browser shows no warning.

Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

5. HTTP to HTTPS Redirect

We verify that your server redirects all HTTP traffic to HTTPS and check the redirect status code. A 307 Temporary Redirect means browsers may retry HTTP on future visits. 308 Permanent Redirect is semantically correct for HSTS preloading and SEO — search engines update their index to the HTTPS URL.

server {
    listen 80;
    server_name example.com www.example.com;
    return 308 https://$host$request_uri;
}

6. Certificate Chain Completeness

A valid TLS handshake requires your server to send the complete chain: your domain certificate, the intermediate CA certificate(s), up to a root CA trusted by the client. Modern browsers cache intermediate certificates, but an incomplete chain fails on Android 7 and older, Windows XP, embedded devices, and CLI tools like curl without system CA bundles.

Common mistake: only deploying the domain certificate without the intermediate CA. This works in Chrome (which caches intermediates) but breaks in curl, Postman, and older mobile browsers — causing silent API failures in production.

# fullchain.pem = domain cert + intermediate(s). NOT cert.pem alone.
ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;