Every website that handles user data — logins, payments, form submissions — must use HTTPS. But HTTPS is only secure when the underlying SSL/TLS certificate is valid, correctly configured, and up to date. An expired certificate triggers browser warnings that tell visitors your site is "not secure," driving them away instantly.
An SSL checker lets you inspect any domain's certificate without a browser or command-line tool — checking expiry, issuer, chain validity, and configuration from anywhere.
Check Any Domain's SSL Certificate
SSL Certificate Checker
Check SSL certificate validity and expiry.
What SSL/TLS Actually Is
Despite the common name "SSL certificate," modern web encryption uses TLS (Transport Layer Security), not the older SSL protocol. SSL 2.0 and 3.0 were deprecated due to serious vulnerabilities. TLS 1.2 and 1.3 are current standards.
The certificate itself is a digital document that:
- Proves identity: Confirms that
example.comis actually owned by the entity the certificate says it is - Enables encryption: Contains the public key used to establish an encrypted connection
- Is signed by a Certificate Authority (CA): A trusted third party that vouched for the identity verification
When your browser connects to a HTTPS website, it verifies all three things before showing the padlock icon.
How to Read an SSL Certificate
An SSL checker returns these key fields:
| Field | What It Means |
|---|---|
| Common Name (CN) | The domain the certificate is issued for |
| Subject Alternative Names (SAN) | Additional domains covered (e.g., www.example.com, *.example.com) |
| Issuer | The Certificate Authority that signed the cert (Let's Encrypt, DigiCert, etc.) |
| Valid From | Certificate issue date |
| Valid Until / Expiry | Date the certificate stops being trusted |
| Days Remaining | How long until expiry |
| Key Algorithm | RSA or ECDSA; key size (2048-bit RSA or 256-bit EC are standard) |
| Signature Algorithm | SHA-256 is current standard; SHA-1 is deprecated |
| Certificate Chain | The chain from your cert → intermediate CA → root CA |
Certificate Types Explained
Domain Validation (DV)
The CA verifies only that you control the domain. Fastest and cheapest (often free with Let's Encrypt). Sufficient for most websites.
Padlock shown: Yes
Identity verified: Domain ownership only
Organisation Validation (OV)
The CA verifies the domain and the legal existence of the organisation. Takes 1–3 days. Used by established businesses.
Padlock shown: Yes
Identity verified: Domain + organisation details
Extended Validation (EV)
The highest verification level — the CA verifies legal identity, physical address, and operational existence. Previously showed the company name in the browser bar (most browsers have phased this out).
Padlock shown: Yes
Still worth it for: High-value financial or e-commerce sites where the certificate's legal identity claim matters
Wildcard Certificates
Covers one domain and all its subdomains: *.example.com covers www.example.com, api.example.com, blog.example.com. One certificate for all subdomains.
Multi-Domain / SAN Certificates
One certificate covering multiple distinct domains: example.com, example.co.uk, exampleshop.com.
Common SSL Problems
1. Expired Certificate
The most common issue. Browsers show a hard-blocking "Your connection is not private" error. Visitors cannot proceed without clicking through a security warning most people won't override.
Fix: Renew the certificate immediately. With Let's Encrypt (free), set up auto-renewal via Certbot or your hosting provider's auto-renewal feature.
2. Hostname Mismatch
The certificate is valid but issued for a different domain. E.g., certificate is for example.com but the visitor is accessing www.example.com and www isn't covered.
Fix: Reissue the certificate to include all variants — both example.com and www.example.com. A wildcard *.example.com also resolves this for subdomains.
3. Incomplete Certificate Chain
Your certificate is valid but your server isn't serving the intermediate CA certificates. Some browsers can recover automatically; others (especially on mobile) may show errors.
Fix: Configure your server to serve the full certificate chain — your cert + all intermediate certificates up to (but not including) the root CA.
4. Mixed Content
The page loads over HTTPS but includes resources (images, scripts, stylesheets) loaded over HTTP. The browser blocks or warns about these.
Fix: Update all resource URLs to use https:// or protocol-relative URLs (//). Use a CSP header to enforce HTTPS for all resources.
5. Self-Signed Certificate
A certificate you generated yourself rather than getting it signed by a trusted CA. Useful for internal development; always triggers browser warnings in production.
Fix: Replace with a CA-signed certificate. Let's Encrypt provides free, trusted DV certificates.
SSL and SEO
Google confirmed HTTPS as a ranking signal in 2014. More importantly:
- Chrome marks all HTTP pages as "Not Secure" in the address bar
- HTTP-only sites get flagged in Google Search Console
- Users who see security warnings have very high bounce rates
An expired or broken SSL certificate effectively takes your site offline for most users — the SEO impact is severe if not fixed within hours.
Auto-Renewal Setup (Critical)
Most SSL certificate problems are caused by forgetting to renew. Let's Encrypt certificates expire every 90 days by design (to encourage automation). Set up auto-renewal:
Certbot (Linux servers):
certbot renew --dry-run # Test auto-renewal
Add to cron: 0 0,12 * * * /usr/bin/certbot renew --quiet
Vercel, Netlify, Cloudflare: Fully automatic — no action needed.
cPanel hosting: Usually has a built-in Let's Encrypt auto-renew toggle in SSL/TLS settings.
Set a calendar reminder 30 days before expiry as a backup regardless of auto-renewal.
Privacy Note
SSL certificate checking queries publicly available certificate data that any browser retrieves automatically when visiting a site. No private data is involved. FluxToolkit does not log the domains you check.
Frequently Asked Questions
How do I know if my SSL certificate is about to expire?
Run an SSL checker on your domain — it shows the expiry date and days remaining. For proactive monitoring, set a calendar alert 30 days before expiry, or use an uptime monitoring service with SSL expiry alerts.
Is Let's Encrypt as trusted as paid SSL certificates?
Yes, for domain validation. Let's Encrypt is a legitimate, widely-trusted CA. Its certificates are recognised by all major browsers. Paid certificates add organisation or extended validation layers, which Let's Encrypt doesn't offer.
Why does Chrome still show "Not Secure" even though I installed an SSL certificate?
This is usually a mixed content issue — the page loads over HTTPS but contains resources (images, scripts) still being requested over HTTP. Open the browser's developer console and check the Security panel for specific mixed content warnings.
What's the difference between TLS and SSL?
SSL (Secure Sockets Layer) is the older protocol — now deprecated and vulnerable. TLS (Transport Layer Security) is the current standard. TLS 1.2 and 1.3 are what browsers actually use. The term "SSL certificate" persists as a colloquialism even though the underlying protocol is TLS.
Does FluxToolkit store the domains I check?
No. SSL checks query public certificate data in real time and results are not logged.
Related Articles
- WHOIS Lookup Guide — Check domain registration details alongside SSL.
- Domain Availability Checker Guide — Find a domain before setting up SSL.
- HTTP Redirect Checker Guide — Verify HTTP→HTTPS redirects are configured correctly.
- HTTP Headers Checker Guide — Check HSTS and security headers alongside SSL.
- DNS Records Lookup Guide — Verify DNS is correctly pointed to your secured server.