SSL & TLS

What Is an SSL Certificate? (HTTPS, TLS, and Browser Trust Explained)

A clear, non-marketing explanation of SSL/TLS certificates: what they prove, how browser trust works, common error messages, and how to fix the most frequent certificate problems.

Check an SSL certificate

Quick summary

  • Certificates encrypt traffic and prove a site's identity.
  • Browsers trust certificates issued by recognized authorities.
  • Most errors come from expiry, name mismatch, or chain issues.
  • Free certificates (Let's Encrypt) work for almost every use case.

What a certificate actually proves

An SSL/TLS certificate binds a public key to a domain name. When your browser connects, it verifies the server holds the matching private key and that a trusted Certificate Authority (CA) signed the certificate. That's how the padlock appears.

How browser trust works

Your operating system and browser ship with a list of trusted root CAs. Each site certificate chains up to one of these roots through intermediate certificates. If any link is missing or expired, you get a warning.

Common certificate errors and what they mean

  • NET::ERR_CERT_DATE_INVALID — expired or wrong system clock.
  • NET::ERR_CERT_COMMON_NAME_INVALID — certificate doesn't cover this hostname.
  • NET::ERR_CERT_AUTHORITY_INVALID — issued by an untrusted CA or self-signed.
  • SSL_ERROR_BAD_CERT_DOMAIN — the SAN list doesn't include your domain.
  • Incomplete chain — server didn't send intermediate certs.

Troubleshooting workflow

Run through these steps in order. Each step links to the right diagnostic tool.

  1. 1

    Run an SSL check

    See expiry, issuer, SANs, chain, and protocol versions in one report.

    Open SSL Checker
  2. 2

    Verify the domain in WHOIS

    Make sure the domain isn't expiring before the certificate.

    Open WHOIS Lookup
  3. 3

    Check DNS for CAA and resolution

    CAA records control who can issue certificates for your domain.

    Open DNS Lookup
  4. 4

    Run a website safety check

    Confirm the site isn't flagged for unrelated trust issues.

    Open Website Safety

Recommended next steps

SSL Certificate Lookup

DNS Records

WHOIS / RDAP

Website Safety

Fix connection errors

Improve website security

Frequently asked questions

Is SSL the same as TLS?

Practically yes. 'SSL' is the legacy name; modern certificates use TLS (currently TLS 1.2 and 1.3). The industry still says 'SSL certificate' out of habit.

Do I need an SSL certificate for a small site?

Yes. Browsers warn on any site without HTTPS, search engines deprioritize them, and free certificates from Let's Encrypt make it trivial.

Why do I get NET::ERR_CERT_DATE_INVALID?

The certificate is expired or your device clock is wrong. Run an SSL check to confirm the expiry date.

What's the difference between DV, OV, and EV certificates?

Domain Validated proves you control the domain. Organization Validated also verifies the company. Extended Validation adds stricter vetting. All offer the same encryption.

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