SSL Certificate Checker

SSL Certificate Checker: Core Features and Principles

Expired or misconfigured website SSL certificates directly trigger browser security warnings, leading to visitor loss and potential business interruption. This tool performs a real-time probe of the SSL/TLS certificate chain returned by the server for a target domain. It parses and displays key fields such as the Certificate Authority (CA), validity period (start and end dates), remaining days, signature algorithm, public key length, and Subject Alternative Names (SAN). Simultaneously, it automatically verifies whether the certificate is within its validity period, if the domain matches, and if the certificate chain is complete and trusted, thereby providing a clear valid or invalid status.

Why Choose Our SSL Certificate Checker?

• No configuration required. Simply enter a domain name to get complete certificate details and a validity check with a single click.
• Intuitively displays the certificate validity period and remaining days, helping users spot expiring certificates in advance and avoid service downtime risks.
• Outputs multi-dimensional information including the certificate chain, encryption algorithm strength, and SAN domain lists, meeting needs from routine checks to in-depth troubleshooting.

Tool Tutorial

1. Enter the domain you want to check in the input box, such as www.example.com. No https:// prefix is needed.
2. Click the "Check" button and wait for the server to return the results.
3. The page will display detailed certificate information, including:
   • Status: Valid or Invalid (including error reasons)
   • Issuer: e.g., DigiCert Inc
   • Validity Period: From 2024-01-01 to 2025-01-01
   • Remaining Days: e.g., 120 days
   • Signature Algorithm and Public Key Information
   • Subject Alternative Names (supported wildcards or subdomains)

Example: Entering google.com yields a result like—Status: Valid, Issuer: GTS CA 1C3, Validity Period: 2024-10-20 to 2025-01-12, Remaining Days: approx. 83 days, Signature Algorithm: sha256WithRSAEncryption.

Frequently Asked Questions (FAQ)

• What are the possible reasons for a certificate being marked as invalid? The most common reasons include: the certificate has expired, the certificate is not yet valid, the domain name does not match the name bound to the certificate, the Certificate Authority is untrusted, or the certificate chain is incomplete/missing intermediate certificates. Please troubleshoot based on the detailed error message returned.
• What should I do if the check result is "Unable to connect"? Please verify that the entered domain is correct and publicly accessible, the server has port 443 open and provides HTTPS services, and there are no firewalls or proxies blocking the connection on your local network. You can try accessing https://[domain] directly in your browser to test connectivity.

Important Notes

This tool only supports checking standard domains (including subdomains) without adding protocol headers. It simulates a standard TLS handshake to fetch the certificate, and the results reflect the true status at the current moment, excluding historical records. The checking process does not store your domain information on our servers, but it is still recommended to avoid entering internal test domains containing sensitive business logic. If a domain is configured with multiple certificates (e.g., in SNI scenarios), the tool will display the certificate corresponding to the default virtual host. Abnormal scenarios such as DNS resolution failures, connection timeouts, or server connection refusals will provide clear error prompts. Please adjust the domain or network environment according to the prompts and try again.

Technical Notes & Recommendations

Conventional RSA 2048-bit certificates are gradually being replaced by ECDSA P-256 for better encryption/decryption performance and security strength. It is recommended to prioritize ECC curves when applying for new certificates. For multi-domain or wildcard domain scenarios, you can confirm the certificate's coverage scope by checking the "Subject Alternative Names" (SAN) field. In production environments, it is advisable to use monitoring scripts to periodically call similar checking logic and automatically trigger alerts 30 days before a certificate expires, preventing downtime caused by human oversight. A typical healthy output example—Domain: api.example.io, Status: Valid, Issuer: Let's Encrypt R3, Remaining Days: 62 days, SAN: api.example.io, *.example.io.