SSL/TLS Certificate Checker

What is an SSL/TLS Certificate?

An SSL/TLS certificate is a digital certificate that authenticates a website’s identity and enables an encrypted connection between a client and a server. When a certificate is installed on a web server, it activates the padlock icon in browsers and allows HTTPS connections. Certificates are issued by Certificate Authorities (CAs) such as Let’s Encrypt, DigiCert, Sectigo, and GlobalSign, which verify that the certificate requester legitimately owns the domain. Modern TLS certificates support key lengths up to 4096 bits and use signature algorithms like SHA-256 with RSA or ECDSA.

Every TLS certificate contains critical fields that network engineers and system administrators need to verify. The Subject field identifies the entity the certificate was issued to, including the Common Name (CN) and Organization (O). The Issuer field identifies the CA that signed the certificate. The Validity period specifies the date range during which the certificate is considered valid, and checking the days remaining is essential for preventing unexpected expiry outages. Subject Alternative Names (SANs) list all domain names the certificate covers, including wildcard entries like *.example.com. The Fingerprint uniquely identifies the certificate for verification purposes.

Regular SSL monitoring is critical for production infrastructure. Expired certificates cause browser security warnings, API failures, and can break automated deployments. Certificate transparency logs now require all publicly trusted CAs to log every certificate they issue, providing an audit trail for domain ownership. Our SSL checker performs a real-time TLS handshake with the target server and returns the raw certificate data exactly as presented, including the complete certificate chain. You can verify that your certificates are correctly installed, check that SANs include all required domains, and ensure your certificate hasn’t expired or been revoked. Use this tool to audit your own domains, verify third-party service certificates, or troubleshoot TLS-related connectivity issues.

Certificate Chain and Validation

A properly configured TLS certificate includes the full certificate chain: the leaf certificate for your domain, one or more intermediate certificates, and the root CA certificate. Browsers and clients build this chain from leaf to trusted root when establishing a connection. Certificate revocation is checked through Certificate Revocation Lists (CRLs) or the Online Certificate Status Protocol (OCSP), though OCSP stapling improves performance by having the server periodically fetch and cache the signed OCSP response. When choosing a certificate, you typically decide between wildcard certificates covering *.example.comand all subdomains at one level, or Subject Alternative Name (SAN) certificates listing specific domains individually. Wildcard certificates simplify management but pose a greater risk if compromised — an attacker gaining access to a wildcard private key can impersonate any subdomain, while SAN certificates provide finer-grained control and isolation.