SSL Certificate Checker

The SSL Checker analyzes any HTTPS website\u2019s SSL/TLS certificate and reports everything you need to know: validity dates, issuer, subject, Subject Alternative Names (SANs), cipher suites, protocol versions supported, and certificate chain completeness. SSL/TLS certificates are the foundation of HTTPS; an expired or misconfigured cert breaks the site for visitors. Monitoring and validating certificates is a routine task for site administrators, security engineers, and anyone managing web properties.

Enter any domain and the tool fetches the certificate directly from the server and reports: certificate validity (not-before and not-after dates), issuer (Let\u2019s Encrypt, DigiCert, Sectigo, etc.), subject and all SANs (which domains the cert covers), signature algorithm, key size, cipher suites supported, TLS protocol versions, and chain completeness (whether intermediate certificates are included). Warnings flag expiring certificates (under 30 days), weak ciphers, outdated protocols (TLS 1.0 and 1.1), missing intermediates, and hostname mismatches. All analysis runs via public API calls to certificate transparency logs and direct probes.

SSL/TLS certificates are X.509 structures signed by Certificate Authorities. Key fields:

Validity: notBefore and notAfter dates. Modern certs are 90 days (Let\u2019s Encrypt default) or up to 1 year (paid CAs). Browsers warn on expired certs; many services break outright.

Subject: the domain(s) the cert is for. CN (Common Name) field plus SubjectAlternativeNames extension. Modern certs rely on SAN because CN is deprecated.

Issuer: the CA that signed the cert. Let\u2019s Encrypt is most common for small sites; DigiCert, Sectigo, GoDaddy for enterprise.

Chain: cert → intermediate CA → root CA. The server must send the cert and intermediates; the browser has root CAs built in. Missing intermediates cause \"unable to verify\" errors in some clients (particularly older Android and custom HTTP clients).

Signature algorithm: SHA-256 is current. SHA-1 is deprecated and rejected by modern browsers.

Key size and algorithm: RSA 2048+ or ECDSA 256+ bits. Weaker keys are rejected.

TLS versions: TLS 1.2 is minimum for modern security. TLS 1.3 is current best. TLS 1.0 and 1.1 are deprecated (PCI DSS forbids them since 2018).

Cipher suites: the algorithms used for encryption, authentication, and MAC in TLS. Modern strong suites: AES-GCM, ChaCha20-Poly1305. Weak: RC4, 3DES, export-grade cipher. The tool flags weak suites.

HSTS (Strict-Transport-Security): HTTP response header telling browsers to always use HTTPS. Not part of the cert but critical for security posture.

Certificate Transparency: all certs are logged in public CT logs. Browsers require CT for Chrome-trusted certs. The tool can query CT logs to find all certs ever issued for a domain.

Grade (A+/A/B/C/D/F): composite score based on protocol, cipher, key strength, and known vulnerabilities. SSL Labs\u2019 grading is the canonical reference.

Compared to SSL Labs\u2019 SSL Test, this tool is faster for quick checks with the same core information. SSL Labs provides more detail and a more thorough vulnerability scan; use it for deep audits.

Compared to openssl s_client command line, this tool has a web UI with clearer formatting and automated warnings. CLI is better for scripting and automation; this tool for interactive checks.

Compared to paid certificate monitoring services (DigiCert, Sectigo dashboards), this tool is for ad-hoc checking rather than continuous monitoring. Use monitoring services for production alerting.