SSL Certificate Checker

Purpose & User Intent: The tool provides an automated check of SSL certificates for a given domain. Target users include site owners, DevOps, security engineers, and developers who need quick visibility into certificate validity without external tooling. It solves the problem of uncertain SSL status, expiry risk, and chain integrity during development, testing, and site deployment. When a domain is entered, the tool retrieves the TLS certificate, analyzes its fields, and returns actionable details.
Core Logic & Features: Required inputs include domain name (string, FQDN, sample: example.com) and optional port (integer, default 443). Outputs include certificate validity status (valid/expired), expiry date, issuer, subject, and chain status. Advanced features include days until expiry, warning thresholds, and multi-host checks. Distinguishes between missing certificates, self-signed certs, and incomplete chains, and flags each condition.
Inputs & Outputs: Inputs: domain_name (string, required, sample: example.com), port (integer, optional, sample: 443). Outputs: status (string: valid/invalid, valid if the certificate is trusted for the domain), expiry_date (date, ISO 8601), issuer (string), subject (string), sha256_fingerprint (string), chain_complete (boolean), tls_version (string), days_to_expiry (integer). Formatting: expiry_date in ISO format; days_to_expiry is negative if expired.
Algorithms & Calculations: TLS handshake to fetch certificate, parse X.509 fields, validate against a trust store, compute days_to_expiry = expiry_date - today. Determine chain integrity by presence of intermediates and trust anchor. Apply UTC times. Error & Edge Cases: Unresolvable domain, port closed, firewall blocking TLS, certificate not valid for domain, missing intermediates, expired cert, clock skew. Return structured status with code and reason.
Industry/Region & Localization: Uses UTC times; supports TLS 1.2/1.3; regional display formats may vary but internal times use ISO; no currency or locale-specific data. Assumptions & Exclusions: Assumes network access from the tool environment; excludes in-depth CA policy validation or revocation checking beyond standard trust store; not a full risk assessment tool.