SSL Certificate Check

An SSL (Secure Sockets Layer) certificate is a digital credential that authenticates a website's identity and enables an encrypted connection between a web server and a browser. Although the term "SSL" is still widely used, modern implementations rely on TLS (Transport Layer Security), the successor protocol that addresses vulnerabilities found in earlier SSL versions.

When a browser connects to an SSL/TLS-protected website, the certificate serves two critical functions: authentication (proving the server is who it claims to be) and encryption (ensuring data transmitted between the client and server cannot be intercepted or tampered with). This is achieved through a cryptographic handshake that establishes a shared session key without transmitting it over the network.

SSL certificates are issued by Certificate Authorities (CAs) — trusted organizations that verify the identity of certificate applicants. There are three validation levels: Domain Validation (DV), which confirms domain ownership; Organization Validation (OV), which verifies the organization's identity; and Extended Validation (EV), which requires rigorous identity checks and was historically shown with a green address bar in browsers.

Every SSL certificate contains a public key, the domain name, the issuer's digital signature, validity dates, and Subject Alternative Names (SANs) for multi-domain coverage. The certificate's security depends on its key size (RSA 2048+ bits or ECDSA P-256+), signature algorithm (SHA-256 minimum), and the overall TLS configuration of the server.

Without a valid SSL certificate, browsers display security warnings that erode user trust, search engines penalize the site's rankings, and sensitive data like passwords and payment information is transmitted in plaintext — exposing it to interception through man-in-the-middle attacks.

TLS 1.3 is the latest version of the Transport Layer Security protocol, finalized in RFC 8446 (August 2018). It represents a major overhaul of the TLS handshake process, removing support for legacy cryptographic algorithms and reducing connection latency.

Key improvements in TLS 1.3 include: a simplified handshake that completes in just one round-trip (1-RTT) compared to two in TLS 1.2; removal of insecure cipher suites like RC4, DES, 3DES, and static RSA key exchange; mandatory Perfect Forward Secrecy (PFS) through ephemeral Diffie-Hellman; and support for 0-RTT resumption for returning connections, enabling near-instant encrypted communication.

TLS 1.3 only supports five cipher suites, all of which use AEAD (Authenticated Encryption with Associated Data) algorithms: AES-128-GCM, AES-256-GCM, ChaCha20-Poly1305, AES-128-CCM, and AES-128-CCM-8. This dramatically reduces the attack surface compared to TLS 1.2, which supports dozens of cipher combinations including some known to be vulnerable.

From a performance perspective, TLS 1.3 reduces HTTPS connection overhead by approximately 100ms on typical networks. For mobile users and high-latency connections, this improvement is particularly noticeable. Major browsers and web servers have supported TLS 1.3 since 2018-2019, and it is now the recommended protocol for all production deployments.

HSTS is a web security policy mechanism that instructs browsers to only connect to a website over HTTPS, preventing protocol downgrade attacks and cookie hijacking. When a server sends the Strict-Transport-Security header, the browser remembers this policy for the specified duration (max-age) and automatically converts any HTTP requests to HTTPS.

A properly configured HSTS header includes three directives: max-age (minimum 31536000 seconds, or 1 year, for preload eligibility), includeSubDomains (extends protection to all subdomains), and preload (signals readiness for inclusion in browser preload lists). Once a domain is added to the HSTS preload list maintained by Chromium, browsers enforce HTTPS for that domain without ever needing to see the header — providing protection from the very first connection.

HSTS protects against several attack vectors: SSL stripping (where an attacker downgrades HTTPS to HTTP), session cookie theft over unencrypted connections, and accidental HTTP links within a site. It is one of the most effective and easy-to-implement security headers available.

OCSP is a protocol used to check the revocation status of an SSL/TLS certificate in real-time. When a browser encounters a certificate, it can query the certificate's OCSP responder (specified in the Authority Information Access extension) to confirm the certificate has not been revoked.

OCSP replaced the older CRL (Certificate Revocation List) approach, which required downloading potentially large lists of revoked certificates. OCSP queries are faster and more efficient, returning a signed response indicating the certificate's status: "good," "revoked," or "unknown."

OCSP Stapling is an optimization where the web server periodically fetches and caches OCSP responses from the CA, then "staples" them to the TLS handshake. This eliminates the client's need to contact the OCSP responder separately, improving privacy (the CA doesn't see which clients visit the site) and performance (one fewer network round-trip). OCSP stapling is recommended for all production servers and is supported by Nginx, Apache, Caddy, and most modern web servers.