SSL Certificate Checker

SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) are cryptographic protocols that encrypt communication between a web browser and a server. An SSL certificate is a digital certificate that authenticates a website's identity and enables an encrypted connection. When you see the padlock icon in your browser's address bar, it means the site has a valid SSL certificate and the connection is encrypted.

Why Check SSL Certificates?

Expired or misconfigured SSL certificates are one of the most common causes of website downtime and security warnings. Browsers display prominent warnings when a certificate has expired, has a domain mismatch, or is issued by an untrusted authority. Regularly checking your SSL certificates helps prevent unexpected outages, maintains user trust, and ensures compliance with security standards. This tool lets you quickly verify any domain's certificate status without installing command-line tools or writing scripts.

Certificate Transparency (CT) Logs

Certificate Transparency is an open framework for monitoring and auditing SSL certificates. When a Certificate Authority (CA) issues a certificate, it must log the certificate in publicly accessible CT logs. This allows domain owners, security researchers, and browsers to detect misissued or fraudulent certificates. This tool queries crt.sh, which aggregates data from multiple CT logs, to retrieve certificate details for any domain. CT logs are a crucial part of the internet's security infrastructure.

Certificate Validity and Expiry

SSL certificates have a defined validity period, typically 90 days (Let's Encrypt) to 397 days (commercial CAs). The maximum certificate lifetime has been progressively shortened by browser vendors, with the current CA/Browser Forum baseline requiring certificates valid for no more than 398 days. Monitoring certificate expiry is critical: an expired certificate immediately triggers browser warnings that can drive visitors away. Best practice is to set up renewal automation or alerts at least 30 days before expiry.

Certificate Issuers

Certificates are issued by Certificate Authorities (CAs), which are trusted entities that verify the identity of certificate applicants. Major CAs include Let's Encrypt (free, automated), DigiCert, Sectigo (formerly Comodo), GlobalSign, and Google Trust Services. The issuer field in a certificate identifies which CA issued it. Browsers maintain a list of trusted root CAs; certificates signed by an untrusted CA will generate security warnings.

Subject Alternative Names (SANs)

The Subject Alternative Name extension allows a single certificate to protect multiple domain names. For example, a certificate for example.com might also include www.example.com, api.example.com, and *.example.com as SANs. Wildcard certificates (using *. prefix) can cover any subdomain at a single level. SANs have replaced the older Common Name (CN) field as the primary method for domain validation in modern browsers.

SSL vs TLS

Although "SSL" is the commonly used term, modern secure connections actually use TLS (Transport Layer Security). SSL was deprecated after version 3.0 due to security vulnerabilities (POODLE attack). TLS 1.2 and TLS 1.3 are the current standards. TLS 1.3, released in 2018, offers improved security and faster handshakes by reducing the number of round trips required to establish a connection. Despite the terminology difference, "SSL certificate" remains the widely understood term for TLS certificates.

How This Tool Works

This SSL checker runs entirely in your browser. When you enter a domain, it queries the crt.sh Certificate Transparency log database to retrieve the most recently issued certificate for that domain. The results include the common name, issuer, validity dates, serial number, and subject alternative names. The visual progress bar shows how much of the certificate's validity period has elapsed, with color coding: green for valid (more than 30 days remaining), yellow for expiring soon (fewer than 30 days), and red for expired certificates. No data passes through SecureBin servers.