SSL Cert Decoder

What is SSL Cert Decoder?

SSL Certificate Decoder parses X.509 certificates in PEM or DER format and displays all fields in human-readable form — subject, issuer, validity dates, serial number, signature algorithm, public key details, Subject Alternative Names (SANs), and extension fields like Basic Constraints and Key Usage. It validates the certificate chain by checking issuer-subject matching, validates date ranges, and identifies common issues like expired certificates, weak signature algorithms (SHA-1), and missing SANs.

DevOps engineers troubleshooting TLS configurations, security engineers auditing certificate properties, and sysadmins managing certificate renewals. The tool runs entirely in your browser — your data never leaves your device.

How to Use SSL Cert Decoder

1. Paste the PEM-encoded certificate (including BEGIN/END markers) into the input
2. View the decoded certificate fields — subject, issuer, validity, SANs, and extensions
3. Check the validation warnings for expired dates, weak algorithms, or missing fields
4. Paste additional certificates to validate the full chain from leaf to root

Common Use Cases

• Inspecting TLS certificates to verify domain coverage and expiration dates
• Debugging certificate chain issues by examining issuer and intermediate certificates
• Verifying that a certificate includes all required Subject Alternative Names
• Checking certificate properties before deployment to catch expired or misconfigured certs

Pro Tips

• Subject Alternative Names (SANs) are more important than the Common Name (CN) for domain validation — modern browsers check SANs first and some ignore CN entirely; always verify your domains appear in the SAN extension
• Certificates signed with SHA-1 are rejected by all major browsers since 2017 — if the decoder flags SHA-1, the certificate must be reissued with SHA-256 or SHA-384 regardless of its expiration date
• Intermediate certificates are the most common cause of chain validation failures — your server must send the full chain (leaf + intermediates) because browsers may not have intermediate CA certificates cached

Why Use This Tool?

• Privacy-first: All processing happens locally in your browser. No data is transmitted to any server.
• No installation required: Works in any modern browser on desktop or mobile.
• Completely free: No account, no credit card, no usage caps.
• Fast and reliable: Powered by modern Web APIs for near-instant results.

Related Concepts

Learn more about related topics: X.509, TLS/SSL, Certificate chain, Public key infrastructure.

Frequently Asked Questions