SSL Certificate Decoder & Inspector

Paste a certificate or enter a website URL to decode its SSL/TLS certificate details. This tool helps admins and developers verify certificate information and troubleshoot security configurations.

Input Method

PEM Certificate

About SSL/TLS Certificates

SSL/TLS certificates are digital certificates that authenticate the identity of a website and enable encrypted connections. They contain information about the certificate holder, the issuing authority, validity period, and cryptographic keys.

Key Components:
- Subject: Information about the certificate owner
- Issuer: The Certificate Authority (CA) that issued the certificate
- Validity: The time period during which the certificate is valid
- Public Key: Used for encrypting data sent to the server
- Extensions: Additional information like alternative domain names

How It Works

This SSL certificate decoder parses X.509 certificates to display their contents in human-readable format. Certificates contain identity information, public keys, and digital signatures that enable secure HTTPS connections.

The decoding process:

Format detection: Identifies whether the certificate is PEM (text with BEGIN/END markers) or DER (binary format).
ASN.1 parsing: Certificates use ASN.1 encoding. The decoder traverses this structure to extract fields.
Field extraction: Pulls out subject, issuer, validity dates, public key info, extensions, and signature.
Validation checks: Verifies dates, checks for self-signed status, and identifies potential security issues.

The tool displays certificate chain information, key algorithms, and extensions like Subject Alternative Names (SANs) that specify which domains the certificate covers.

When You'd Actually Use This

Troubleshooting HTTPS Errors

Diagnose certificate warnings by examining expiry dates, domain mismatches, or chain issues.

Verifying Certificate Before Deployment

Check that a newly issued certificate has correct domains, dates, and key sizes before installing on servers.

Security Auditing

Review certificates across your infrastructure to identify weak algorithms, expiring certs, or misconfigurations.

Understanding Certificate Structure

Learn what information certificates contain and how to interpret fields like CN, SAN, and key usage extensions.

Debugging TLS Connection Issues

Compare server certificates with expected values when clients report certificate validation failures.

Checking Certificate Transparency

Verify certificate details match what's logged in CT logs, ensuring no unauthorized certificates were issued.

What to Know Before Using

This tool decodes but doesn't validate trust

It shows certificate contents but doesn't verify the certificate chain against trusted root CAs. A valid-looking cert might still be untrusted.

PEM vs DER format matters

PEM is text format with -----BEGIN CERTIFICATE----- markers. DER is binary. Most web servers use PEM; Windows often uses DER (.cer files).

Certificate files may contain multiple certs

Bundle files include the server cert plus intermediate CA certs. This tool typically shows the first certificate (your server cert).

Private keys are NOT in certificates

Certificates only contain public keys. Private keys are separate files (.key, .p12, .pfx). Never share private keys - they're secret.

Self-signed certificates have security implications

Self-signed certs aren't verified by a CA. Browsers warn about them. They're fine for internal use but not for public websites.

Common Questions

What's the difference between subject and issuer?

Subject is who the certificate belongs to (your domain). Issuer is who signed/issued it (the Certificate Authority). For self-signed certs, these are identical.

What are Subject Alternative Names (SANs)?

SANs list all domains the certificate covers. Modern certs use SANs instead of Common Name (CN). A cert can cover multiple domains via SANs.

How do I know if a certificate is about to expire?

Check the 'Not After' date. Browsers typically warn 30 days before expiry. Plan renewal at least 2 weeks ahead to avoid service disruption.

What key size is considered secure?

RSA 2048-bit minimum (3072+ recommended). ECDSA P-256 or higher. RSA 1024 is deprecated and insecure. Check the 'Public Key Algorithm' field.

What does 'self-signed certificate' mean?

The certificate was signed by its own private key, not by a trusted CA. Browsers don't trust these by default. Fine for internal/testing use.

Can I extract the public key from a certificate?

Yes! The certificate contains the public key. This tool displays it. The corresponding private key is separate and must be kept secret.

What are certificate extensions?

Extensions add extra information: Key Usage (what the key can do), Extended Key Usage (TLS server, code signing), SANs, CRL/OCSP endpoints, and more.

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