What Is an X.509 Certificate?
X.509 is the standard format for public key certificates used in SSL/TLS, S/MIME, and other security protocols. An X.509 certificate binds a public key to an identity (domain name, organization) and is signed by a Certificate Authority (CA). Certificates contain information about the subject, issuer, validity period, public key, signature algorithm, and extensions like Subject Alternative Names (SAN).
How to Use This Certificate Decoder
- 1Paste a PEM-encoded certificate (starting with -----BEGIN CERTIFICATE-----) into the input area.
- 2Click Decode Certificate to parse and display the certificate details.
- 3Review the subject, issuer, validity dates, and signature algorithm.
- 4Check Subject Alternative Names (SAN) for all covered domains.
- 5Copy SHA-256 or SHA-1 fingerprints for certificate pinning or verification.
Common Use Cases
Debug SSL/TLS Issues
Quickly inspect server certificates to diagnose SSL handshake failures or certificate mismatch errors.
Verify Certificate Details
Confirm the subject, issuer, and validity of certificates before deployment.
Certificate Pinning
Extract SHA-256 fingerprints for HTTP Public Key Pinning (HPKP) or mobile app certificate pinning.
Frequently asked questions
Is my certificate data safe?
Yes. All parsing runs entirely in your browser. Your certificate data never leaves your device.
What certificate formats are supported?
This tool supports PEM-encoded X.509 certificates. If you have a DER-encoded certificate, convert it to PEM first using openssl.