Online X.509 Certificate Decoder
Paste a PEM certificate to inspect its common X.509 fields, validity, fingerprint, extensions, and subject alternative names locally. It helps platform engineers inspect a PEM certificate’s identity and validity before troubleshooting TLS hostnames or deployment configuration.
What is an X.509 certificate decoder?
An X.509 decoder reads the ASN.1 structure inside a certificate and presents fields that are otherwise difficult to inspect in PEM form, such as the subject, issuer, validity window, and SANs.
How to use this certificate decoder
Paste the complete PEM block, including its BEGIN and END lines, and choose Decode certificate. The result panel displays extracted fields and a SHA-256 fingerprint calculated by Web Crypto.
Decode locally and verify separately
This tool helps inspect a certificate but does not verify trust, revocation, or the complete chain. Use your operating system, browser, or deployment platform for those security checks.
How to use Certificate Decoder for related tasks
Paste the PEM block, decode the certificate, and compare the subject, SANs, issuer, validity window, and fingerprint with the service configuration.
Related tools: RSA Key Pair Generator, AES-256 Encrypt & Decrypt.
Frequently asked questions
Is the certificate sent to a server?
No. PEM decoding happens locally using browser JavaScript and Web Crypto. The certificate text is never uploaded or stored.
What certificate details are shown?
The decoder extracts common issuer and subject names, validity timestamps, a SHA-256 fingerprint, subject alternative names, and key-usage extension labels when those extensions are present.
Can this validate whether a certificate is trusted?
No. It decodes the certificate structure but does not build a trust chain, verify revocation, or replace a browser/server TLS configuration check.
How do I check certificate expiration and SANs?
Decode the PEM and inspect the validity dates and subject alternative names; trust-chain and revocation checks require a separate TLS tool.