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SECURITY

X.509 Certificate Decoder

Inspect X.509 certificate fields such as subject, issuer, validity dates, SANs, key usage, public key, signature algorithm, and SHA-256 fingerprint without uploading the certificate.

Tool statusRuns in your browser
Preview
Or drop a certificate filechoose a file
.pem, .crt, .cer · up to 2.0 MB. Read in your browser, never uploaded.
Or drop a certificate filechoose a file
.der, .cer, .crt · up to 2.0 MB. Read in your browser, never uploaded.

This decodes certificate information only. It does not validate trust, hostname, DNS, revocation, or certificate chains.

Results

This decodes certificate information only. It does not validate trust, hostname, DNS, revocation, or certificate chains.

How it works

  1. Enter your input

    Fill in the values, paste your text, or upload the file this tool works with.

  2. See results instantly

    Most tools update live as you type; a few use a single button. Either way, the result appears right on this page.

  3. Use your results

    Copy, download, or share what the tool produces — you're always in control of the output.

Privacy and processing — Runs locally in your browser. This tool does not upload your input.

What is X.509 Certificate Decoder?

Paste or load a PEM/DER certificate to inspect its subject, issuer, validity period, public key, fingerprint, and common extensions.

UNDERSTAND THE TOOL

How to read an X.509 certificate

Certificate information

The decoder reads fields encoded inside the certificate itself, such as Subject, Issuer, serial number, validity dates, Subject Alternative Names, Key Usage, and Basic Constraints.

Trust is different

A certificate can be decoded without proving that it is trusted for a domain. Trust requires hostname context, a certificate chain, revocation information, and platform trust roots, none of which this local decoder verifies.

Common uses

  • Check when a certificate expires.
  • Inspect SAN entries before configuring TLS.
  • Compare issuer and public key details during debugging.

What is parsed locally

Subject and issuer distinguished names, validity dates, serial number, public key details, common extensions, and the SHA-256 fingerprint are decoded from the certificate bytes.

Certificate parsing notes

PEM text or DER bytes are parsed in your browser to show certificate fields and a SHA-256 fingerprint; the tool does not validate a live TLS connection.

Frequently Asked Questions

Does this prove a certificate is trusted?

No. It decodes certificate data but does not validate trust, hostname, DNS, revocation, or certificate chains.

Can I load DER certificates?

Yes, DER files can be loaded from your device and parsed locally when they are not malformed.

What is the SHA-256 fingerprint?

It is a digest of the certificate bytes, useful for comparing certificates without sharing the full certificate.

Why can an expired certificate still be decoded?

Decoding only reads the certificate bytes. Expiration affects whether a certificate should be trusted, not whether its fields can be parsed.

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