X.509 Certificate Decoder

What is a X.509 Certificate Decoder?

Decode PEM-encoded X.509 SSL/TLS certificates (`-----BEGIN CERTIFICATE-----`). Inspect Subject Name, Issuer CA, Expiration Validity Dates, Subject Alternative Names (SANs), Public Key algorithm, and SHA-256 fingerprints.

Why Use This Tool?

SSL Certificate Renewal Auditing: Inspect SSL certificate expiration dates before HTTPS certificates expire.
SAN Domain Verification: Verify which subdomains are covered by Subject Alternative Name (SAN) fields.
Certificate Authority Inspection: Inspect root and intermediate Issuer CA details.

How to Use

Paste PEM certificate string (starting with `-----BEGIN CERTIFICATE-----`) or upload a `.crt`/`.pem` file.
The ASN.1 parser decodes certificate attributes in real time.
View Common Name (CN), Issuer, Valid From/To dates, SANs, and SHA-256 Fingerprint.

Real Working Example

Input:

PEM Certificate: -----BEGIN CERTIFICATE-----
MIIF... (Web Server SSL Cert)

Output Result:

Common Name (CN): devdeskapp.com
Issuer: Let's Encrypt Authority X3
Valid Until: November 15, 2026 (Valid for 89 Days)
SANs: devdeskapp.com, *.devdeskapp.com
Fingerprint (SHA-256): 4B:2A:8D:3E:...

Important Technical Details & Security Standards

ASN.1 DER Certificate Parser: Parses X.509 v3 structures, Object Identifiers (OIDs), extensions, and public key parameters.
Expiration Warning Indicator: Flags certificates expiring within 30 days.
Local Memory Execution: Zero network overhead—certificates are decoded locally.

Related Security & Crypto Tools

CSR Generator: Generate Certificate Signing Requests.
Self-Signed Certificate Generator: Create self-signed TLS certificates.
RSA Key Pair Generator: Generate RSA key pairs.

Frequently Asked Questions

What is an X.509 certificate?

An X.509 certificate is a digital certificate standard used in TLS/HTTPS to bind a public key to an identity (domain name or organization).

What are Subject Alternative Names (SANs)?

SANs are extension fields in SSL certificates allowing a single certificate to secure multiple domain names and wildcards.

Are my uploaded certificates or PEM strings sent to a server?

No, decoding executes 100% locally inside your web browser.

Is it free?

Yes, 100% free.

What does a SHA-256 fingerprint mean?

A certificate fingerprint is a unique SHA-256 cryptographic hash of the entire binary certificate file used for pinning and verification.

Can it decode intermediate and root CA certificates?

Yes, server, intermediate, and root CA X.509 certificates are parsed cleanly.