# X.509 Certificate Decoder Parse an X.509 SSL/TLS certificate (paste PEM or upload .pem/.crt) and inspect subject, issuer, validity period with expiry status, public key, signature algorithm, serial, SANs, key usage and fingerprint. Like openssl x509 -text, visualised. Runs locally. > Canonical page: https://elysiatools.com/en/tools/certificate-decoder - **Category:** Security - **Keywords:** x509, certificate, ssl, tls, pem, decoder, parser, csr, openssl, https, subject, issuer, san, fingerprint ## Overview Decodes an X.509 certificate so you can read everything `openssl x509 -text` would show — without the command line. Input: paste a PEM certificate (`-----BEGIN CERTIFICATE-----`) or upload a `.pem` / `.crt` file. Only the first certificate block is parsed. What you get: - **Validity status** at a glance: VALID / EXPIRES SOON / EXPIRED / NOT YET VALID, with days remaining. - **Subject & Issuer** Distinguished Names, with a self-signed badge when they match. - **Public key**: type (RSA/EC/Ed25519) and bit size. - **Signature algorithm** (e.g. `sha256WithRSAEncryption`). - **Serial number**, version, CA flag. - **SHA-256 fingerprint**. - **Extensions**: key usage, extended key usage (serverAuth/clientAuth/…), and Subject Alternative Names (the full domain/IP list). Powered by Node's `crypto.X509Certificate` plus `node-forge` for signature-algorithm and readable extended-key-usage names. The certificate is parsed locally and never transmitted. ## Inputs - **Certificate (paste PEM)** (textarea): -----BEGIN CERTIFICATE----- MIID... -----END CERTIFICATE----- - **Or upload .pem / .crt file** (file) ## When to use - When you need to verify the expiration date and validity status of an SSL/TLS certificate without using command-line tools. - When you want to inspect the Subject Alternative Names (SANs) of a certificate to ensure all required domains are covered. - When you need to check if a certificate is self-signed by comparing the subject and issuer distinguished names. ## How it works - Paste your PEM-encoded certificate text starting with `-----BEGIN CERTIFICATE-----` into the input field, or upload a `.pem`, `.crt`, or `.cer` file. - The tool processes the certificate locally in your browser using secure cryptographic parsing libraries. - Review the structured output displaying validity badges, public key details, signature algorithms, and certificate extensions. ## Use cases - Checking the expiration date and remaining validity days of a web server certificate. - Verifying the public key type (RSA/EC/Ed25519) and bit size of a newly generated certificate. - Auditing the Key Usage and Extended Key Usage extensions to confirm the certificate's intended application. ## Frequently asked questions ### Is my certificate data sent to a server? No. The certificate is parsed entirely locally in your browser and is never transmitted over the network. ### What file formats does this decoder support? It supports PEM-encoded text inputs and file uploads with `.pem`, `.crt`, or `.cer` extensions. ### Can I decode multiple certificates at once? No, the tool only parses the first certificate block detected in the input. ### How does the tool determine if a certificate is self-signed? It compares the Subject and Issuer Distinguished Names; if they match, it flags the certificate as self-signed. ### Can this tool decode private keys or CSRs? No, this tool is specifically designed to decode and inspect X.509 public certificates. ## Related tools - [JavaScript Deobfuscator](https://elysiatools.com/en/tools/javascript-deobfuscator): Deobfuscate and analyze obfuscated JavaScript code to improve readability and understanding - [JWT Inspector & Verifier](https://elysiatools.com/en/tools/jwt-inspector-verifier): Decode a JWT (header + payload) with syntax highlighting, diagnose standard claims, and verify the signature with HS*/RS*/PS*/ES*/EdDSA keys — plus a tampered-token forgery demo - [Batch Image Watermark](https://elysiatools.com/en/tools/batch-watermark): Add watermarks to multiple images with text or image watermarks, position control, and batch processing - [QR Code Decoder](https://elysiatools.com/en/tools/qr-code-decoder): Decode QR codes from images and extract the contained text or URLs - [File Type Detector (Magic Number)](https://elysiatools.com/en/tools/file-type-detect): Identify a file by its magic-number byte signature, not the extension. 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