🔒 SSL/TLS Certificate Decoder
Decode and inspect PEM-encoded certificates. Everything runs in your browser.
Last updated: May 18, 2026 · By Λ
Free SSL/TLS Certificate Decoder
Instantly decode and inspect SSL/TLS certificates in PEM format. View the full subject and issuer details, serial number, validity period with expiration countdown, signature algorithm, public key info, key usage, extended key usage, Subject Alternative Names (SANs), and SHA-1/SHA-256 fingerprints. Supports certificate chains with multiple certificates. The ASN.1 DER structures are unpacked by JavaScript shipped with this page, meaning a pasted certificate is read on your own machine and goes no further. Upload a .pem or .crt file or paste the certificate text directly.
What is an SSL/TLS Certificate Decoder?
An SSL/TLS certificate decoder reads the contents of a digital certificate and displays its fields in a human-readable format. SSL (Secure Sockets Layer) and its successor TLS (Transport Layer Security) certificates are used to establish encrypted connections between web browsers and servers. Each certificate contains information about the domain it secures, the organization that owns it, the certificate authority that issued it, the validity period, the public key, and various extensions that define how the certificate should be used.
This free tool parses PEM-encoded certificates directly in your browser using pure JavaScript ASN.1 decoding. It extracts and displays the subject, issuer, serial number, validity dates, signature algorithm, public key details, key usage, extended key usage, Subject Alternative Names (SANs), and SHA-1/SHA-256 fingerprints. It also handles certificate chains with multiple certificates. Your certificate data stays completely private because nothing is sent to any remote server during the decoding process.
How to Use This Tool
- Paste your PEM-encoded certificate into the text area. The certificate should start with "-----BEGIN CERTIFICATE-----" and end with "-----END CERTIFICATE-----". You can also upload a .pem or .crt file directly.
- Click "Decode" to parse the certificate. If your input contains multiple certificates (a certificate chain), each one will be decoded separately with navigation buttons to switch between them.
- Review the decoded fields organized by section: subject details, issuer information, validity period, public key specifications, extensions, and fingerprints.
- Check the validity banner at the top to quickly see whether the certificate is currently valid, expired, or approaching expiration. The expiration countdown shows exactly how much time remains.
Key Features
- Complete Certificate Details - Displays subject, issuer, serial number, validity period, signature algorithm, public key type and size, key usage, extended key usage, and all Subject Alternative Names.
- Expiration Countdown - Shows a clear validity banner indicating whether the certificate is valid, expiring soon, or already expired, along with a precise countdown timer.
- Certificate Chain Support - Handles PEM files containing multiple certificates, allowing you to inspect each certificate in the chain individually.
- Fingerprint Display - Shows both SHA-1 and SHA-256 fingerprints for certificate identification and verification, with one-click copying.
- File Upload Support - Upload .pem or .crt files directly instead of pasting text, making it convenient to inspect certificates exported from servers or certificate managers.
Frequently Asked Questions
What is a PEM-encoded certificate?
PEM (Privacy Enhanced Mail) is the most common format for storing SSL/TLS certificates. It is a Base64-encoded version of the binary DER format, wrapped between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----" header lines. You can obtain a PEM certificate from your web server configuration, certificate provider dashboard, or by using command-line tools like OpenSSL.
Is it safe to paste my SSL certificate here?
Yes. SSL certificates contain only public information and are designed to be shared openly. They do not include your private key, which is the sensitive component that must be kept secret. Additionally, this tool processes everything in your browser with no server communication, so your certificate data never leaves your device.
What are Subject Alternative Names (SANs)?
SANs are an extension in SSL certificates that list all the domain names and IP addresses the certificate is valid for. A single certificate can secure multiple domains (like example.com, www.example.com, and mail.example.com) by listing each one as a SAN entry. Modern browsers require the domain to appear in the SAN list, not just the Common Name (CN) field.
What is the difference between SHA-1 and SHA-256 fingerprints?
Both are cryptographic hash values computed from the certificate's binary data, used to uniquely identify a specific certificate. SHA-256 produces a longer, more secure hash and is the current standard. SHA-1 is considered weak for signing purposes but is still commonly displayed for backward compatibility and quick certificate identification when comparing against known fingerprints.