Reading a raw PEM certificate by eye is nearly impossible — it’s just base64. When you need to quickly check who issued a cert, when it expires, or which domains it covers, you normally reach for openssl x509 -text. This tool gives you the same core fields directly in the browser.
What Is the SSL Certificate Decoder?
SSL Certificate Decoder is one of the tools in WebdevToolbox’s Security & Crypto category. Paste a PEM-encoded X.509 certificate and instantly see its subject, issuer, validity window, serial number, signature algorithm, public key algorithm, and Subject Alternative Names.
How to Use It
- Open the tool at /tools/ssl-decoder.
- Paste a certificate, including the
-----BEGIN CERTIFICATE-----/-----END CERTIFICATE-----markers. - Read the decoded fields on the right, updated live as you paste or edit.
- Check the expiry status banner for an at-a-glance valid/expired read with days remaining.
Key Features
- Pure client-side ASN.1/DER parsing — a small hand-written parser reads the certificate structure directly; no library or server call involved.
- Expiry status at a glance — see immediately whether a cert is valid and how many days are left.
- Subject Alternative Names — lists the DNS names covered by the certificate.
- Human-readable algorithm names — common OIDs like
sha256WithRSAEncryptionare mapped to readable labels. - 100% private — the certificate text never leaves your browser.
When to Use the SSL Certificate Decoder
- Debugging expired certs — quickly confirm a
notAfterdate without shelling out toopenssl. - Verifying SANs — check which domains a certificate actually covers before deploying it.
- Reviewing a cert someone sent you — paste and inspect without installing anything.
Try It Now
Open SSL Certificate Decoder directly from anywhere on the site using ⌘K and typing the tool name. No login required — everything runs offline-capable in your browser.
Part of the Security & Crypto collection on WebdevToolbox — free, browser-based developer tools with no login required.