Certificate decoder
Read an SSL/TLS certificate's subject, issuer, validity, and SANs.
A certificate decoder parses an SSL/TLS certificate and presents its contents clearly, including the subject and issuer, the validity dates, the serial number, the signature algorithm, and the subject alternative names it covers. Reading these fields lets you confirm a certificate is issued to the right names, signed properly, and not close to expiring. It is the quickest way to understand exactly what a certificate authorizes and when it lapses. Rather than checking expiry by hand, SJ Monitor's SSL monitoring can watch your live certificates and warn you well before they expire.
Frequently asked questions
What can I learn from decoding a certificate?
You can see who it was issued to and by, its valid-from and valid-to dates, the signature algorithm, and every hostname it covers through its SANs. That tells you exactly what the certificate authorizes.
How do I know if a certificate is about to expire?
The validity end date in the decoded output shows when it lapses, but tracking that manually is error-prone. SJ Monitor's SSL monitoring watches expiry for you and alerts you in advance.
Why does the SAN list matter?
Browsers validate hostnames against the subject alternative names, so a missing name causes certificate errors even if the certificate is otherwise valid. Confirming SANs prevents those mismatches.
What certificate format can I paste in?
A PEM-encoded certificate — the Base64 block between the BEGIN CERTIFICATE and END CERTIFICATE lines. If you have a binary DER or PKCS#7 file, convert it to PEM first.
Does the signature algorithm matter?
Yes. SHA-256 with RSA or ECDSA is the modern standard; anything using SHA-1 is deprecated and rejected by browsers. The decoder shows the algorithm so you can confirm it's current.
Is it safe to paste a certificate here?
Yes — a certificate contains only public information (the public key and identity fields). It never includes the private key, so decoding one exposes nothing sensitive.
More SSL/TLS & security tools tools
Want this checked automatically and around the clock? Create a free SJ Monitor account and we'll alert you the moment something changes.