X.509 certificate decoder
Paste a PEM certificate, chain or CSR and read its subject, SANs, validity, key, extensions and fingerprints.
Input
Result
The result will appear here.A certificate is a DER blob wrapped in Base64, and the facts you need from it — which names it covers, when it expires, who issued it, what its fingerprint is — are buried in ASN.1. Paste one or more PEM blocks and this decoder lays them out the way openssl x509 -text would, but readable: subject and issuer, validity with the days left, subject alternative names, key type and size, signature algorithm, basic constraints, key usage and extended key usage, and SHA-1 and SHA-256 fingerprints. Paste a whole chain and it checks that each certificate is signed by the next. Certificate signing requests decode too, including the SANs they ask for. Parsing is done by @peculiar/x509 from PeculiarVentures (MIT), the team that also maintains PKI.js.
How it works
- Every -----BEGIN CERTIFICATE----- and -----BEGIN CERTIFICATE REQUEST----- block is decoded in order; private keys and other PEM blocks in the paste are counted and left alone.
- For a chain in the usual leaf-first order, each certificate's issuer name is compared with the next subject and its signature is verified with the next certificate's public key.
- Fingerprints are the SHA-1 and SHA-256 of the DER bytes, printed as colon-separated hex, so they compare directly with browsers, openssl and certificate-pinning configs.
- A CSR's self-signature is checked as well, which catches a request that was edited or copied incompletely before you pay a CA for it.
Where your data goes
Nowhere. This tool runs entirely in your browser: the text you paste is processed by the page and is never transmitted to a server or written to a log.
This tool is free and needs no account. Its results exist only in your open page and are not saved anywhere.
What it costs
This tool is free, with no sign-in and no points.
Common questions
- Does this tell me whether the certificate is trusted?
- No. It reads what the certificate says and checks signatures between the certificates you paste, but it does not check revocation (OCSP or CRLs) or whether the root is in a browser's trust store — both need the network or a trust list. For a live site, the browser gives that verdict itself (the Security panel in its developer tools explains it), and a scanner such as SSL Labs checks the whole configuration.
- Why does my browser reject a certificate whose CN matches the domain?
- Because browsers match host names against the subject alternative names only and have ignored the common name for years; Chrome stopped using it in 2017. If the SAN list shown here does not contain the exact host (or a wildcard covering one level of it), the certificate will fail, whatever the CN says.
- How long can a public TLS certificate be valid now?
- Much shorter than it used to be. The CA/Browser Forum has voted to cut the maximum lifetime in steps, reaching 47 days in 2029, which is why automated renewal with ACME clients like certbot has become the normal way to run TLS. The days-left figure here helps you spot a certificate that automation forgot.
- What order should a chain file be in?
- Leaf first, then each intermediate, each signed by the one after it; the root is normally left out because clients already have it. The 'Issued by next' line shows where an order is wrong or an intermediate is missing — the most common reason a site works in one browser and fails in curl or on older Android.
- Is it safe to paste certificates here?
- Certificates are public by design — every TLS client receives them — so there is nothing secret in one. Do not paste private keys; this tool does not need them and skips any it finds.
The open-source behind it
This tool runs on PeculiarVentures/x509, released under MIT. If you need the same behaviour inside your own program, that is the library to reach for.
PeculiarVentures/x509Also known as
- certificate decoder
- x509 decoder
- pem certificate viewer
- ssl certificate checker
- csr decoder
- certificate fingerprint sha256