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SSL Certificate Decoder
Paste a PEM certificate (-----BEGIN CERTIFICATE-----) or a whole chain, or open a .crt, .cer, .pem or .der file, to see who it was issued to, who signed it, when it expires, which domains it covers and every extension. Certificates are parsed locally with a built-in ASN.1 decoder.
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How to use the SSL Certificate Decoder
- Paste the certificate including the BEGIN and END lines, or press Open file to load a .crt, .cer, .pem or .der file. Press Load sample to try a two-certificate chain.
- Read the summary: the common name, status badge (valid, expired or not yet valid) and the days remaining.
- Check the domains under Subject Alternative Names; browsers use this list, not the common name.
- Review the issuer and chain: when you paste a chain, each certificate shows whether its signature was verified by the next one.
- Copy the fingerprints or download a single certificate from a chain as a .crt file.
What is inside an X.509 certificate
An SSL/TLS certificate is an X.509 v3 structure encoded in DER (binary ASN.1) and usually wrapped in Base64 as PEM. It has three parts: the to-be-signed data, the signature algorithm and the issuer's signature over that data.
| Field | What it tells you |
|---|---|
| Subject | Who the certificate was issued to (CN, organization, country). For domain-validated certificates usually only the CN. |
| Issuer | The certificate authority (CA) that signed it. It must equal the subject of the next certificate in the chain. |
| Validity | Not Before and Not After in UTC. Under CA/Browser Forum rules, public TLS certificates issued since 15 March 2026 may last at most 200 days; the limit drops to 100 days in March 2027 and 47 days in March 2029. |
| Serial number | Unique per issuer; used in revocation lists and OCSP. |
| Subject Public Key Info | The key the server proves it owns during the TLS handshake (RSA 2048+ or ECDSA P-256/P-384). |
| Subject Alternative Name | The host names and IP addresses the certificate is valid for. Browsers check only this list. |
| Key Usage / Extended Key Usage | What the key may be used for, for example Digital Signature and TLS Web Server Authentication. |
| Basic Constraints | CA:TRUE marks a certificate authority that may sign other certificates; CA:FALSE marks an end-entity (leaf) certificate. |
| Authority Information Access | Where to download the issuer certificate (CA Issuers) and check revocation (OCSP). |
| Signed Certificate Timestamps | Proof that the certificate was logged in public Certificate Transparency logs, required by Chrome and Safari. |
Common certificate problems and how to spot them
- Expired certificate: the status badge turns red. Renew it and reload the web server; with ACME clients such as Certbot, check that the renewal timer runs.
- Name mismatch: enter the host name in Check host name. If it is not covered, the SAN list is missing that name or a wildcard is used for the bare domain or a deeper sub-domain.
- Incomplete chain: servers must send the leaf certificate followed by the intermediate. If you paste the server's full chain and the signature of the leaf shows Not checked, the intermediate is missing.
- Wrong order: the summary warns if the chain is not ordered leaf first, then intermediates, which some clients require.
- Weak algorithms: SHA-1 or MD5 signatures and RSA keys below 2048 bits are rejected by modern browsers.
Command-line equivalents
- Show a certificate:
openssl x509 -in cert.pem -text -noout - Fingerprint:
openssl x509 -in cert.pem -noout -fingerprint -sha256 - Convert DER to PEM:
openssl x509 -inform der -in cert.cer -out cert.pem - Fetch a server's chain:
openssl s_client -connect example.com:443 -servername example.com -showcerts </dev/null
Frequently asked questions
How do I get the PEM certificate of a website?
In Chrome or Edge, click the icon left of the address bar, choose Connection is secure, then Certificate is valid, open the Details tab and press Export. In Firefox use the padlock, Connection secure, More information, View Certificate, then download PEM (cert) or PEM (chain). On the command line: openssl s_client -connect example.com:443 -showcerts.
Is my certificate uploaded?
No. The certificate is decoded by JavaScript in your browser, and fingerprints are computed with the WebCrypto API. Certificates are public documents, but even so nothing you paste leaves the page. Never paste a private key into any website; if you do here, the tool refuses to process it.
Why does the common name not match my domain?
Modern browsers ignore the common name (CN) and only check the Subject Alternative Name (SAN) extension. A certificate is valid for a host name only if that name, or a matching wildcard such as *.example.com, appears in the SAN list. A wildcard covers one level only: *.example.com matches www.example.com but not example.com or a.b.example.com.
What is a certificate fingerprint?
A hash of the complete DER-encoded certificate, usually SHA-256. It uniquely identifies the certificate and is used for certificate pinning, comparing certificates and looking them up in Certificate Transparency logs. It is not stored inside the certificate; it is calculated from it.
What do DV, OV and EV mean?
They are validation levels shown by the Certificate Policies extension. Domain Validated (2.23.140.1.2.1) certificates only prove control of the domain; Organization Validated (2.23.140.1.2.2) and Extended Validation (2.23.140.1.1) certificates also verify the company named in the subject.
What does the signature check prove?
When you paste a chain, the tool verifies each certificate's signature with the public key of the next certificate. A passed check proves that the next certificate's key signed it. It does not check whether the root is trusted by your operating system or whether the certificate was revoked.
Can it decode a CSR or private key?
This decoder reads X.509 certificates. If you paste a certificate signing request (BEGIN CERTIFICATE REQUEST) or a key, the tool tells you what it is instead of guessing.
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