SSL Certificate Decoder — Expiry & SAN Inspector
Decode X.509 SSL/TLS certificates and RSA private keys in your browser. View subject, issuer, SANs, validity dates, key type, serial number, and SHA-256/SHA-1 fingerprints. Optionally check if a certificate and private key match.
How to Use SSL Certificate Decoder — Expiry & SAN Inspector
How to Use SSL Certificate & Key Decoder
Certificate Tab
Step 1: Paste Your PEM Certificate
Copy the contents of your .crt, .cer, or .pem file and paste it into the Certificate input, including the -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- headers.
You can get the PEM from:
- A
.crtor.pemfile on your server - OpenSSL:
openssl s_client -connect example.com:443 < /dev/null | openssl x509 -out cert.pem - Your browser: click the padlock → Certificate → copy the PEM
Step 2: Click Decode Certificate
The tool decodes and displays:
- Subject — CN, Organisation, Country, and other fields
- Issuer — the certificate authority that signed this cert
- Validity — Not Before / Not After dates with an expiry badge
- Key Info — key type (RSA/EC), key size in bits
- SANs — all DNS names, IPs, and email addresses this cert covers
- Fingerprints — SHA-256 and SHA-1 fingerprints for verification
- Flags — Self-Signed and CA Certificate badges where applicable
Private Key Tab
Step 1: Paste Your PEM Private Key
Paste an RSA private key in PKCS#1 (-----BEGIN RSA PRIVATE KEY-----) or PKCS#8 (-----BEGIN PRIVATE KEY-----) format.
Step 2: Click Decode Private Key
The tool shows key type, size, and a SHA-256 fingerprint of the derived public key.
Certificate & Key Match
If you have decoded both a certificate and a private key, a Check Match button appears. Click it to verify that the certificate was issued for that private key — the tool compares the public key fingerprint from the certificate against the one derived from the private key.
Tips
- Certificates with no SANs are rejected by modern browsers — the CN field alone is not sufficient
- Use Landscape orientation for wide tables
- A SHA-256 fingerprint mismatch after renewal means the key was rotated — update both cert and key on the server
Frequently Asked Questions
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