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The SSL Certificate Decoder utility parses and displays detailed information from X.509 SSL/TLS certificates. You can inspect certificate metadata, verify validity periods, examine public key information, and view cryptographic fingerprints.

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Features

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Certificate information extraction

The decoder extracts and displays comprehensive certificate data:
  • Version: X.509 certificate version (v1, v2, or v3)
  • Serial Number: Unique certificate identifier in hexadecimal
  • Signature Algorithm: Cryptographic algorithm used for signing (SHA256-RSA, ECDSA, etc.)
  • Subject: Entity the certificate was issued to (CN, O, OU, C, ST, L)
  • Issuer: Certificate Authority that issued the certificate
  • Validity Period: Not Before and Not After dates with expiration status
  • Public Key: Algorithm and key size information
The decoder supports PEM-encoded certificates, which are the standard format starting with -----BEGIN CERTIFICATE----- and ending with -----END CERTIFICATE-----.

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Certificate extensions

View important X.509v3 extensions: Subject Alternative Names (SAN):
  • DNS names covered by the certificate
  • IP addresses
  • Email addresses
Key Usage:
  • digitalSignature
  • keyEncipherment
  • dataEncipherment
  • keyCertSign
  • crlSign
Extended Key Usage:
  • serverAuth (TLS/SSL server authentication)
  • clientAuth (TLS/SSL client authentication)
  • codeSigning
  • emailProtection
  • timeStamping
Subject Alternative Names (SANs) are crucial for modern certificates. A single certificate can cover multiple domains through SANs.

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Certificate fingerprints

Generate cryptographic fingerprints using multiple hash algorithms:
  • SHA-1: Legacy fingerprint (still widely displayed)
  • SHA-256: Current standard fingerprint
  • SHA-384: Enhanced security fingerprint
  • SHA-512: Maximum security fingerprint
Fingerprints are displayed in standard colon-separated hexadecimal format (e.g., AB:CD:EF:12:34:...).

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Validity checking

Automatic validation of certificate validity:
  • Valid: Certificate is within its validity period
  • Expired: Current date is after the Not After date
  • Not Yet Valid: Current date is before the Not Before date
  • Days Remaining: Countdown to expiration
This tool checks certificate dates but does not verify the certificate chain, revocation status, or cryptographic signatures. Full validation requires a complete certificate chain and CRL/OCSP checking.

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Use cases

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Certificate inspection

Examine certificates before deployment:
  1. Paste the certificate in PEM format
  2. Verify the subject matches your domain
  3. Check the validity period covers your needs
  4. Confirm the issuer is your expected CA
  5. Review SANs to ensure all domains are covered

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Expiration monitoring

Track when certificates will expire:
  • View the exact expiration date and time
  • See days remaining until expiration
  • Plan certificate renewal timing
  • Avoid service disruptions from expired certificates
Certificates typically need renewal 30-90 days before expiration. Set up monitoring to track expiration dates proactively.

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Troubleshooting SSL/TLS issues

Diagnose certificate-related problems: Common issues you can identify:
  • Wrong certificate installed (check subject CN)
  • Expired certificate (check validity dates)
  • Missing domain in SAN (check alternative names)
  • Weak signature algorithm (check signature algorithm)
  • Wrong CA issuer (check issuer information)

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Certificate comparison

Compare certificates across environments:
  • Development vs. production certificates
  • Old vs. new certificates during renewal
  • Different servers to ensure consistency
  • Certificate fingerprints for verification
Use the fingerprints to verify certificate identity. SHA-256 fingerprints are the current standard for certificate pinning and verification.

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Security auditing

Review certificate security properties:
  • Signature algorithm strength (prefer SHA-256 or higher)
  • Public key algorithm and size (prefer RSA 2048+ or ECDSA)
  • Extended Key Usage matches intended use
  • Key Usage permissions are appropriate
  • Certificate lifespan is reasonable (prefer shorter validity periods)

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Certificate formats

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PEM format

The decoder accepts PEM-encoded certificates: 
-----BEGIN CERTIFICATE-----
MIIDdTCCAl2gAwIBAgILBAAAAAABFUtaw5QwDQYJKoZIhvcNAQEFBQAwVzELMAkG
A1UEBhMCQkUxGTAXBgNVBAoTEEdsb2JhbFNpZ24gbnYtc2ExEDAOBgNVBAsTB1Jv
...
-----END CERTIFICATE-----
You can obtain certificates in PEM format from web browsers, servers, or using OpenSSL commands like openssl s_client -connect example.com:443 -showcerts.

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Getting certificates

From a browser:
  1. Click the padlock icon in the address bar
  2. View certificate details
  3. Export or copy the certificate
From a server using OpenSSL: 
openssl s_client -connect example.com:443 -showcerts < /dev/null 2>/dev/null | openssl x509 -outform PEM
From a file: 
cat certificate.crt

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Understanding certificate fields

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Subject and Issuer DN

Distinguished Names (DN) contain:
  • CN (Common Name): Primary domain or entity name
  • O (Organization): Company or organization name
  • OU (Organizational Unit): Department or division
  • C (Country): Two-letter country code
  • ST (State): State or province
  • L (Locality): City or locality

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Validity period

Certificates have strict time bounds:
  • Not Before: Certificate becomes valid at this time
  • Not After: Certificate expires at this time
  • Both times are in UTC/ISO format
Certificate validity is checked against the system clock. Ensure your system time is accurate for correct validation results.

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Public key algorithms

Common algorithms you’ll see:
  • RSA: Traditional algorithm, 2048-bit or 4096-bit keys
  • EC (Elliptic Curve): Modern algorithm, smaller keys with equivalent security
  • DSA: Older algorithm, less common
  • Ed25519: Modern elliptic curve algorithm
The decoder performs ASN.1 DER parsing of the certificate structure. This is a complex binary format defined by X.509 standards.

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