Free X.509 Certificate Decoder — Read and Analyze SSL/TLS Certificates Online

Paste any PEM or raw Base64-encoded certificate and see every field extracted
and formatted. Our certificate parser runs entirely in your browser using a
client-side ASN.1 decoder — nothing is uploaded, no account is required, and
there is no limit on usage.

What the Certificate Decoder Extracts

Uploading a PEM certificate (the kind you get from -----BEGIN CERTIFICATE-----
through -----END CERTIFICATE-----) reveals:

  • Serial number in hexadecimal format — the unique identifier assigned by
    the issuing CA, useful for revocation checking and audit logs.
  • Validity period — both notBefore and notAfter dates with an automatic
    calculation of days remaining. Expired certificates and those expiring within
    30 days are clearly flagged.
  • Subject Distinguished Name — the full X.500 directory name with each
    Relative Distinguished Name attribute shown alongside its OID.
  • Issuer Distinguished Name — who issued the certificate, parsed attribute
    by attribute. Useful for verifying chain-of-trust and confirming the correct
    intermediate CA.
  • Subject Alternative Names — every DNS name and IP address covered by this
    certificate. Essential for confirming wildcard coverage and multi-domain
    certificates.
  • Public key — algorithm family (RSA or Elliptic Curve), key size, and EC
    curve name when applicable. Verify that your certificate meets minimum key
    length requirements.
  • Signature algorithm — the algorithm the CA used to sign this certificate,
    displayed as both the OID dotted string and the common name (e.g.
    sha256WithRSAEncryption).
  • SHA-256 fingerprint — computed from the DER body and displayed for
    Certificate Transparency log cross-referencing.

When to Use an SSL Certificate Decoder

A TLS certificate parser helps in several real-world situations:

  • Inspect a certificate before installation — after receiving a signed
    certificate from a Certificate Authority, decode it to confirm the CN, SANs,
    key type, and validity window are correct before deploying to production.
  • Debug chain-of-trust issues — when a browser reports an untrusted
    connection, decode each certificate in the chain to identify missing
    intermediates or expired cross-signatures.
  • Verify certificate transparency — copy the SHA-256 fingerprint and search
    crt.sh to confirm the certificate appears in public CT logs.
  • Check key compliance — organizational policies often mandate minimum RSA
    2048-bit keys or prohibit certain signature algorithms (e.g. SHA-1). Use the
    decoder to audit compliance on any certificate file.
  • Prepare for migration — when moving from RSA to ECDSA certificates or
    switching CAs, verify that newly issued certificates match the expected
    parameters before cutting over traffic.

Input Format Support

The tool auto-detects your input format:

  • PEM — standard -----BEGIN CERTIFICATE----- / -----END CERTIFICATE-----
    with Base64 body. This is the default output of most CAs, web servers, and
    the OpenSSL command line.
  • Raw Base64 — the DER body encoded in Base64 without PEM headers. Useful
    when you’ve extracted the certificate data programmatically.

Privacy and Security

The entire decoding process runs inside your web browser using JavaScript.
Certificate data is never transmitted to any server. You can safely decode
internal PKI certificates, production server certificates, and private CA
material without exposing sensitive information to a third party.

OpenSSL Alternative

The command-line equivalent is:

openssl x509 -in crtmgr.com.crt -noout -text

Our web tool produces the same information with structured formatting and
click-to-copy fields, making documentation and sharing easier.

Related Tools

  • SSL Certificate Checker — query a live domain and see the
    certificate it’s currently serving, including days-remaining countdown and SANs.
  • Certificate Converter — switch between PEM and raw Base64
    formats for use in Kubernetes Secrets, Terraform variables, or API integrations.
  • CSR Decoder — decode a Certificate Signing Request to verify
    the subject and public key before submitting it to a Certificate Authority.
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