Verify that a certificate and private key match by comparing their public key modulus fingerprints.
Certificate Key Matcher answers one narrow deployment question: does the RSA public key inside an X.509 certificate correspond to the supplied RSA private key? It extracts the modulus from each input, represents each modulus as a fingerprint, and compares those fingerprints. A match indicates shared RSA key material despite the files having different contents. The common surprise is that this is not certificate validation: a matching certificate may still be expired, revoked, untrusted, or issued for another hostname.
An X.509 certificate contains a public key, while an RSA private key also records the modulus associated with its public component. The matcher reduces both accepted inputs to those modulus values. It therefore compares the mathematical key material rather than comparing the PEM text, certificate serial number, subject name, issuer, or surrounding metadata.
Each extracted modulus is represented by a fingerprint suitable for direct comparison. Equal fingerprints produce a match result; different fingerprints indicate that the certificate and private key do not share the same RSA modulus. This modulus fingerprint is distinct from a certificate fingerprint, which identifies the encoded certificate and changes when certificate fields or signatures change.
No. It means the certificate and private key produced the same RSA modulus fingerprint. Certificate validity is a separate question. Check the validity period, subject alternative names, issuer chain, signature, revocation status where applicable, and intended key usage before deployment. A correctly paired certificate can still fail any of those checks.
No. This tool performs the certificate and key comparison in the browser, so the pasted private key is not uploaded by the tool. The key should still be treated as sensitive. Use a trusted device and browser environment, avoid shared clipboards, and clear copied key material after completing the check.
The behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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