HPKP
HPKP (HTTP Public Key Pinning, RFC 7469) lets a site declare which public key fingerprints it trusts, blocking man-in-the-middle attacks that rely on CA mis-issuance.
Purpose
It makes browsers trust only the pinned fingerprints, so even a compromised CA issuing a fraudulent certificate cannot enable a MITM.
Mechanism
The site sends a Public-Key-Pins header with the SHA-256 fingerprint of its public key plus a backup; the browser thereafter trusts only certificates matching those fingerprints. max-age controls the pin's lifetime.
Use cases
- Certificate public key pinning
- Protection against CA compromise
- Man-in-the-middle defense
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