What does Key Pinning involve?

Key Pinning involves storing the server's certificate within the client to ensure the integrity of connections made to that server. This technique is employed primarily to protect against man-in-the-middle attacks, where an attacker might try to impersonate the server. By pinning the expected certificate or public key to the client, it can verify that the server it communicates with is indeed the legitimate entity, rather than an unauthorized one.

When a client connects to a server, it checks that the server's certificate matches the stored certificate. If there is a mismatch, the client can take steps to terminate the connection or warn the user, thereby preventing potential security breaches.

This method enhances security significantly by not relying solely on the trustworthiness of a Certificate Authority (CA) or the entire certificate chain, focusing instead on the known good certificate directly. This approach can help reduce risks associated with compromised CAs or fraudulent certificates.

The other options do not relate directly to the concept of Key Pinning, as they involve different aspects of certificate management and validation processes.