In public key authentication, the supplicant must prove knowledge of which private key?

• A. The supplicant's public key
• B. The supplicant's private key
• C. The true party's private key
• D. The verifier's private key

Answer: (C)

In public key authentication, you prove you are who you claim by showing possession of the private key that matches your public key. The verifier sends a challenge, and you sign that challenge with your private key. Anyone can verify the signature using the corresponding public key, but only the true holder of the private key could produce a valid signature. That’s why the requirement is proving knowledge of the true party’s private key—the private key that belongs to the identity being authenticated. The public key alone can’t prove who you are, because it’s not secret and can’t demonstrate possession of its pair. The verifier’s private key isn’t used in this process, and the authentication hinges on demonstrating knowledge of the private key associated with the claimed public key.