Which technique is used to prove the user's identity in public-key authentication?

• A. Encrypt with the recipient's public key
• B. Sign with the user's private key
• C. Verify with the user's private key
• D. Encrypt with the verifier's public key

Answer: (B)

Public-key authentication relies on digital signatures to prove you control the private key tied to your public key. In this approach, a challenge from the verifier is signed with your private key and returned; the verifier then uses your public key to verify the signature. Because only you should possess the private key, a successful signature verification confirms you are the legitimate owner, and thus your identity within the system (assuming the public key is properly bound to you, e.g., via a certificate). The other options don’t establish identity: encrypting with the recipient’s public key keeps the message confidential to that recipient; verifying with the private key isn’t feasible because the private key is kept secret; and encrypting with the verifier’s public key protects data for the verifier rather than proving who you are.