In public key encryption, the sender uses the recipient's public key for which purpose in public key encryption?

• A. Public key encryption for confidentiality
• B. Public key encryption for authentication
• C. Both A and B
• D. Neither A nor B

Answer: (A)

The key idea is that using the recipient’s public key to encrypt a message is about keeping the contents secret. In public key cryptography, the public key is distributed openly so anyone can use it to encrypt a message for that recipient. Only the recipient, who holds the matching private key, can decrypt it. That provides confidentiality: others cannot read the message even though they could have encrypted it for the recipient.

Authentication, meaning proving who sent the message, works differently—typically with a digital signature. A sender signs with their private key, and others verify the signature with the sender’s public key. Encrypting with the recipient’s public key does not prove who sent it, so it doesn’t establish authentication by itself.

So using the recipient’s public key to encrypt serves confidentiality, making that option the best answer.