Rina wants to send Naseem a message that only Naseem can read. Which key does she encrypt it with?

Answer

Naseem's public key

Options - A. Naseem's public key - B. Naseem's private key - C. Rina's private key - D. Rina's public key Why - A. Correct. Only the matching private key reverses it, and Naseem alone holds that. - B. Rina does not have it, and never will — a private key is generated locally and never distributed. - C. That is mode (b). It authenticates Rina but everyone holding her public key can read the result. - D. Everyone can reverse that with her private key... which only she has, so nobody but Rina could read it — including Naseem. Confidentiality always runs toward the recipient: encrypt with the key the recipient publishes, decrypt with the key only the recipient holds.

Stallings & Brown 5e ch2 §2.3; ch21 §21.1