When Joshua sends a message to Larry, Joshua will use ________ to encrypt the message.

• A. the public key
• B. Joshua's public key
• C. Larry's public key
• D. Larry's private key

Answer: (C)

In public-key cryptography, to send a confidential message, you encrypt it with the recipient’s public key so that only the recipient can decrypt it with their private key. So Joshua should use Larry’s public key to encrypt the message. That way Larry can decrypt it using his private key. Using Joshua’s public key wouldn’t let Larry read the message, and using Larry’s private key isn’t appropriate for confidentiality because private keys are meant to stay secret with the owner (encryption with a private key is essentially a signature, not a confidential delivery).