When Carol sends a message to Bob, Bob will use ________ to decrypt the message.

• A. the private key
• B. Carol's private key
• C. Bob's private key
• D. Carol's public key

Answer: (C)

Public-key cryptography uses a pair of keys: a public key for encryption and a private key for decryption. When Carol sends a confidential message to Bob, she encrypts it with Bob's public key, and Bob decrypts it with his private key. So the key Bob uses to decrypt is his own private key. Using Carol’s private key would be like a signature, not decryption for a message sent to Bob, and using Carol’s or Bob’s public key would not decrypt the message in this confidentiality scenario.