A message is encrypted using public key cryptography to send a message from…

Correct answer: Sender encrypts the message using the receiver’s public key.

Key idea: Use the receiver's public key to ensure only the receiver can decrypt the message.

• Encryption for confidentiality: Encrypt with the recipient’s public key so only the recipient’s private key can decrypt.

• Why encrypting with the sender’s public key is wrong: That would require the sender’s private key to decrypt, so the recipient could not read it; public keys are not used for decrypting.

• Role of sender’s public key: It is used to verify a digital signature created with the sender’s private key (authenticity), not to decrypt messages addressed to the receiver.

• Receiver’s keys: The receiver decrypts using their private key. Public keys are for encrypting to someone or verifying signatures; private keys are for decrypting or signing.