(Two people want to communicate through secure email.

To ensure confidentiality so that only the intended recipient can decrypt an email, the sender must encrypt in a way that only the recipient can reverse. In public key cryptography, that means encrypting with the recipient’s public key. The recipient is the only party who should possess the matching private key, so only they can decrypt the ciphertext. This pattern is fundamental to PKI-based secure email systems such as S/MIME and OpenPGP: the sender looks up or is provided the recipient’s certificate/public key, encrypts the message (often by encrypting a randomly generated symmetric session key with the recipient’s public key), and the recipient uses their private key to recover the session key and decrypt the content. Encrypting with the sender’s private key would not provide confidentiality; it resembles signing because anyone with the sender’s public key could “decrypt” it. Encrypting with a private key of the recipient is also incorrect because private keys are not shared and should never leave the recipient’s control. Therefore, the correct key to encrypt the message so only the friend can decrypt it is the recipient’s public key.