Joe, a user, wants to send Ann, another user, a confidential document electronically. Which of the following should Joe do to ensure the document is protected from eavesdropping?

Encrypting with Joe's private key performs a digital signature operation (providing authenticity/non-repudiation), not confidentiality, because anyone with Joe's widely available public key could decrypt it.

Encrypting with Joe's public key would mean only Joe could decrypt it with his private key, so Ann would be unable to read the document at all.

Ann's private key must remain secret and is never shared; if Joe had access to Ann's private key to encrypt with it, the entire security model would already be compromised.

Encrypting with Ann's public key ensures that only Ann, who possesses the corresponding private key, can decrypt and read the document. This is the foundational principle of asymmetric (public-key) encryption for confidentiality: the public key is freely shared and used to encrypt, while the private key is kept secret and used to decrypt, preventing any eavesdropper from reading the content.