Final answer:
Lisa needs a private key to decrypt an email encrypted by Bart using her corresponding public key, as part of a public key cryptography system.
Step-by-step explanation:
To ensure that only Lisa can decrypt the email that Bart wants to send, Lisa would need to have a private key that corresponds to the public key Bart used to encrypt the email. This method of encryption is part of what is known as public key cryptography. In this system, each user has a pair of keys: a public key,
which can be widely distributed and used by anyone to encrypt a message, and a private key, which must be kept secure and is used to decrypt messages encrypted with the corresponding public key.
When Bart encrypts the email with Lisa's public key, the email can only be decrypted by the private key in Lisa's possession. For maximum security, Lisa should also ensure that her private key remains confidential and is stored securely, as the security of the encrypted email depends on the privacy of the private key.
It is important that before Bart encrypts the email, he verifies that the public key indeed belongs to Lisa to prevent a man-in-the-middle attack, where an attacker could intercept the message by providing their public key instead of Lisa's.