Other attacks include the use of viruses and Trojan horses. Eve might design a virus that infects PGP software and sits quietly inside Alice's computer. When Alice uses her private key to decrypt a message, the virus would wake up and make a note of it. The next time that Alice connects to the Internet, the virus would surreptitiously send the private key to Eve, thereby allowing her to decipher all subsequent messages sent to Alice. The Trojan horse, another software trick, involves Eve designing a program that appears to act like a genuine encryption product, but which actually betrays the user.

–The Code Book,
Simon Singh

What is the central idea of the passage?

No one should ever use PGP software or viruses.
People can invade your Internet privacy even if you code your information.
The Trojan horse is a really clever way to invade someone’s privacy.
Alice should find a new way to send her private messages.