Final answer:
A PGP virus is a type of virus that targets PGP software used for email encryption. The virus infects a computer and remains dormant until the user decrypts a message with their private key, at which point it steals the key and sends it to the attacker. This is different from Trojan horse attacks, which involve deceiving users into unknowingly installing malicious software.
Step-by-step explanation:
A PGP virus refers to a specific type of virus that targets PGP software, which is used for email encryption. In this scenario, Eve designs a virus that infects Alice's computer and remains dormant until Alice uses her private key to decrypt a message. The virus then secretly notes the private key and sends it to Eve when Alice connects to the Internet, allowing Eve to decipher all subsequent messages sent to Alice.
The PGP virus described in the question is an example of a virus that targets specific software or systems. Unlike general viruses that may infect various types of hosts, this virus specifically targets the PGP software and utilizes its functionality to compromise the security of encrypted messages.
In contrast to viruses, Trojan horse attacks involve disguising malicious software as legitimate or desirable programs. Rather than spreading and infecting other systems like viruses, Trojan horse attacks aim to deceive users into unknowingly installing harmful software that can compromise their computer's security.