Mario starts up the tool to show me how it works. A little box appears on his laptop screen, politely asking me to name my Trojan. I call it the 'Clive' virus. Then it asks me what I'd like the virus to do. Shall the Trojan horse format drive C:? Yes, I click. Shall the Trojan horse overwrite every file? Yes. It asks me if I'd like to have the virus activate the next time the computer is restarted and I say yes again.
Then it's done. The generator spits out the virus on to Mario's hard drive, a tiny 3k file. Mario's generator also displays a stern notice warning that spreading your creation is illegal. The generator, he says, is just for educational purposes, a way to help curious programmers learn how Trojans work.
But, I could ignore that advice. I could give this virus an enticing name, like 'britney-spears-wedding-clip. mpeg' to fool people into thinking it's a video. If I were to email it to a victim and if he clicked on it and didn't have up-to-date anti-virus software, then disaster would strike his computer. The virus would activate. It would quietly reach into the victim's Microsoft Windows operating system and insert new commands telling the computer to erase its own hard drive.
The next time the victim started up his computer, the machine would find those new commands, assume they were part of the normal Windows operating system and guilelessly follow them. Everything on his hard drive would vanish - emails, pictures, documents, games. Mario drags the virus over to the trash bin on his computer's desktop and discards it. 'I don't think we should touch that,' he says hastily.