How do viruses function from a programming point of view? [closed]

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I have always been fascinated by computer viruses. For years I have tired to learn about them, but due to their nature people are unwilling to give many details.

For what it is worth I'm not a hacker and am not trying to build a virus.

If anyone is willing to answer this question I want to know what makes a virus a virus and how they are different from spyware.

How can they install themselves onto a computer without you noticing?

And how do worms work? How can a program replicate and move on its own? Does it contain its source code within it? And does it interface with other programs or just assess the hardware directly to spread ?

EDIT: Wha开发者_如何学Got language would they be written in? Would you use assembly/C++ types of languages or create them as scripts in lua?

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Well, a worm is simply a self-replicating piece of software. Imagine a program that copies its executeable over some link to another computer and launches it there. That's not that much magic.

A virus is simply a worm which infects other executeables, i.e. it does not replicate its own image, but it "backpacks" it to a different application's image and uses that application's execution flow to get initiated.

The user does not notice anything if there are no side-effects, and no UI interaction. If the user is a technically more competent than the average end-user, this is very hard to achieve. Some malwares host the target system in a virtual machine so you as the user have a hard time to see anything suspicious as long as you don't figure you look at a virtual machine. Like Neo, awaking from the Matrix.

As there is no limit to what you can implement in what language, there is no language of choice. Naturally, a low-level and natively-compiled language is more versatile to do what a virus/worm must do to stay low-profile. However, there are worms and viruses written in assembly language, Basic, C, Delphi, JavaScript, whatever -- there is nothing you can not imagine here.

Spyware has similar requirements, but different goals. While a virus, and a worm, usually spreads around, either for no reason or to drop some kind of payload at some point, spyware wants to either "phone home" or open the target system so it can be attacked, i.e. inspected, easier, usually in order to get hold of a victim's data that is secret, personal, or otherwise interesting.

Hope this quick answer helps a bit. You can google more details easily at bing :)