(Understand what is a virus)

I am convinced that computer viruses are not evil and that programmers have a right to create them, posses them and experiment with them. That kind of a stand is going to offend a lot of people, no matter how it is presented.
    Mark Ludwig
    February 22, 1996

  In this section I would like to warn the reader that he will have the oppurtunity to see viruses from a different point of view. A person
who listens to different ideas at the risk of offense, and who at least considers that he might be wrong, can but gain from it. If you are offended by something in this article, please be critical - both of the article and yourself - and don't fall into a rut, by letting someone else tell you how to think.

And God saw that it was good.
And God blessed them, saying "Be fruitful and multiply"

Genesis 1:21,22

From the start I want to stress that I do not advocate anyone’s going out and infecting an innocent party’s computer system with a malicious virus designed to destroy valuable data or bring their system to a halt. That is not only wrong, it is illegal. If you do that, you could wind up in jail or find yourself being sued for millions.

However this does not mean that it is illegal to create a computer virus and experiment with it, even though I know some people wish it was.

If you do create a virus, though, be careful with it. Make sure you know it is working properly or you may wipe out your own system by accident. And make sure you don’t inadvertently release it into the world, or you may find yourself in a legal jam . . . even if it was just an accident. Someone who loses a year’s worth of work may not be so convinced that it was an accident. And soon it may be illegal to infect a computer system (even your own) with a benign virus which does no harm at all. The key word here is responsibility. Be responsible. If you do something destructive, be prepared to take responsibility. Many people think of viruses as sort of a black art but actually is not. Many years ago, "primitive people" were killing any who was crazy because they couldn't understand what was wrong with them. Fear made them killers and destroyer of life. Being afraid of something that you don't really understand what it does and how it does it is not the solution. When you don't understand something you have questions, these question must be anwsered by those who has the knowledge. Nowadays, this kind of information relies in many books and definitely is available free on the Internet.

We all should understand that computer viruses are functionally similar to living organisms. Biology can teach us a lot about them, both how they behave and how to make them better. However computer viruses also have the potential to teach us something about living organisms. We can create and control computer viruses in a way that we cannot yet control living organisms.

Learning how to write a virus may not make you more employable, or give you new techniques to incorporate into programs. So why waste time with them, unless you need them to sow chaos among your enemies?

Let me try to answer that: Ever since computers were invented in the 1940’s, there has been a brotherhood of people dedicated to exploring the limitless possibilities of these magnificent machines. This brotherhood has included famous mathematicians and scientists, as well as thousands of unnamed hobbyists who built their own computers, and programmers who love to dig into the heart of their machines. As long as computers have been around, men have dreamed of intelligent machines which would reason, and act without being told step by step just what to do. For many years this was purely science fiction. However, the very thought of this possibility drove some to attempt to make it a reality. Thus “Artificial Intelligence” was born. Yet AI applications are often driven by commercial interests, and tend to be colored by that fact. Typical results are knowledge bases and the like—useful, sometimes exciting, but also geared toward putting the machine to use in a specific way, rather than to exploring it on its own terms. The computer virus is a radical new approach to this idea of “living machines.” Rather than trying to design something which poorly mimics highly complex human behavior, one starts by trying to copy the simplest of living organisms. Simple one-celled organisms don’t do very much. The most primitive organisms draw nutrients from the sea in the form of inorganic chemicals, and take energy from the sun, and their only goal is apparently to survive and to reproduce. They aren’t very intelligent, and it would be tough to argue about their metaphysical aspects like “soul.” Yet they do what they were programmed to do, and they do it very effectively. If we were to try to mimic such organisms by building a machine— a little robot—which went around collecting raw materials and putting them together to make another little robot, we would have a very difficult task on our hands. On the other hand, think of a whole new universe—not this physical world, but an electronic one, which exists inside of a computer. Here is the virus’ world. Here it can “live” in a sense not too different from that of primitive biological life. The computer virus has the same goal as a living organism—to survive and to reproduce. It has environmental obstacles to overcome, which could “kill” it and render it inoperative. And once it is released, it seems to have a mind of its own. It runs off in its electronic world doing what it was programmed to do. In this sense it is very much alive.

There is no doubt that the beginning of life was an important milestone in the history of the earth. However, if one tries to consider it from the viewpoint of inanimate matter, it is difficult to imagine life as being much more than a nuisance. We usually assume that life is good and that it deserves to be protected.

However, we must realize that computer viruses are not inherently destructive. The essential feature of a computer program that causes it to be classified as a virus is not its ability to destroy data, but its ability to gain control of the computer and make a fully functional copy of itself. It can reproduce. When it is executed, it makes one or more copies of itself. Those copies may later be executed, to create still more copies, ad infinitum. Not all computer programs that are destructive are classified as viruses because they do not all reproduce, and not all viruses are destructive because reproduction is not destructive. However, all viruses do reproduce. The idea that computer viruses are always destructive is deeply ingrained in most people’s thinking though. The very term “virus” is an inaccurate and emotionally charged epithet. The scientifically correct term for a computer virus is “self-reproducing automaton,” or “SRA” for short. This term describes correctly what such a program does, rather than attaching emotional energy to it.

Leaving metaphysical questions like “soul” aside, a living organism can be differentiated from non-life in that it appears to have two goals:
(a) to survive, and
(b) to reproduce.
Actually, a computer virus has the same two goals as a living organism: to survive and to reproduce.

The simplest of living organisms depend only on the inanimate, inorganic environment for what they need to achieve their goals. They draw raw materials from their surroundings, and use energy from the sun to synthesize whatever chemicals they need to do the job. The organism is not dependent on another form of life which it must somehow eat, or attack to continue its existence. In the same way, a computer virus uses the computer system’s resources like disk storage and CPU time to achieve its goals. Specifically, it does not attack other self-reproducing automata and “eat” them in a manner similar to a biological virus. Instead, the computer virus is the simplest unit of life in this electronic world inside the computer.

Computer viruses are admirable life forms inside a computer box. They are looking for hosts as a real virus does and attach them self on it in order to survive and reproduce. We should not be afraid of them but we sould have the knowledge how to deal with them. Combination of ignorance, inexperience and fear-provoking reports of danger is the perfect formula for mass hysteria.

Parts for the text above have been taken from
“The little black book of viruses”, Mark A. Ludwig, 1996

Find More Information

Diagrams and a general view of "how a virus works and what it does" can be found here
Also, you can find an index with the most popular viruses and trojans allong with a description of what they do and how they work. Probably a sort tutorial of "How to create a backdoor" is what you are looking for! Take a look at this web page for a second opinion around the subject!