1.3. Automated Replicating Code: The Theory and Definition of Computer Viruses
Cohen provided a formal mathematical model for computer viruses in 1984. This model used a Turing machine. In fact, Cohen's formal mathematical model for a computer virus is similar to Neumann's self-replicating cellular automata model. We could say, that in the Neumann sense, a computer virus is a self-reproducing cellular automata. The mathematical model does not have much practical use for today's researcher. It is a rather general description of what a computer virus is. However, the mathematical model provides significant theoretical foundation to the computer virus problem.
Here is Cohen's informal definition of a computer virus: "A virus is a program that is able to infect other programs by modifying them to include a possibly evolved copy of itself."
This definition provides the important properties of a computer virus, such as the possibility of evolution (the capability to make a modified copy of the same code with mutations). However, it might also be a bit misleading if applied in its strictest sense.
This is, by no means, to criticize Cohen's groundbreaking model. It is difficult to provide a precise definition because there are so many different kinds of computer viruses nowadays. For instance, some forms of computer viruses, called companion viruses, do not necessarily modify the code of other programs. They do not strictly follow Cohen's definition because they do not need to include a copy of themselves within other programs. Instead, they make devious use of the program's environmentproperties of the operating systemby placing themselves with the same name ahead of their victim programs on the execution path. This can create a problem for behavior-blocking programs that attempt to block malicious actions of other programsif the authors of such blockers strictly apply Cohen's informal definition. In other words, if such blocking programs are looking only for viruses that make unwanted changes to the code of another program, they will miss companion viruses.
Cohen's mathematical formulation properly encompasses companion viruses; it is only the literal interpretation of the single-sentence human language definition that is problematic. A single-sentence linguistic definition of viruses is difficult to come up with.
Integrity checker programs also rely on the fact that one program's code remains unchanged over time. Such programs rely on a database (created at some initial point in time) assumed to represent a "clean" state of the programs on a machine. Integrity checker programs were Cohen's favorite defense method and my own in the early '90s. However, it is easy to see that the integrity checker would be challenged by companion viruses unless the integrity checker also alerted the user about any new application on the system. Cohen's own system properly performed this. Unfortunately, the general public does not like to be bothered each time a new program is introduced on their systems, but Cohen's approach is definitely the safest technique to use.
Dr. Cohen's definition does not differentiate between programs explicitly designed to copy themselves (the "real viruses" as we call them) from the programs that can copy themselves as a side effect of the fact that they are general-purpose copying programs (compilers and so on).
Indeed, in the real world, behavior-blocking defense systems often alarm in such a situation. For instance, Norton Commander, the popular command shell, might be used to copy the commander's own code to another hard drive or network resource. This action might be confused with self-replicating code, especially if the folder in which the copy is made has a previous version of the program that we overwrite to upgrade it. Though such "false alarms" are easily dealt with, they will undoubtedly annoy end users.
Taking these points into consideration, a more accurate definition of a computer virus would be the following: "A computer virus is a program that recursively and explicitly copies a possibly evolved version of itself."
There is no need to specify how the copy is made, and there is no strict need to "infect" or otherwise modify another application or host program. However, most computer viruses do indeed modify another program's code to take control. Blocking such an action, then, considerably reduces the possibility for viruses to spread on the system.
As a result, there is always a host, an operating system, or another kind of execution environment, such as an interpreter, in which a particular sequence of symbols behaves as a computer virus and replicates itself recursively.
Computer viruses are self-automated programs that, against the user's wishes, make copies of themselves to spread themselves to new targets. Although particular computer viruses ask the user with prompts before they infect a machine, such as, "Do you want to infect another program? (Y/N?)," this does not make them non-viruses. Often, novice researchers in computer virus labs believe otherwise, and they actually argue that such programs are not viruses. Obviously, they are wrong!
When attempting to classify a particular program as a virus, we need to ask the important question of whether a program is able to replicate itself recursively and explicitly. A program cannot be considered a computer virus if it needs any help to make a copy of itself. This help might include modifying the environment of such a program (for example, manually changing bytes in memory or on a disk) orheaven forbidapplying a hot fix to the intended virus code itself using a debugger! Instead, nonworking viruses should be classified as intended viruses.
The copy in question does not have to be an exact clone of the initial instance. Modern computer viruses, especially so-called metamorphic viruses (further discussed in Chapter 7, "Advanced Code Evolution Techniques and Computer Virus Generator Kits"), can rewrite their own code in such a way that the starting sequence of bytes responsible for the copy of such code will look completely different in subsequent generations but will perform the equivalent or similar functionality.