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Computer Viruses

In: Computers and Technology

Submitted By SBelton08
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Definition of virus

"Computer viruses" is the term that defines the class of programs which illegally explore resources of systems. It is an executable code able to reproduce itself. Viruses are an area of pure programming, and, unlike other computer programs, carry intellectual functions on protection from being found and destroyed. They have to fight for survival in complex conditions of conflicting computer systems. Computer viruses have pervaded popular culture at least as successfully as they have the world's computer population. Computer viruses replicate by attaching themselves to a host a program or computer and co-opting the host's resources to make copies of themselves. Symptoms can range from unpleasant to fatal. Computer viruses spread from program to program and computer to computer. There are other computer pathogens, such as the "worms" that occasionally afflict networks, and the "Trojan horses" that put a friendly face on malicious programs, but viruses are the most common computer ill by far.

Types of viruses.

There are different types of viruses, and they have already been separated into classes and categories. For instance: dangerous, harmless, and very dangerous. No destruction means a harmless one, tricks with system halts means a dangerous one, and finally with a devastating destruction means a very dangerous virus.
But viruses are famous not only for their destructive actions, but also for their special effects, which are almost impossible to classify. Some virus-writers suggest the following: funny, very funny and sad or melancholy (keeps silence and infects). But one should remember that special effects must occur only after a certain number of contaminations. Users should also be given a chance to restrict execution of destructive actions, such as deleting files, formatting hard disks. Thereby virus can be considered to be a useful program, keeping a check on system changes and preventing any surprises such as of deletion of files or wiping out hard disks.
Computer viruses can trace their pedigree to John Von Neumann's studies of self-replicating mathematical automata in the 1940s. Although the idea of programs that could infect computers dates to the 1970s, the first well-documented case of a computer virus spreading "in the wild" occurred in October 1987, when a code known as the "Brain" virus appeared on several diskettes at the University of Delaware. Today viruses afflict at least a million computers every year. Users spend several hundred million dollars annually on antiviral products and services, and this statistic is growing rapidly.
Most viruses attack personal computers (PCs). More than 10,000 viruses have appeared so far, and programmers generate another six every day. Fortunately, only a handful have been detected far from born place. There are three main classes of PC viruses (and the categories for other systems are analogous): file infectors, boot-sector viruses and macro viruses. Roughly 85 percent of all known viruses infect files containing applications such as spreadsheet programs or games. When a user runs an infected application, the virus code executes first and installs itself independently in the computer's memory so that it can copy itself into subsequent applications that the user runs. Once in place, the virus returns control to the infected application; the user remains unaware of its existence. Eventually a infected program will make its way to another computer through a shared diskette or network, and the infection cycle will begin again.
Boot-sector viruses, which account for about 5 percent of known PC viruses, reside in a special part of a diskette or hard disk that is read into memory and executed when a computer first starts. The boot sector normally contains the program code for loading the rest of a computer's operating system. Once loaded, a boot-sector virus can infect any diskette that is placed in the drive. It also infects the hard disk, so that the virus will be loaded into memory whenever the system is restarted. Boot viruses are highly effective.
The third category, macro viruses, are independent of operating systems and infect files that are usually regarded as data rather than as programs. Many spreadsheet, database and word-processing programs can execute scripts—prescribed sequences of actions-- embedded in a document. Such scripts, or macros, are used to automate actions ranging from typing long words to carrying out complicated sequences of calculations. And virus writers have created scripts that insert copies of themselves in other documents. Macro viruses can spread much more rapidly than other kinds of viruses because many people share "data" files freely--consider several workers swapping drafts of a jointly written report. "Concept," the first macro virus observed in the wild, infected its first Microsoft Word document late in 1995 and is now the most prevalent virus in the world. Today more than 1,000 macro viruses are known. As well as basic replication code, viruses can contain whatever other code the author chooses. Some viruses may simply print a message or display an image, but others will damage programs and data. Even those without malicious payloads can cause damage to systems whose configuration differs from what the virus designer expected. For instance, the "Form" virus, which usually produces only a slight clicking noise once a month, overwrites one disk directory sector in a way that is harmless to older PCs but lethal to newer ones that arrange disk information differently.

Antivirus Technology.

Antiviral software has existed since shortly after computer viruses first appeared. Generic virus-detection programs can monitor a computer system for virus like behaviour (such as modification of certain crucial files or parts of main memory), and they can periodically check programs for strange modifications. Such software can even detect unknown viruses, but it can also be prone to false alarms because some legitimate activities resemble viruses at work. Scanning programs, in contrast, can search files, boot records and memory for specific patterns of bytes indicative of known viruses. To stay current, they must be updated when new viral strains arise, but they only rarely raise false alarms. The viral signatures these programs recognize are quite short: typically 16 to 30 bytes out of the several thousand that make up a complete virus. It is more efficient to recognize a small fragment than to verify the presence of an entire virus, and a single signature may be common to many different viruses. Most computer- virus scanners use pattern-matching algorithms that can scan for many different signatures at the same time: the best can check for 10,000 signatures in 10,000 programs in less than 10 minutes.
Once a virus has been detected, it must be removed. One brutal but effective technique is simply to erase the infected program, much as certain types of immune cells destroy an infected cell. Body cells are generally easy to replace, but computer programs and documents are not so expendable. As a result, antiviral programs do their best to repair infected files rather than destroy them. (They are aided in this endeavour by the fact that computer viruses must preserve their host program essentially intact to remain undetected and multiply.) If a virus-specific scanning program detects an infected file, it can usually follow a detailed prescription, supplied by its programmers, for deleting viral code and reassembling a working copy of the original. There are also generic disinfection techniques that work equally well for known and unknown viruses. One method we developed gathers a mathematical fingerprint for each program on the system. If a program subsequently becomes infected, our method can reconstitute a copy of the original.
Virus-specific detection and removal techniques require detailed analysis of each new virus as it is discovered. Experts must identify unusual sequences of instructions that appear in the viral code but not in conventional programs--a process that relies on carefully developed knowledge and intuition. They also must develop a prescription for verifying and removing the virus from any infected host. To keep up with the influx of half a ten new viruses a day, antiviral technologists have developed automated tools and procedures to assist human virus experts or even replace them. Antiviral specialists have developed a statistical technique to extract high- quality signatures very quickly. We started by measuring the frequencies of short byte sequences in a large group of legitimate programs. When a new virus is sent to Centre, special software finds the sequence of viral bytes that is statistically least likely to appear in a legitimate program.

This method is much faster than analysis by hand, and tests suggest that it produces signatures that are less prone to false alarms than those selected by expert humans. Our signature- extraction method is somewhat analogous to the outmoded "template" theory of the immune system, according to which antibodies mould themselves to a particular foreign invader—our signatures are made specifically for each new virus we encounter.
Stephanie Forrest of the University of New Mexico and her collaborators at Los Alamos National Laboratory have developed an alternative that is more faithful to the currently accepted "clonal selection" theory of the immune system, in which the body generates an huge range of immune cells and then mass-produces the ones that turn out to recognize a pathogen. Their scheme generates code signatures randomly, without reference to any particular virus. Each signature is checked against existing code on the system; if it does not match anything, it is retained in a huge database. Finding one of these signatures in a program is a sure sign that the program has been modified, although further analysis is required to determine whether a virus is at fault.…...

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