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Metamorphic code is used by some viruses when they are about to infect new files, and the result is that the next generation will never look like current generation. The mutated code will do exactly the same thing (under the interpretation used), but the child's binary representation will typically be completely different from the parent's.
The first known polymorphic virus was written by Mark Washburn. The virus, called 1260, was written in 1990. A better-known polymorphic virus was created in 1992 by the hacker Dark Avenger as a means of avoiding pattern recognition from antivirus software. A common and very virulent polymorphic virus is the file infecter Virut.
Polymorphic code was the first technique that posed a serious threat to virus scanners. Just like regular encrypted viruses, a polymorphic virus infects files with an encrypted copy of itself, which is decoded by a decryption module. In the case of polymorphic viruses, however, this decryption module is also modified on each infection.
Oligomorphic code, also known as semi-polymorphic code, is a method used by a computer virus to obfuscate its decryptor by generating different versions of it, in order to evade detection by antivirus software. It is similar to, but less sophisticated than, polymorphic code. [1]
November: The term "virus" is re-coined by Frederick B. Cohen in describing self-replicating computer programs. In 1984 Cohen uses the phrase "computer virus" (suggested by his teacher Leonard Adleman) to describe the operation of such programs in terms of "infection". He defines a "virus" as "a program that can 'infect' other programs by ...
A polymorphic engine (sometimes called mutation engine or mutating engine) is a software component that uses polymorphic code to alter the payload while preserving the same functionality. Polymorphic engines are used almost exclusively in malware , with the purpose of being harder for antivirus software to detect.
Software distributors use executable compression for a variety of reasons, primarily to reduce the secondary storage requirements of their software; as executable compressors are specifically designed to compress executable code, they often achieve better compression ratio than standard data compression facilities such as gzip, zip or bzip2 [citation needed].
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