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7.2 Evolution of Code

Virus writers continually challenge antivirus products. Their biggest enemies are the virus scanner products that are the most popular of current antivirus software. Generic AV solutions, such as integrity checking and behavior blocking, never managed to approach the popularity of the antivirus scanner.

In fact, such generic virus detection models need a lot more thought and technology in place under Windows platforms. These technologies were beaten by some of the old DOS viruses in the DOS days. As a result, some people draw the incorrect conclusion that these techniques are not useful.

Scanning is the market's accepted solution, regardless of its drawbacks. Thus it must be able to deal with the escalating complexity and emerging number of distributed and self-distributing malware.

Although modern computing developed extremely quickly, for a long time binary virus code could not catch up with the technological challenges. In fact, the DOS viruses evolved to a very complex level until 1996. At that point, however, 32-bit Windows started to dominate the market. As a result, virus writers had to go back years in binary virus development. The complexity of DOS polymorphism peaked when Ply was introduced in 1996 with a new permutation engine (although the metamorphic virus, ACG, was introduced in 1998). These developments could not continue. New 32-bit infection techniques had to be discovered by the pioneer virus writers and later on Win32 platforms.

Some virus writers still find the Windows platforms far too challenging, especially when it comes to Windows NT/2000/XP/2003. The basic infection techniques, however, have already been introduced, and standalone virus assembly sources are distributed widely on the Internet. These sources provide the basis of new mass-mailing worms that do not require major skills—just cut and paste abilities.

In the following sections, we will examine the basic virus code obfuscation techniques, from encrypted viruses to modern metamorphic techniques.

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