An oligomorphic engine is generally used by a computer virus

Computer virus

A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

 to generate a decryptor for itself in a way comparable to a simple polymorphic engine

Polymorphic code

In computer terminology, polymorphic code is code that uses a polymorphic engine to mutate while keeping the original algorithm intact. That is, the code changes itself each time it runs, but the function of the code will not change at all...

. It does this by randomly selecting each piece of the decryptor from several predefined alternatives.
The pieces used to build the decryptor are usually too common to be detected with signatures. However, most oligomorphic viruses aren't able to generate more than just a few hundred different decryptors, so detecting them with simple signatures is still possible. Another method to detect an oligomorphic decryptor is to make a signature for each possible piece of code, group pieces that can substitute each other together and scan the file for a chain of decryptor pieces from alternating groups. Emulation may be used to detect the virus, but it can take more resources than necessary.

See also

• Timeline of notable computer viruses and worms
  Timeline of notable computer viruses and worms
  This is a timeline of noteworthy computer viruses, worms and Trojan horses.- 1966 :* The work of John von Neumann on the "Theory of self-reproducing automata" is published...
  
  
• Polymorphic code
  Polymorphic code
  In computer terminology, polymorphic code is code that uses a polymorphic engine to mutate while keeping the original algorithm intact. That is, the code changes itself each time it runs, but the function of the code will not change at all...
  
  
• Metamorphic code
  Metamorphic code
  In computer virus terms, metamorphic code is code that can reprogram itself. Often, it does this by translating its own code into a temporary representation, editing the temporary representation of itself, and then writing itself back to normal code again. This procedure is done with the virus...
  
  
• Self-modifying code
  Self-modifying code
  In computer science, self-modifying code is code that alters its own instructions while it is executing - usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, thus simplifying maintenance...
  
  
• Alphanumeric code
  Alphanumeric code
  In general, in computing, an alphanumeric code is a series of letters and numbers which are written in a form that can be processed by a computer....
  
  
• Shellcode
  Shellcode
  In computer security, a shellcode is a small piece of code used as the payload in the exploitation of a software vulnerability. It is called "shellcode" because it typically starts a command shell from which the attacker can control the compromised machine. Shellcode is commonly written in...
  
  
• Software cracking
  Software cracking
  Software cracking is the modification of software to remove or disable features which are considered undesirable by the person cracking the software, usually related to protection methods: copy protection, trial/demo version, serial number, hardware key, date checks, CD check or software annoyances...
  
  
• Security cracking
• Obfuscated code
  Obfuscated code
  Obfuscated code is source or machine code that has been made difficult to understand for humans. Programmers may deliberately obfuscate code to conceal its purpose or its logic to prevent tampering, deter reverse engineering, or as a puzzle or recreational challenge for someone reading the source...