ViruXine.W32 virus hides their presence by changing the underlying executable code. This Virus code mutates while keeping the original algorithm intact, the code changes itself each time it runs…

ViruXine.W32 virus hides their presence by changing the underlying executable code. This Virus code mutates while keeping the original algorithm intact, the code changes itself each time it runs, but the function of the code (its semantics) will not change at all. Here is a section of the Virus code:

What is this technique called?