Virus mask algorithm
How do viruses disguise?
Users have been struggling with computer viruses since the very moment they appeared. When you detect and delete a virus you not only protect your computer in such a way but also help to ensure other computers' safety for they could have got infected with this virus in the process of data transfer from your computer. It goes without saying that people creating computer viruses are perfectly aware of it and try to use various methods to protect their brainchildren. The most common method they resort to is virus disguising.
As you know the main principle of any anti-virus software is revealing a virus as to the known pattern of its source code. Anti-virus software scans all files one by one without any exception looking for a code pattern peculiar to already known viruses. All code patterns of known viruses are stored in the database of any antivirus. If anti-virus software detects such code it immediately alerts the user.
Virus creators are doing their best to disguise this code to prevent antivirus from detecting it. There are some virus disguise methods.
Polymorphic viruses (also known as ghost viruses)
Such viruses don't have invariable permanent codes. It one takes several copies of the same virus they will notice that their codes has nothing in common. The thing is that such virus is spread encrypted and the decrypting algorithm is hidden it it. This algorithm is random, which means that the virus looks different every time. Such viruses can't be detected by comparing their code string to the database information.
In cases of ghost viruses anti-virus software uses especially devised method of heuristic analysis.
Invisible stealth viruses
Such viruses can't be detected with the help of operating system means. The screen will display a file that is not infected with a virus. Why this happens? The thing is that stealth viruses removes its body from the infected file when it is opened and a user sees a clen noninfected file. As soon as the file is closed the virus infects it again.
In order to detect such a virus anti-virus software uses specials means that don't depend on the operating system.