Another common stealth technique changes a file, but displays its size as it was before infection. An encrypted virus enjoys the advantages of other encrypted material - chiefly, a dramatically reduced risk of complete discovery. Initially, encrypted viruses appear not as viruses, but as nondescript gibberish - but when an infected program is executed, a small piece of plain, unencrypted code decrypts the rest of the virus, which proceeds to do its damage. When and if an encrypted virus is detected, it is very difficult to dissect. Because it is not as subject to reverse engineering as unencrypted viruses are, the encrypted variety is better able to evade researchers, who try to determine a virus' structure and the precise scope of its payload.
Encryption is most useful when coupled with a polymorphic strategy.
Polymorphism A polymorphic virus tries to evade detection by altering its structure or its encryption techniques. Each time an infection occurs, a polymorphic virus changes its form, confusing antivirus scanners. Famous Viruses New Zealand New Zealand, also called Stoned or Marijuana, is a stealth boot-record infector. It may also damage floppy disks. Stoned was originally created by a New Zealand student, who claimed that his program was never meant to get out into the wild.