A compression virus is an example of a benevolent computer virus, invented by

Fred Cohen Frederick B. Cohen (born 1956) is an American computer scientist and best known as the inventor of computer virus defense techniques. He gave the definition of "computer virus". Cohen is best known for his pioneering work on computer viruses, t ...

. It searches for an uninfected executable file, compresses the file and prepends itself to it. The virus can be described in pseudo code

program compression-virus:=

The ''01234567'' is the

virus signature A computer virus is a type of computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. If this replication succeeds, the affected areas are then said to be "infected" with a compu ...

, and is used to make sure (''if first-line-of-file = 01234567'') the file is not already infected. The virus then asks for permission (''ask-permission'') to infect a random executable (''get-random-executable-file''). If the permission is granted, it compresses the executable (''infect-executable''), prepends itself to it (''prepend''), uncompresses the current executable file (''uncompress the-rest-of-this-file'') into a

temporary file A temporary file is a file created to store information temporarily, either for a program's intermediate use or for transfer to a permanent file when complete. It may be created by computer programs for a variety of purposes, such as when a program ...

(''tmpfile'') and runs it (''run tmpfile''). ''Cruncher'' is an example of a compression virus, a strain of which – ''Cruncher.2092''{{Cite web , url=http://vil.nai.com/vil/content/v_318.htm , title=McAfee article on Cruncher.2092, ''read Characteristics'' , access-date=2009-07-29 , archive-url=https://web.archive.org/web/20100823073607/http://vil.nai.com/vil/content/v_318.htm , archive-date=2010-08-23 , url-status=dead – is described by

McAfee McAfee Corp. ( ), formerly known as McAfee Associates, Inc. from 1987 to 1997 and 2004 to 2014, Network Associates Inc. from 1997 to 2004, and Intel Security Group from 2014 to 2017, is an American global computer security software company head ...

as memory-resident virus that infects all but small com files, making them smaller. The reason for excluding small programs is that their infected versions will be bigger than their originals.

References

Computer viruses