Compression virus is an example of a benevolent 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...

 invented by Fred Cohen

Fred Cohen

Frederick B. Cohen is an American computer scientist and best known as the inventor of computer virus defense techniques.In 1983, while a student at the University of Southern California's School of Engineering , he wrote a program for a parasitic application that seized control of computer...

. It searches for an uninfected executable

Executable

In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

 file, compresses the file and prepends itself to it. The virus can be described in pseudo code

program compression-virus:=

{01234567;



subroutine infect-executable:=

 {loop:file = get-random-executable-file;

 if first-line-of-file = 01234567 then goto loop;

 compress file;

 prepend compression-virus to file;

 }



main-program:=

 {if ask-permission then infect-executable;

 uncompress the-rest-of-this-file into tmpfile;

 run tmpfile;}

}

The 01234567 is the virus signature, 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

Executable

In computing, an executable file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a data file that must be parsed by a program to be meaningful. These instructions are traditionally machine code instructions for a physical CPU...

 (get-random-executable-file). If permission granted, it compresses

Data compression

In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....

 the executable (infect-executable), prepends itself to it (prepend), uncompresses the current executable file (uncompress the-rest-of-this-file) into a temporary file

Temporary file

Temporary files may be created by computer programs for a variety of purposes; principally when a program cannot allocate enough memory for its tasks, when the program is working on data bigger than the architecture's address space, or as a primitive form of inter-process communication.- Auxiliary...

(tmpfile) and runs it (run tmpfile).

Cruncher is an example of a compression virus , a strain of which - Cruncher.2092 is described by McAfee

McAfee

McAfee, Inc. is a computer security company headquartered in Santa Clara, California, USA. It markets software and services to home users, businesses and the public sector. On August 19, 2010, electronics company Intel agreed to purchase McAfee for $7.68 billion...

 as memory-resident

Terminate and Stay Resident

Terminate and Stay Resident is a computer system call in DOS computer operating systems that returns control to the system as if the program has quit, but keeps the program in memory...

 virus that infects all but small com

COM file

In many computer operating systems, a COM file is a type of executable file; the name is derived from the file name extension .COM. Originally, the term stood for "Command file", a text file containing commands to be issued to the operating system , on many of the Digital Equipment Corporation mini...

files, making them smaller. The reason for excluding small programs is that their infected versions will be bigger than their originals.