A compression virus is an example of a benevolent computer virus, invented by Fred Cohen. It searches for an uninfected executable file, compresses the file and prepends itself to it. The virus can be described in pseudo code^[1]

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 (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 (tmpfile) and runs it (run tmpfile).

Cruncher is an example of a compression virus,^[2] a strain of which – Cruncher.2092^[3] – is described by McAfee 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.

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