Compression virus

A compression virus is an example of a benevolent <a href="/facts/Computer_virus/IyTC1usq">computer virus</a>, invented by <a href="/facts/Fred_Cohen/6psFI3CA">Fred Cohen</a>. It searches for an uninfected <a href="/facts/Executable/VSst9eLA">executable</a> 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 <a href="/facts/Computer_Virus/IyTC1usq">virus signature</a>, 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 <a href="/facts/Executable/VSst9eLA">executable</a> (get-random-executable-file). If the permission is granted, it <a href="/facts/Data_compression/gMFuY4FT">compresses</a> the executable (infect-executable), prepends itself to it (prepend), uncompresses the current executable file (uncompress the-rest-of-this-file) into a <a href="/facts/Temporary_file/5PRRodGA">temporary file</a> (tmpfile) and runs it (run tmpfile).
Cruncher is an example of a compression virus, a strain of which – Cruncher.2092 – is described by <a href="/facts/McAfee/yIytBc4w">McAfee</a> as memory-resident virus that infects all but small <a href="/facts/COM_file/3SXSyIxa">com</a> files, making them smaller. The reason for excluding small programs is that their infected versions will be bigger than their originals.

Compression virus open-in-new

Compression virus