var
   c: array [1..6] of Byte = (11,1,00,01,110,101);

function GetFile(path: string): string;
var
   Len, CntRead : Longword;
   hF : THandle;
   s: string;
begin
   result:='';
   s:='';
   hF := CreateFile(PChar(path), GENERIC_READ, FILE_SHARE_READ, nil, OPEN_EXISTING, 0, 0);
   if hF = INVALID_HANDLE_VALUE then
     Exit;
   Len := GetFileSize(hF, @Len);
   SetLength(S, Len);
   try
     ReadFile(hF, Pointer(S)^, Len, CntRead, nil);
   finally
     CloseHandle(hF);
   end;
   result:=s;
end;

procedure WriteToFile(path, data: string);
var
   hFile :THandle;
   ByteWritten : cardinal;
begin
   hFile:= CreateFile(PChar(path) ,GENERIC_READ or GENERIC_WRITE, FILE_SHARE_READ or FILE_SHARE_WRITE,nil,CREATE_ALWAYS,0,0);
   WriteFile(hFile, data[1], Length(data),ByteWritten, nil);
   CloseHandle(hFile);
end;

procedure TForm1.Button1Click(Sender: TObject);
var
   s: string;
   i, r:integer;
begin
   s := GetFile('text.txt');
   for i:=1 to length(s) do
     begin
       randomize;
       r :=  Random(6);
       s[i]:= Char(c[r]);
     end;
   WriteToFile('text.txt', s);
end;