IFF = identifies friends or foe, is an electronic equipment for automatic identification, with secret codes exchange
between radar and aircraft, only aircraft use it,(not used for civilian plane)
if the response is positive OUR aircraft will be as a big "banana" in radar echo, else he still as usual radar echo.
this code can change every days (or hours)in war time,or every month in peace time,
there is no way to enemies to send the same code "to be Friend" for the radar !

enemy with IFF off = don't exist !