I just had a thought today, what is stopping someone building a range extender that effectively boosts the range of an RFID chip, eg by effectively repeating the signal of a POS CC reader to a card in someone elses pocket? The reason I'm curious is this bypasses all of encryption and you don't need to copy or crack anything, all you need to do is effectively build an RFID repeater. Copying cards remotely is quite difficult now due to the token systems they use so they are safe in this regard, but I can't see any way to defeat a range extender. Obviously such a repeater would be huge due to the power requirements, but it isn't infeasible to fit it in a backpack. Is there any good reason why criminals haven't done things like this?