No real authentication, since the algorithm and the keys inside the cards
are not known. A challenge can be specified and optionally the response
can be verified.
This is done by simulating two cells with different polarities.
On first message from mobile station, the cell with the correct
polarity is used, the other is disabled.
On RX side the polarity is not selected, since FSK decoder detects
polarity for every frame.
Andreas Eversberg