I suggested one of the Bitcoin ATM guys to use two boards. One board is
   connected like normally to networks and accessories and the like. That
   one board also has a custom connection to the other board. The other
   board contains all the secrets and performs all the important
   functions. The one board just communicates.
   The advantage is that once the big bad guys crack your baseband, your
   chipset, your system on a chip's trapcards, etc. your secrets are still
   safe. If you sign all the packets that pass through the communication
   board you can truly abstract away from almost every possible hack that
   both the boards could be vulnerable for. (Do you trust your silicon?)