Another option is to use different type of radio, like the RFM69W which has onboard AES encryption.
Some of the security aspects raised here are already covered here: http://lowpowerlab.com/blog/2013/10/02/raspberrypi-home-automation-gateway . There are 5 articles in total.
The author also created a arduino library for the RFM69W. (http://lowpowerlab.com/blog/2013/06/20/rfm69-library)

How difficult would it be to support this radio ?