MySensors will have native support for sha204 and the
source for usage and personalization of sha204 will be
much more elaborate than this example so it does not
provide additional understanding for the chip.
Taken from the sha204_library folder in
https://github.com/jimblom/sha204-Breakout
Commit 8e1ac05ef987b1869605ad067db2bdbaec1c109f
ATSHA204 is a crypto cirquit from Atmel that enables
SHA256-based MAC and HMAC calculations as well as
secure key storage and random number generation.
This can be used to sign messages passed between
sensor nodes and prevent both man-in-the-middle and
replay attacking.