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.
