This is a trimmed-down version of Peter Knight's excellent Cryptosuite library for Arduino, supporting secure hashing and hashed message authentication using SHA-256 and HMAC-SHA-256.

To help fit complex applications within the limited code space of the Arduino, this version strips out the SHA-1 functionality -- it wasn't required for my OpenSesame code.

https://github.com/Cathedrow/Cryptosuite

The original also contains a test suite and further exposition on the use of hash functions.

Installation:
  Make a 'libraries' directory with your Arduino folder if you do not already have one.
  Rename this folder 'Sha' and move it into that directory.
  Restart Arduino IDE to rescan for new libraries.