ATSHA204 emulated signing backend

ATSHA204 emulated signing backend. The emulated ATSHA204
implementation offers pseudo random number generation and
HMAC-SHA256 authentication compatible with a "physical"
ATSHA204.

NOTE: Key is stored in clear text in the Arduino firmware.
Therefore, the use of this back-end could compromise the key
used in the signed message infrastructure if device is lost
and its memory dumped.

The seed for the random number generated is sampled from an
analog pin. The pin is set in MyConfig.h using RANDOMSEED_PIN.
The pin should be unconnected.

The key to use for HMAC calculations must match the key used by
other nodes this node will exchange signed messages with. The key
for ATSHA204 emulator is set in MyConfig.h using hmacKey.

libraries/MySensors/utility/sha256.h

@@ -0,0 +1,40 @@
+// This code is taken from https://github.com/Cathedrow/Cryptosuite (Peter Knight) and is lightly modified for MySensors use
+#ifndef Sha256_h
+#define Sha256_h
+
+#include <inttypes.h>
+
+#define HASH_LENGTH 32
+#define BLOCK_LENGTH 64
+
+union _buffer {
+  uint8_t b[BLOCK_LENGTH];
+  uint32_t w[BLOCK_LENGTH/4];
+};
+union _state {
+  uint8_t b[HASH_LENGTH];
+  uint32_t w[HASH_LENGTH/4];
+};
+
+class Sha256Class
+{
+  public:
+    void init(void);
+    void initHmac(const uint8_t* secret, int secretLength);
+    uint8_t* result(void);
+    uint8_t* resultHmac(void);
+    virtual void write(uint8_t);
+  private:
+    void pad();
+    void addUncounted(uint8_t data);
+    void hashBlock();
+    uint32_t ror32(uint32_t number, uint8_t bits);
+    _buffer buffer;
+    uint8_t bufferOffset;
+    _state state;
+    uint32_t byteCount;
+    uint8_t keyBuffer[BLOCK_LENGTH];
+    uint8_t innerHash[HASH_LENGTH];
+};
+
+#endif