Refactor signing backend (#690)

Cleaned up the AES driver and removed some cppcheck isues
in the process.

drivers/ATSHA204/ATSHA204.cpp

@@ -177,7 +177,6 @@ static uint8_t swi_receive_bytes(uint8_t count, uint8_t *buffer)
 
 static uint8_t sha204p_receive_response(uint8_t size, uint8_t *response)
 {
-	uint8_t count_byte;
 	uint8_t i;
 	uint8_t ret_code;
 
@@ -189,6 +188,7 @@ static uint8_t sha204p_receive_response(uint8_t size, uint8_t *response)
 
 	ret_code = swi_receive_bytes(size, response);
 	if (ret_code == SWI_FUNCTION_RETCODE_SUCCESS || ret_code == SWI_FUNCTION_RETCODE_RX_FAIL) {
+		uint8_t count_byte;
 		count_byte = response[SHA204_BUFFER_POS_COUNT];
 		if ((count_byte < SHA204_RSP_SIZE_MIN) || (count_byte > size)) {
 			return SHA204_INVALID_SIZE;

drivers/ATSHA204/sha256.cpp

@@ -32,6 +32,16 @@ const uint8_t sha256InitState[] PROGMEM = {
 	0x19,0xcd,0xe0,0x5b  // H7
 };
 
+Sha256Class::Sha256Class()
+{
+	memset(keyBuffer, 0, BLOCK_LENGTH);
+	memset(innerHash, 0, HASH_LENGTH);
+	memset(buffer.b, 0, BLOCK_LENGTH);
+	memset(state.b, 0, BLOCK_LENGTH);
+	bufferOffset = 0;
+	byteCount = 0;
+}
+
 void Sha256Class::init(void)
 {
 	memcpy_P(state.b,sha256InitState,32);

drivers/ATSHA204/sha256.h

@@ -19,6 +19,7 @@ union _state {
 class Sha256Class
 {
 public:
+	Sha256Class(); // Constructor
 	void init(void);
 	void initHmac(const uint8_t* secret, int secretLength);
 	uint8_t* result(void);