Sha256 calculator for library use

The signing infrastructure now supports calculating sha256.
This feature does not require signing support to be activated.
Fixes #315

libraries/MySensors/core/MySigning.cpp

@@ -18,6 +18,7 @@
  */
 
 #include "MySigning.h"
+#include "drivers/ATSHA204/sha256.h"
 
 #if defined(MY_DEBUG_VERBOSE)
 #define SIGN_DEBUG(x,...) debug(x, ##__VA_ARGS__)
@@ -308,3 +309,22 @@ bool signerVerifyMsg(MyMessage &msg) {
 #endif // MY_SIGNING_REQUEST_SIGNATURES
 	return verificationResult;
 }
+
+static uint8_t sha256_hash[32];
+Sha256Class _soft_sha256;
+
+void signerSha256Init(void) {
+	memset(sha256_hash, 0, 32);
+	_soft_sha256.init();
+}
+
+void signerSha256Update(const uint8_t* data, size_t sz) {
+	for (size_t i = 0; i < sz; i++) {
+		_soft_sha256.write(data[i]);
+	}
+}
+
+uint8_t* signerSha256Final(void) {
+	memcpy(sha256_hash, _soft_sha256.result(), 32);
+	return sha256_hash;
+}

libraries/MySensors/core/MySigning.h

@@ -581,5 +581,32 @@ bool signerSignMsg(MyMessage &msg);
  */
 bool signerVerifyMsg(MyMessage &msg);
 
+/**
+ * @brief Initialize a hash calculation session.
+ *
+ * Any ongoing calculation session will be terminated/reset.
+ */
+void signerSha256Init(void);
+
+/**
+ * @brief Add data to an ongoing hash calculation session.
+ *
+ * More data can be added by repeated calls to this function.
+ *
+ * @param data Buffer with data to add.
+ * @param sz Size of data buffer.
+ */
+void signerSha256Update(const uint8_t* data, size_t sz);
+
+/**
+ * @brief Finalize an ongong hash calculation session and return the hash.
+ *
+ * The returned hash size is always 32 bytes. Buffer can be assumed to be valid
+ * until @ref signerSha256Init() is called again.
+ *
+ * @returns Buffer with 32-byte hash.
+ */
+uint8_t* signerSha256Final(void);
+
 #endif
 /** @}*/