Added XTEA and Pearson Hashing.

Improved message encryption for RF24.

drivers/PearsonHash/PearsonHash.cpp

@@ -0,0 +1,27 @@
+
+#include "PearsonHash.h"
+
+
+
+// Permutation function for Pearson hashing (uses less space than a lookup table)
+// Needs to be bijective, which it is
+uint8_t PearsonHash::_pearsonPermutation(uint8_t n) {
+	return (n*121) % 256;
+}
+
+
+uint16_t PearsonHash::calcHash16(const void* data, uint16_t len) {
+	uint8_t h[2] = {  _pearsonPermutation((((uint8_t*)data)[0]+1)%256), _pearsonPermutation(((uint8_t*)data)[0]) };
+	for (uint16_t i = 0; i < len; i++) {
+		auto _tmp = h[0];
+		h[0] = _pearsonPermutation(h[1] ^ ((uint8_t*)data)[i]);
+		h[1] = _pearsonPermutation(_tmp ^ ((uint8_t*)data)[i]);
+	}
+	void* _tmp = h; // To avoid the type punning warning
+	return *((uint16_t*)_tmp);
+}
+
+
+bool PearsonHash::checkHash16(uint16_t crc, const void* data, uint16_t len) {
+	return crc == calcHash16(data, len);
+}