Add encryption for the sent packet

weather-sensor/firmware/encryption.c

@@ -0,0 +1,118 @@
+#include <stdint.h>
+
+#define LENGTH_OF_BLOCK 8
+
+void xxtea_Encrypt(uint32_t * data, uint8_t dataLength, const uint32_t key[4]);
+void xxtea_Decrypt(uint32_t * data, uint8_t dataLength, const uint32_t key[4]);
+
+/* The data packets are encrypted using the xxtea algorithm. */
+void Encrypt(uint32_t * data, uint8_t dataLength, uint64_t salt, const uint32_t key[4])
+{
+	/* This function assumes that the dataLength is a multiple of the length of the
+	 * salt (8)
+	 *
+	 * The data is encrypted using cipher block chaining:
+	 * (https://upload.wikimedia.org/wikipedia/commons/8/80/CBC_encryption.svg)
+	 */
+
+	uint8_t i;
+	uint8_t * previousCipherBlock = (uint8_t*) &salt;
+	uint8_t * currentPlaintextBlock = (uint8_t*) data;
+
+	for (i = 0; i < dataLength/LENGTH_OF_BLOCK; i++)
+	{
+		/* XOR of current plain text block and previous cipher block */
+		//for (j = 0; j < LENGTH_OF_BLOCK; j++)
+		//{
+		//	currentPlaintextBlock[j] ^= previousCipherBlock[j];
+		//}
+		*((uint64_t*) currentPlaintextBlock) ^= *((uint64_t*) previousCipherBlock);
+
+		/* Encrypt the block */
+		xxtea_Encrypt((uint32_t*) currentPlaintextBlock, LENGTH_OF_BLOCK, key);
+
+		/* Setup for next block */
+		previousCipherBlock = currentPlaintextBlock;
+		currentPlaintextBlock += LENGTH_OF_BLOCK;
+	}
+
+}
+
+
+void Decrypt(uint32_t * data, uint8_t dataLength, uint64_t salt, const uint32_t key[4])
+{
+	/* This function assumes that the dataLength is a multiple of the length of the
+	 * salt (8)
+	 *
+	 * The data is encrypted using cipher block chaining:
+	 * (https://upload.wikimedia.org/wikipedia/commons/2/2a/CBC_decryption.svg)
+	 */
+
+	/* The data is decrypted from back to front as front to back cannot be done in
+	 * place
+	 */
+	uint8_t i;
+	uint8_t * previousCipherBlock = (uint8_t*) data + dataLength - 2 * LENGTH_OF_BLOCK;
+	uint8_t * currentCipherBlock = (uint8_t*) data + dataLength - LENGTH_OF_BLOCK;
+
+	for (i = dataLength/LENGTH_OF_BLOCK; i > 0; i--)
+	{
+		/* Decrypt the block */
+		xxtea_Decrypt((uint32_t*) currentCipherBlock, LENGTH_OF_BLOCK, key);
+
+		/* XOR of the decrypted block with cipher block in front of it */
+		*((uint64_t*) currentCipherBlock) ^= *((uint64_t*) previousCipherBlock);
+
+		/* Setup for next block */
+		currentCipherBlock = previousCipherBlock;
+		if (previousCipherBlock != (uint8_t*) data)
+		{
+			previousCipherBlock -= LENGTH_OF_BLOCK;
+		}
+		else
+		{
+			previousCipherBlock = (uint8_t*) &salt;
+		}
+	}
+}
+
+void xxtea_Encrypt(uint32_t * data, uint8_t dataLength, const uint32_t key[4])
+{
+    uint32_t sum = 0, z, y, e;
+    uint8_t i = 6 + 52/(dataLength/4), r;
+    const int16_t n = dataLength / 4 - 1;
+
+    z = data[n]; // left neighbour for the first round
+    do {
+        // cycle
+        sum += 0x9e3779b9ul;
+        e = (sum >> 2) & 3;
+        for (r = 0; r <= n; r++) {
+            // round
+            y = data[(r+1) % (n + 1)]; // right neighbour
+            data[r] += ((z>>5 ^ y<<2) + (y>>3 ^ z<<4)) ^ ((sum^y) + (key[(r^e) & 3] ^ z));
+            z = data[r]; // left neighbour for the next round
+        }
+    } while (--i);
+}
+
+void xxtea_Decrypt(uint32_t * data, uint8_t dataLength, const uint32_t key[4])
+{
+    uint32_t sum, z, y, e;
+    int16_t i = 6 + 52/(dataLength/4), r;
+    const int16_t n = dataLength / 4;
+
+    sum = (uint32_t) ((uint64_t) (6 + 52/(dataLength/4)) * 0x9e3779b9ul);
+    y = data[0];
+    do {
+        // cycle
+        e = sum >> 2;
+        for (r = n-1; r >= 0; --r) {
+            // round
+            z = data[(r+n-1) % n];
+            data[r] -= ((z>>5 ^ y<<2) + (y>>3 ^ z<<4)) ^ ((sum^y) + (key[(r^e) & 3] ^ z));
+            y = data[r];
+        }
+        sum -= 0x9e3779b9;
+    } while (--i);
+}

weather-sensor/firmware/encryption.h

@@ -0,0 +1,8 @@
+#ifndef ENCRYPTION_H
+#define ENCRYPTION_H
+
+
+void Encrypt(uint32_t * data, uint8_t dataLength , uint64_t salt, const uint32_t key[4]);
+void Decrypt(uint32_t * data, uint8_t dataLength, uint64_t salt, const uint32_t key[4]);
+
+#endif

weather-sensor/firmware/main.c

@@ -16,10 +16,13 @@
 #include "bme280_defs.h"
 #include "pin_programming.h"
 #include "crc.h"
+#include "encryption.h"
 
 
 
 uint8_t ownId;
+const uint8_t encryptionKey[16] = {0x9e, 0x37, 0x79, 0xb9, 0x9b, 0x97, 0x73, 0xe9, 0xb9, 0x79, 0x37, 0x9e, 0x6b, 0x69, 0x51, 0x56}; /* TODO: use exernal file with the keys */
+uint64_t salt;
 
 char bool_case = 0;
 int timer = 0;
@@ -92,6 +95,9 @@ int main (void)
 	Initialize_BME280();
 	Set_Up_Power_Save_Mode();
 
+	/* Initialize the salt */
+	salt = 0xFFFFFFFFFFFFFFFFull;
+	salt &= ~ownId;
 
 	/* Delay the change of the operating frequency by the function Enter_Power_Save_Mode for the
 	 * first function pass. If it is changed before the ISP can flash the MCU the clocks of the ISP
@@ -116,8 +122,8 @@ int main (void)
 
 			memset((uint8_t*)&reportPacket, 0, sizeof(reportPacket)); //Reinitialize the buffer with zeros
 
-			reportPacket.senderId = ownId;
-			//reportPacket.salt; /* TODO */
+			salt &= ~(1ull<<55);
+			reportPacket.salt = salt;
 			reportPacket.payload.values.packetIdentifier.elementCount = 3;
 			reportPacket.payload.values.packetIdentifier.packetType = PACKET_TYPE_REPORT;
 
@@ -134,7 +140,13 @@ int main (void)
 			reportPacket.crc = crc;
 
 			/* Encrypt the packet */
-			/* TODO */
+			/* TODO: 
+			 * - increment the salt for every packet
+			 * - Receive salt from the base station
+			 */
+			Encrypt((uint32_t*) &reportPacket.payload.buffer,
+			        PACKET_BUFFER_LENGTH + sizeof(crc),
+			        salt, (uint32_t*) encryptionKey);
 
 			NRF24L01_Send_Message((uint8_t*)&reportPacket, sizeof(reportPacket));
 

weather-sensor/firmware/nrf24l01.h

@@ -55,8 +55,7 @@ typedef struct __attribute__((packed)) BITFIELD_PACKET_COUNT_ELEMENT
 
 typedef struct __attribute__((packed)) PACKET
 {
-	uint8_t senderId;
-	uint8_t salt[7];
+	uint64_t salt; /* 1 byte device id, 7 bytes remainder */
 	union {
 		struct {
 			BITFIELD_PACKET_COUNT_ELEMENT packetIdentifier;