add key and use AVR crypto lib. payload format changed

arduino_nano/main.c

@@ -35,6 +35,11 @@
 #include "usart.h" // basic USART functions
 #include "nrf24.h" // nRF24L01 functions
 
+// AES encryption functions
+#include "aes_types.h"
+#include "aes128_enc.h"
+#include "aes_keyschedule.h"
+
 /* variables */
 bool query_pzem004 = false; // flag to query the PZEM-004 power meter
 volatile uint8_t timer_seconds = 0; // how many seconds have passed
@@ -86,6 +91,8 @@ void io_init(void)
 int main(void)
 {
 	io_init(); // initialize IOs
+	uint8_t nrf24_payload[1+4*4] = {0}; // measurement values to be sent: ID byte + encrypted 4x4 float values (voltage, current, power, energy)
+	nrf24_payload[0] = conf.id; // set ID
 	
 	//printf(PSTR("welcome to the spar counter power meter monitor\n"));
 	
@@ -99,7 +106,18 @@ int main(void)
 	for (uint8_t i=0; i<sizeof(cmd); i++) {
 		usart_putchar_nonblocking(cmd[i],NULL);
 	}
-	
+
+	/* start encryption */
+	aes128_ctx_t ctx; // the context where the round keys are stored
+	aes128_init(conf.key, &ctx); // generating the round keys from the 128 bit key
+	// XOR data with IV for AES CBC mode
+	for (uint8_t i=0; i<sizeof(nrf24_payload)-1 && i<sizeof(conf.iv); i++) {
+		nrf24_payload[i+1] ^= conf.iv[i];
+	}
+	aes128_enc(&nrf24_payload[1], &ctx); // encrypt data
+	memcpy(conf.iv,&nrf24_payload[1],sizeof(conf.iv)); // save IV back to continue CBC mode
+	nrf24_transmit(nrf24_payload,sizeof(nrf24_payload)); // transmit empty packet to sync CBC
+
 	/* start timer to periodically query the power meter */
 	timer_seconds = 0; // restart seconds counter
 	TCCR1B |= (1<<CS12)|(0<<CS11)|(0<<CS10); // set prescale to 256 (starting the timer)
@@ -194,28 +212,17 @@ int main(void)
 		if (send_values) { // send the stored values from the power meter using the nRF24L01
 			action = true; // an action is performed
 			send_values = false; // clear flag
-			uint8_t message[3+6+6+6+6]; // create TLV based message
-			uint8_t i = 0;
-			message[i++] = 0; // type: message source
-			message[i++] = 1; // length
-			message[i++] = 1; // value: home 1
-			message[i++] = 1; // type: voltage
-			message[i++] = 4; // length
-			memcpy(&message[i],&voltage,4); // value
-			i += 4; // value
-			message[i++] = 2; // type: current
-			message[i++] = 4; // length
-			memcpy(&message[i],&current,4); // value
-			i += 4; // value
-			message[i++] = 3; // type: power
-			message[i++] = 4; // length
-			memcpy(&message[i],&power,4); // value
-			i += 4; // value
-			message[i++] = 4; // type: energy
-			message[i++] = 4; // length
-			memcpy(&message[i],&energy,4); // value
-			i += 4; // value
-			nrf24_transmit(message,i);
+			memcpy(&nrf24_payload[1+0],&voltage,4); // populate voltage
+			memcpy(&nrf24_payload[1+4],&current,4); // populate current
+			memcpy(&nrf24_payload[1+8],&power,4); // populate power
+			memcpy(&nrf24_payload[1+12],&energy,4); // populate energy
+			// XOR data with last data for AES CBC mode
+			for (uint8_t i=0; i<sizeof(nrf24_payload)-1 && i<sizeof(conf.iv); i++) {
+				nrf24_payload[i+1] ^= conf.iv[i];
+			}
+			aes128_enc(&nrf24_payload[1], &ctx); // encrypt data
+			memcpy(conf.iv,&nrf24_payload[1],sizeof(conf.iv)); // save data back to continue CBC mode
+			nrf24_transmit(nrf24_payload,sizeof(nrf24_payload)); // transmit empty packet to sync CBC
 		}
 		if (nrf24_flag) {
 			action = true; // an action was performed 

arduino_nano/main.h

@@ -20,9 +20,15 @@
  
 /* spark counter configuration */
 struct configuration {
+	uint8_t id; // the identity of the node
 	uint8_t rx_addr[5]; // nRF24 receiving address
 	uint8_t tx_addr[5]; // nRF24 transmit address
 	uint8_t channel; // nRF24 channel
 	uint8_t request_period; // how many seconds between power meter requests
+	uint8_t key[16]; // the AES 128 bits key
+	uint8_t iv[16]; // the initialisation vector for CBC encryption
+};
+struct configuration conf = {1,{1,'h','o','m','e'},{0,'h','o','m','e'},42,5,
+	{0x00,0x11,0x22,0x33,0x44,0x55,0x66,0x77,0x88,0x99,0xaa,0xbb,0xcc,0xdd,0xee,0xff},
+	{0xff,0xee,0xdd,0xcc,0xbb,0xaa,0x99,0x88,0x77,0x66,0x55,0x44,0x33,0x22,0x11,0x00}
 };
-struct configuration conf = {{1,'h','o','m','e'},{0,'h','o','m','e'},42,5};