diff --git a/AVR/main.c b/AVR/main.c
index 64a92e1..849f32c 100644
--- a/AVR/main.c
+++ b/AVR/main.c
@@ -1,59 +1,47 @@
-/* I2C Echo Example */
 #include "../avr-i2c-slave/I2CSlave.h"
 #include <util/delay.h>
 #include <avr/io.h>
 #include <avr/interrupt.h>
+
 #define I2C_ADDR 0x20
 
-volatile uint8_t data;
-volatile uint8_t send=0;
-void I2C_received(uint8_t received_data)
-{
-  data = received_data;
+volatile uint8_t challenge = 0; // Use 0 as 'challenge not sent' condition
+
+uint8_t check_challenge(uint8_t arg) {
+  return (arg >> 3) ^ (arg << 3);
 }
 
-void I2C_requested() {
-  I2C_transmitByte(send);
+void I2C_received(uint8_t received_data) {
+  if (challenge != 0) {
+    if (check_challenge(received_data) == check_challenge(challenge)) { // Check validity of response
+      // Open lock
+      PORTC |= 2 << 1;
+      _delay_ms(5000);
+      PORTC = 0;
+    }
+  }
+  challenge = 0;
 }
-void I2C_challenge() {
-  I2C_transmitByte(send);
+
+void I2C_requested() {
+  challenge = (uint8_t)(TCNT1 & 0xff); // Use content of timer as pseudo-random number
+  if (challenge == 0) // Prevent 'challenge' from being 0, which has a special meaning
+    challenge = 1;
+  I2C_transmitByte(challenge);
 }
 
 void setup() {
   // set received/requested callbacks
   I2C_setCallbacks(I2C_received, I2C_requested);
-
   // init I2C
   I2C_init(I2C_ADDR);
   // init output gpio
-  DDRC = 0b00000100;  // PC2
+  DDRC = 0b00000100; // PC2
   TCNT1 = 0;
   TCCR1B |= (1 << CS10) | (1 << CS11); // No prescaling
-
 }
+
 int main() {
   setup();
-  int challenge = 0;
-  unsigned int i = 0;
-  // Main program loop
-  while(1) {
-  	if(data == 1) {
-		i = TCNT1;
-		srand(i);
-		send = rand();
-	  while(data != send+3) {
-  		I2C_challenge();
-	  }
-	  send = 17;
-	  I2C_challenge();
-	  PORTC |= 2<<1;			
-	  _delay_ms(5000);
-	  PORTC = 0;		 
-	  data = 0;
-
-}/*	else {		
-		send = 0;
-		I2C_challenge();
-	}*/
-  }
+  while (1); // All the work is done in the callbacks
 }