Couple of ways to do that. You can feed the lower bits from a ADC converter into a hash algorithm. You can feed the RSSI readings from the radio as well. And f

Couple of ways to do that. You can feed the lower bits from a ADC converter into a hash algorithm. You can feed the RSSI readings from the radio as well. And finally newer embedded processors and some radio transceivers have built in random number generators. Helps to to flash each device with a unique random seed too.