433 mhz RF comms using external AVR MCU (advice on serial protocol needed)

If you have people monitoring the 433mhz band around your home with an interest to decode the data you're sending, I think you have a more fundamental issue to address

To be honest that is my feeling as well for things that I make, but if your radio became popular then it's actually very possible you may find people 'wardriving' trying to find homes that use it.

For instance it's trivial to write a bit of code that will turn all remote control sockets using this protocol on or off. If you got a powerful 433Mhz transmitter I reckon you could annoy a lot of people - I don't know what radius it'd cover, but I bet it's miles if you pushed a few watts out (illegally). In a city that would affect a lot of people.

And LightwaveRF that is being sold in a lot of retail stores in Europe has pretty bad problems too. While you can't really brute-force it (24 bit address), it's trivial to sniff - and you can then actually disable people's light switches!