Simple (but good) Message Encryption?

What about a sequence number ? This should allow the receiver to identify if a message is missing.

Yes, useful for flagging lost data but i wouldn't recommend for validating data, that is, you don't want to rely on having to receive every sequential packet. It's probably not as easy to recover a missing packet as it is to just design with expectation of missing samples. Recovery means having to queue each data packet until you get an acknowledgement for each, which means memory resources and additional overhead for handshaking too, pretty significant for small devices like Espruino.

It can be useful for rolling a secret code too, essentially what garage door openers do.