Obviously YOU don't understand. Without "sharing" the encryption key with the other side, the other size won't know how to decrypt it without having to hack it. I specifically said, "besides the primary means of communications."
Just as an example to prove you wrong, one method of doing so would be to randomly generate these numbers (as suggested previously) and have them added to the key for a "paired" device, store this information (maybe even encrypted with an unshared key outside of the code) in the EEPROM or SPIFFS and either use a one-time pairing method to send that private key, or enter it into the other device manually (by hand, directly, through serial or web interface).
To start with, your flaw was assuming that I would do something however you see it in your mind, when there are probably thousands of ways I could do it. I never said that whatever I was doing was going to be random every single time, or just a random one-time thing each time the two ends/devices were paired together and could be re-paired any time to change it.
What I said is not wrong in the slightest bit. You just have no idea what I'm going to do, because I haven't given ANY details on it, and like to assume things.
So, please, move along and bother someone else with your all-knowing attitude. Because, I'm really tired of people saying stupid crap like this.