Or just have the smart key ECU and the recipient ECU use a rolling code or even a 1 time shared secret. The other ECUs can learn the rolling code in the factory

Or just have the smart key ECU and the recipient ECU use a rolling code or even a 1 time shared secret. The other ECUs can learn the rolling code in the factory, or in after-service with the left door open, right blinker on, hood open, and horn tapped 8 times, and then wait 20 minutes. Without the key to see what the code is, no injector can spoof the frame. With the after-market procedure making tons of noise and spectacle, and a nice long wait for the police to arrive, the thieves can't replace the key ECU. With the system being simple, no key provisioning is needed, no non-public information, just an extra page in the manual and a software update.