That's true, but it's usually pretty simple to reverse engineer, and that's been done for a lot of cars. CAN message are generally sent out at fixed time interv

That's true, but it's usually pretty simple to reverse engineer, and that's been done for a lot of cars. CAN message are generally sent out at fixed time intervals, and correspond to some physical or control value, with a simple linear mapping between some physical unit and the value in the message. You collect some CAN data, change something like pressing the accelerator, see what changes, record results. Just search for "aftermarket ECU", you'll find lots of them; generally for high-performance cars that gearheads want to soup up further, but the same basic principle applies to most ECUs.