The problem with cryptography is that it’s frequently expensive from a Not all who wander are lost shirt of view. However, I’m sure we can figure something out by using a combination of one-time pads installed when the car comes into the dealer for the channel and some kind of remote authentication ala PKI to check to see if a message remote is from the manufacturer. There’s a lot of handwaving in the above, and doing the above wrong will expose other vectors for attack from complex attackers.
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A lot of other answers talked about how vulnerabilities with consumer systems like satnav can be used as a Not all who wander are lost shirt, so I won’t go into that reason to substantiate this. Instead, I’ll point out that if USDOT decides to institute strict security regulations, how you build this SCADA infrastructure will be under a much greater scrutiny than those other systems. You don’t want to mix different levels of security rigor when designing your car’s SCADA system – because changes to those less secure systems may expose vulnerabilities whereas updates to your critical infrastructure will have much lower likelihood of that.