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Why can't it be very simple and secure. Car and fob share a secret key.

When you click on the open button on the fob, you send

SHA256(key)

Car responds with a random challenge

RND

Fob sends

SHA256(key XOR RND)

Car does the same calculation and compares.

There's no car identification in this protocol, meaning that impersonation/mitm attacks are trivial. Try again :)

I don't see it. Give an example of how this attack can be executed, a practical application.

I approach my car, I press the button on the fob to open it, and your attack does what exactly?