[rmunn]

If I was assigned the task of arguing that in court (though it would be really stupid to assign me, a non-lawyer, that task), I'd probably argue that it's not circumventing a locked door when you use the actual key in the lock; "circumventing" refers to picking the lock. It could still be unauthorized access if you stole the key, but that's a different thing than circumventing, and this law forbids circumventing.

Likewise, if the encryption key is sitting on disk next to the encrypted data, it's not "circumventing" the encryption to use that key. And if you handed me the disk without telling me "Oh, you're only allowed to use certain files on the disk" then it's fair to assume that I'm allowed to use all the files that you put on the disk before handing it to me, therefore not unauthorized access.

That argument might fail depending on what's in the EULA for the car's diagnostic software (which I haven't seen), but I feel it would be worth trying. Especially if you think you can get a sympathetic jury.