How do car keys open car doors?
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It depends on the car door. If you need to inserte the key it is like any other lock with a key. There are electrical actuators in the door of modern cars that can open or lock other doors when you use the key on one door.
If it is a remote then the key transmits a message to the car with radio waves that the car understands and and activates the actuators.
Exactly how it does depends on the exact system that is used. One way is to have a shared secret that both the car and the keyfob know.
The keyfob can then encrypt a message that only the car can decrypt and understand. The message can contain a counter that is increased by one every time the button is pressed. The car will only unlock of the counter is higher then what was used last time, that way i a thief recodes the signal it can’t be reused.
You can increase security by having an accurate close in each, The time can be in the message. That stops someone from triggering the keyfob away from the car and then repeats the sent message at the car and steel it. The problem with this that clocks do not always at the exact right rate.
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