How do car keys open car doors?
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Most car keys nowadays have what’s called a „rolling code“. Every time you press the button on the car key it sends a binary code to the car that is compared to the same list the car has. There’s different ways to do this, the most common way is that you have an initial seed and an algorithm (perhaps a pseudo-Random number generator) generating the next code from the previous one, or what’s less common and the older approach is a long ordered list of codes that is large enough for the lifecycle of the car. If you press the button while your car is not in reach and you come back, the car car can still recognise the key by comparing it to the next codes.
I assume you are asking about the remote locking feature, and not the actually key and keyhole, which works kinda like any other door key. When you push the button on your key, it transmits a code over a short range radio. A receiver in your car is listening out for this radio frequency, and if it gets the right code it tells the car to lock or unlock itself. Now, if your key and car used the same code every time, it wouldn’t be terribly hard for someone who knows a thing or two about radios and computers to intercept the transmission, copy the code, and then use it to break into your car when you leave. To combat this, the key and your car have a really long list of access codes. The key always sends the next code in the list, and every time your car sees a code it recognizes, it locks/unlocks, and then “crosses that code out,” so to speak. Now anyone who intercepted that code won’t be able to do anything with it.
Old style key: The same as with house keys. The keys have a distinct form that pushes some pins in the keyhole in a certain position, and when all pins are at the right position, the key can be turned, unlocking the door. In cars this happens mechanically or by triggering an actuator (small motor for example).
Keyfob with remote control: there’s sometimes still a key that works following the same principle as above, but there’s also a small radio transmitter (the first ones used infrared light, like classic TV remotes) that sends a code when the button is pressed. The car “listens” and when it hears its own code, it unlocks the door through actuators.
Keyless entry systems: they also have a small radio or sometimes, just an RFID tag (like a radio, but passive, aka only works when its antenna receives power from the car’s own antenna). When you are close to the car, your “key” is in transmission range to the car unlocking the door, again through actuators in each door. For RFID sometimes one needs to be very close, or put the card on a certain spot.
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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