# Wouldn't putting an electronic key inside a small Faraday cage render it completely useless?

Am I missing something? The Engadget article Kia made a tiny Faraday cage to protect your wireless key from thieves says:

Many existing keyless entry systems aren't secure, but few people are likely to replace their cars just to reduce the chances of a determined thief making off with their ride. Kia UK has an official stopgap solution, though. It's taking a cue from third parties and releasing KiaSafe, a case that serves as a minuscule Faraday cage to block the key's wireless signals. There's nothing particularly special to it -- it's ultimately a metal-lined pouch -- but that's all might you need to prevent someone from swiping your car while you're asleep.

I'm confused in more than one way.

1. I'd thought that the point of a Faraday cage for RF signals is to block RF inside from getting out and RF outside from getting in. So then you'd have to take it out of the Faraday cage to use it and then of course the usual intercept mechanisms can still take place.
2. If your radiating source is positioned flat up against or even a fraction of a wavelength away from (at least a wire mesh) Faraday cage, don't you then get significant leakage anyway?
• 1: I agree 2: probably but it might just be the case that the signal isn't strong enough even if the pouch doesn't block everything so the car doesn't respond (by unlocking the doors). I'd personally prefer to have a button to unlock the car. – Bimpelrekkie Jul 4 at 8:30
• @jsotola it all depends on the how the key works and there's a variety of different modes of operation, e.g. single-use code transmitters, passive RFID, active RFID, etc. The article doesn't make it clear, but the current answer and comments there shed more light on that. – uhoh Jul 4 at 12:10
• sorry, i was under the impression that you have knowledge about how the key works, since you did not ask about it. – jsotola Jul 4 at 12:22
• The car manufacturers could fix this by putting a time measurement circuit in the key that counts the nanoseconds a pulse takes to go from the car to the key and back. Fairly high speed electronics needed, but a GPS has basically that thing. – Rich Jul 5 at 3:22
• Or... don't transmit anything while the user isn't pressing the button... </rocketscience> – Lundin Jul 5 at 12:22

The idea of that case is to protect the keys while you're sleeping. You need to take the keys out when you want to use them to drive the car.

The main problem is trying to cope with key-relay car theft. This tends to happen at night, with thieves making use of the fact most people keep their keys near the front door. So the key signal just needs to be relayed from just the other side of the wall (where the key's signal just about reaches), to the car. Then the car can then be unlocked, started and driven away. The idea of the case, is you put the key in the case, a lot less RF gets out, so the signal is so low that it cannot be relayed to steal your car.

Then you'll need to take the key out of the case in order to get in your car. So your key-less entry and start isn't quite as easy as it would otherwise be. So yes, the keys are out of the case then, but you are in a position to see/use the car, so that should stop anyone stealing your car.

Alternative solution to this is my prefer ed one: keep keys further away from the door.

• This key case is for the wireless keys, used in key-less entry and key-less start options. These features are getting quite common in cars, are standard on quite a few higher end vehicles. But security is poor. – Puffafish Jul 4 at 8:33
• I understand what it's for, but what is it? Is it a transmitter, or an active RFID, or a passive RFID? Does the thief broadcast some type of signal to read it while you sleep, or does it spontaneously transmit? Is it possible to clarify further in your answer how the key works? Thanks! – uhoh Jul 4 at 8:36
• I have a key-less start, the key sits in my pocket, I press the start button in the car, and the car starts. This kind of key can be RFID, or actively transmitting all the time. The key-less entry options tend to transmit constantly. So it's always blasting out the code to unlock the car, but only at low power, so short distances. So you are near the car when it's unlocked. Unless someone is relaying the unlock signal. – Puffafish Jul 4 at 8:41
• And all these problems are caused by the tendency of people being so lazy they don't even want to push a single button on their key to unlock the car. (or the manufacturer's assumption that people really want this - at least give us the option to set the key to only unlock the car with the press of a button, instead of automatically by proximity) – vsz Jul 5 at 4:28
• @vsz the truly frustrating part is that they don't give us an option/setting to do something smarter-also why do cars continue to work if the key is not inside? Most have a special location where a battery-less key can sit and still work so there is no reason for this. it's like they are designing the cars to be hacked and stolen. Perhaps if we passed laws making the manufacture responsible for the replacement of cars stolen due to poor security things would be different--but without any motivation they will continue to make really stupid design choices that sell due to "Coolness" factors – Bill K Jul 5 at 16:15

Wouldn't putting an electronic key inside a small Faraday cage render it completely useless?

That is entirely the point of putting it in a small Faraday cage. You'd take it out of the cage to access your car or drive.

Some new-fangled keys chat with the car. The car asks 'are you there?', and the key replies 'yup!'. Which is all fine and dandy when you are near the car and want it to work.

Unfortunately, when you are asleep, and your keys are in your jacket pocket hanging in the hall, some well-equipped car thieves might put one end of an RF relay link near your car, and the other end next to your front door. When the car asks 'are you there?', thanks to the link the key can hear, and the car hear its answer. Next morning, there your car isn't. The Faraday cage blocks the unwanted RF access.

• Oh!! Yes I see now, thanks! I'm thinking that now since the key doesn't have a button, you put it in an RF-proof container, which seems to me to be just a different kind of button. – uhoh Jul 4 at 14:19

Addressing the 2nd part of your question

1. If your radiating source is positioned flat up against or even a fraction of a wavelength away from (at least a wire mesh) Faraday cage, don't you then get significant leakage anyway?

Richard Feynmann showed the attenuation of a parallel-metal-wires Faraday cage, with wire spacing of D distance, with L spacing between the wires and the circuit to be shielded, to be AT LEAST

$$2 \pi \frac{D}{L}$$

in NEPERS.

Thus 1mm wire spacing, and 1mm distance from wires to circuit, provides 6.28 nepers which with 8.6 dB/neper == 54 dB.

If the circuit is 2mm inside the wires, at least 108dB.

Why are many IR receivers in metal cages?

• I see! I guess as long as the spacing is a fraction of a wavelength or smaller, the expression is independent of wavelength, which is probably true in this case. Thanks! And thanks for the interesting linked question and excellent answer there as well. – uhoh Jul 5 at 23:05