The German automobile club ADAC did a test with several cars which open doors and start the engine with a "keyless entry" system. You don't have to push a button on your car key. If you get near your car, key and car will recognise each other. If you pull the door handle the car will open. Inside the car your push the ignition button and the engine starts.

The security relies on the distance between key and car. Car thieves have built a repeater to tunnel the radio signals over long distances. One thief stands near the key and the other near the car. Then the car will open. The distance between car and key can easily be several hundreds of meters. Lots of cars are stolen this way.

  • How could car manufacturers solve this problem or is this an unpatchable design flaw?
  • Are there any mitigations a car owner could take in place?
  • How should wireless physical access control look like for cars?
  • 3
    I am making assumptions, but you still have a physical key on it if the wireless system doesn't work, right? Isn't there a battery you can remove on your device?
    – Yuriko
    Commented Mar 17, 2016 at 10:44
  • 4
    I dont think it can be patched. I'd get a pocket with metal inlay for the key (effectively a Faradays cage) so that the car unlocks only if i intetionally pull out the key.
    – marstato
    Commented Mar 17, 2016 at 17:02
  • 8
    Can you edit your question to clarify what you are asking? Are you asking what car manufacturers can do, or are you asking what car owners can do? Who is the "you" in the question?
    – D.W.
    Commented Mar 17, 2016 at 17:45
  • 4
    @Yuriko, Chevy Volt owner here. Opening the door with the metal key will trip the alarm. Once inside the car, you'd have to open a compartment on top of the dash, remove whatever junk you have stored there, remove the rubber liner from the floor of the compartment, and insert the metal key in the keyhole that is thus revealed in order to silence the alarm and start the car.... Or something like that, anyway. I haven't yet had to actually do that. Commented Mar 17, 2016 at 19:50
  • 6
    I'm no fan of car thieves, but I gotta say, that's pretty clever. Commented Mar 18, 2016 at 17:01

7 Answers 7


If you truly fear an RFID proximity relay attack, such that you would keep the key fob in a Faraday cage in your pocket, you must recognize that you will be giving up all the convenience of proximity, plus other inconveniences. You'll have to carry some kind of larger container, and you'll have to open it to take out the key to use the remote or the key.

I see two clear alternatives. If you are willing to give up proximity but don't want to sacrifice the other conveniences, there is a much cleaner solution than a band-aid tin in your pocket: disable the proximity system in your vehicle. That way, no external attacker can attack the fob in your pocket. You retain the convenience of being able to use the normal RF functions to lock and unlock the doors with the buttons. Check your owner's manual for instructions.

The other option is to pass the risk to a third party: buy theft insurance, and don't worry if someone attacks the proximity system; hooks up a tow truck; smashes your window; or any of the many other attack vectors. You give up no convenience, only money. And you're protected regardless of the form of theft.

  • Thank you for your great answer. At the moment the only "secure" way is to deactivate the system. There are other great workarounds, described in the other answers, but this one should work best, because you fall back just one technology level. Great hint to transfer the risk to an insurrance. This is the way to go.
    – honze
    Commented Mar 20, 2016 at 13:04

From a layman point, Yes its a design flaw and yes the signals are boosted to unlock the cars from far far away. This is knows as Relay Station Attack(RSA). Some of the ways to mitigate such attacks are:

  • measuring Group delay time to detect illegal high values
  • measuring Third-order intercept point to detect illegal Intermodulation products
  • measuring Field strength of the Electric field
  • measuring response time of 125 kHz LC circuit
  • using a more complex Modulation (i.e. Quadrature amplitude modulation) which can't be demodulated and modulated by a simple relay station
  • putting a physical on/off switch on the key

I don't think these mitigations can be used by the car owner themselves as there is quite technical detail behind it.

Taken from wikipedia. Smart keys and Security requirements

  • 7
    A passive or nearly-passive repeater doesn't need to demodulate and remodulate if only the frequency range used is known; it can blindly retransmit on the output side whatever is received on the input side. You only need to demodulate (but even that can be done in a separate signal path) if you want to analyze the signal on the air to know whether it makes sense to retransmit or not.
    – user
    Commented Mar 17, 2016 at 14:49
  • @MichaelKjörling Repeaters will always be bound by the speed of light. If the endpoints transmit data simultaneously (on different frequences if necessary) and reliably measures which outgoing bits and incoming bits are transferred simultaneously, then it will be possible for the legitimate endpoints to measure a fairly accurate distance between each other. Then the car just has to refuse unlocking wirelessly if the distance is measured as more than say 20m. Whether the hardware to do this can be embedded in a key is another question.
    – kasperd
    Commented Mar 17, 2016 at 17:00
  • 5
    @kasperd Of course there are ways to make use of a repeater more difficult. My comment was in response to the statement that QAM "can't be demodulated and modulated by a simple relay station", pointing out that a repeater wouldn't even need to demodulate the signal.
    – user
    Commented Mar 17, 2016 at 22:50
  • 1
    RSA is an unfortunate TLA
    – craq
    Commented Mar 19, 2016 at 9:53

I have a car with such a keyless entry system. My dad saw a report about those ADAC findings on TV and he had an idea which works:

enter image description here enter image description here

By placing the key in a steel can like the one above, the car does not detect the key any longer, so I figure range extenders would not pick up the key's signal either. Of course, this is annoying as it defeats the whole purpose of not having to take they key out of my backpack when I want to open the car and drive. But I guess this is still better than coming back to where I left the car, only to find it gone.

  • 40
    Pocket size Faraday cage? :)
    – PTwr
    Commented Mar 17, 2016 at 11:20
  • 19
    well, guess i gotta start smoking then...
    – Gigala
    Commented Mar 17, 2016 at 12:05
  • 20
    (Who)Germany sells cigarettes in Faraday cages?
    – KDecker
    Commented Mar 17, 2016 at 12:40
  • 1
    I have always wondered why no one makes a snap-on key cover which disables the buttons on a standard "digitial" key (to avoid "butt dialing" your car). Making it also be a passable Faraday cage would be a plus.
    – Hot Licks
    Commented Mar 17, 2016 at 12:46
  • 7
    I'm thinking an Altoids can might be equally appropriate? Not sure if it's thick enough though. I'm personally dreading the day I get a car with passive-entry or keyless-start features. Tinfoil hat gets all kinds of tingly just thinking about it.
    – Iszi
    Commented Mar 17, 2016 at 19:28

You could use a Faraday cage to create a physical security layer. By having a box with a built in cage to the correct specifications where the keys are stored, you remove the potential for this attack to work.

Keyless entry systems have many more avenues of attack however, so if you're seriously concerned about vehicle security you may wish to store it in a garage, purchase manual vehicle security items like wheel, pedal or gear locks, install an aftermarket immobilizer or install active tracking systems. There is a (possibly apocryphal) story of a tracking company who have a 99.9% success rate with recovering vehicles. the one they haven't recovered is in an African nation under the "ownership" of a local warlord, hence the lack of recovery!

Remember - any car can be stolen. if someone really wants your car all they'll do is literally pick it up and stick it on the back of a flatbed truck. The main point of most vehicle security is to delay theft attempts, rather than to completely defeat them.

  • 7
    Anti-theft systems have a great deterrence effect. The thieves do not want your car, they want a car (unless they are Nicholas Cage), so they will always go after an easier mark. Commented Mar 17, 2016 at 11:48
  • 3
    Indeed, as @Mindwin says - it is unlikely your car would be a target to such an advanced attack as "lifting onto a flatbed" there are very few occasions where your car will be the one thieves want, rather than a car (or a Ferrari, or an Aston Martin).Taking basic precautions goes a long way to migitgating these attacks. If your car is particularly rare, or has particular options or paintjobs that make it appealing to steal, there's always a chance you could be a victim of a targeted theft, however, so if this is the case explore additional layers of security.
    – Miller86
    Commented Mar 17, 2016 at 11:59
  • 1
    "if someone really wants your car all they'll do is literally pick it up and stick it on the back of a flatbed truck" What if you chain it to the ground with a carbon nanotube chain? Commented Mar 17, 2016 at 13:58
  • 7
    @PyRulez Asphalt is removable, given enough effort.
    – user
    Commented Mar 17, 2016 at 14:51
  • 32
    @PyRulez: In that case they are probably stealing your carbon nanotube chain, which the 2 ton car is protecting by somewhat anchoring it in place.
    – dotancohen
    Commented Mar 17, 2016 at 15:16

Most of the answers I'm seeing (e.g., Faraday cages) defeat the purpose of the wireless key: to unlock and start the car without having to grab the key out of your pocket.

Why not have a time dependent encryption system work both ways? When the door handle is touched, the car sends a public key signal that changes with time (in ns), which the key incorporates into its own signal which it sends back to the car. If the car does not receive the key signal within so many ns (c is 1 foot per ns), it doesn't unlock.

I don't know much about the inner workings of the passive key signal, but if this isn't possible with watch-battery powered technology then cell phones or other mobile devices may have to replace the smart key, which of course introduces a litany of other security questions.

  • 1
    Thats a great idea imho! But i think it is very expensive to put a measuring device into a car that is capable of nanosecond precision and resolution.
    – marstato
    Commented Mar 20, 2016 at 19:47
  • 1
    Using the speed of light would be a physically unbreakable verification of the distance between your car and your key, but it's certainly not easy to implement. It takes multiple microseconds for the car to send a cryptographic challenge and for the key to compute and send a response. Working a 10-20ns delay limit into this in such a way as to make it guaranteed to be impossible to extend the range sounds like a tough engineering challenge.
    – RomanSt
    Commented Mar 21, 2016 at 0:18

You should also think about what other stupidity your car manufacturer could've done. What if the key isn't even necessary, because the "security" is so weak that the authentication system could be brute forced remotely in a reasonable amount of time ?

The only solution would be to reverse engineer the firmware of each control module in the car and patch their security flaws. Since this is pretty much impossible unless you have a team of engineers and unlimited amount of money, your best bet is just to pay for some good insurance and cross your fingers, or buy your car from a vendor that has a good security track record.

  • 1
    This applies equally to wireless keyfobs as commonly used, not just entirely keyless entry.
    – user
    Commented Mar 18, 2016 at 14:49
  • How easy is it to crack a wireless key fob, today? Do you just have to know a shared secret? Like a password or a code? A few years ago the secret was only 4 digits long, if i remember correctly.
    – honze
    Commented Mar 20, 2016 at 13:11

We may need a 2 phase security. Along with the keyless entry system add a strong biometrics authentication too. For instance, the drivers door is still pulled to open by him. Let us have a system in the door handle which detects his finger prints and if he/she is in the authenticated list; the door opens else triggers an alarm.

  • It's perfectly possible to enter the car from the passenger's side and climb over into the driver's seat. Not particularly inconspicious, but perfectly doable.
    – user
    Commented Mar 18, 2016 at 14:51
  • What if the driver is wearing gloves? Commented Mar 18, 2016 at 17:03
  • @Micheal, Most of the cars will be centrally locked and needs some kind of initiation from the driver. but what you have said is still an issue. Will make my answer more detailed to handle that scenario.
    – Kannan_SJD
    Commented Mar 21, 2016 at 9:28
  • @iamnotmaynard, yes that is a drawback too. I just meant thumb as an eg, you could go with eye or something. But that will reduce the convenience. Will improve my answer taking all this into account.
    – Kannan_SJD
    Commented Mar 21, 2016 at 9:39

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