How secure are car remote keys? When you lock or unlock your car, can someone spoof or make another remote key?

How do car remote keys work? Do they use some kind of private/public keys, encryption?

Are aftermarket alarm remote keys less secure than the manufacturer remote keys? I am mostly interested to find out if it is possible for someone to sit in a car park and listen for remote key signals and then use them?

1 Answer 1


More details here.

High-end manufacturers use expensive challenge-response schemes (the key sends a request, the car answers with a challenge, and the key sends a reply derived from the challenge with some algorithm). Even so, such algorithms are proprietary, usually not reviewed, and could well be an example of "rolling your own crypto". There are solutions, but to quote the paper below,

Note, however, that a physical implementation of mathematically secure ciphers still can be vulnerable to side-channel (typically power analysis) attacks. Also the key distribution/management, when wrongly implemented like with the Keeloq's manufacturer key, can introduce a single point of failure into the commercial cryptosystem. In other words, the chain is as strong as its weakest element, so the marketing headword 'employes AES' does by far not mean that the product as whole is secure.

Cheaper manufacturers use a rolling scheme employing a PRNG. Quite often, the car has a "guard period" after receiving a code, in which it will actively not recognize any code, to avoid bruteforcing. Several of these schemes actually relied on the secrecy of the algorithm, and have been broken (link to theory, practice and source code):

Recently it has been demonstrated how the manufacturer key can be extracted from a receiver device by a physical side-channel cryptanalysis and how a particular remote control can be cloned (either knowing a matching manufacturer key without physical access to the remote, or without manufacturer key but requiring physical access to the remote)

Still cheaper manufacturers employ a rolling scheme between N codes, which is vulnerable to replay attacks (you get one code) or "stalking" the garage until you get enough codes.

Another vulnerability of remotes is jamming. You stalk a parking lot and fill the 433 MHz band with noise. Out of every ten people that lock their cars with a remote, nine of them will notice that the car lock didn't actually engage. They'll try again, blaming the batteries on the key fob, and finally lock the car manually (or succeed by transmitting from a few inches, which will further convince them it's the fob battery's fault). Maybe one driver in ten will walk away blissfully unaware that his car was actually left unlocked. You can spot him from afar (he's the one who did not turn back) and rob him blind: no need for sophisticated crypto at all. A 433 MHz transmitter and a white noise generator are enough.

Case in point

Your aftermarket key uses a KeeLoq HCS200. This has been proved and confirmed to be insecure:

KeeLoq remote keyless entry systems are widely used for access control purposes such as garage openers or car door systems. We present the first successful differential power analysis attacks on numerous commercially available products employing KeeLoq code hopping. Our new techniques combine side-channel cryptanalysis with specific properties of the KeeLoq algorithm. They allow for efficiently revealing both the secret key of a remote transmitter and the manufacturer key stored in a receiver. As a result, a remote control can be cloned from only ten power traces, allowing for a practical key recovery in few minutes. After extracting the manufacturer key once, with similar techniques, we demonstrate how to recover the secret key of a remote control and replicate it from a distance, just by eavesdropping on at most two messages. This key-cloning without physical access to the device has serious real-world security implications, as the technically challenging part can be outsourced to specialists. Finally, we mount a denial of service attack on a KeeLoq access control system. All proposed attacks have been verified on several commercial KeeLoq products

This is not so much due to a shortcoming in KeeLoq's algorithm but in its practical implementation by the vendor. As such, there are claims that an aftermarket key can actually be fixed:

If your receiver device contains a specialized hardware KeeLoq decoder, it should be possible to flash your own randomly selected device code into that chip and matching remotes, following the instructions in appropriate data sheets. In this way you circumvent the problem with the learning algorithm and manufacturer key.

(Along with a bounty of information, the page explains why my old garage remote happened to also open the University gate, as I discovered one sleepy morning when I inadvertently picked up the wrong remote - different on the outside, the two receivers must have been identical on the inside).

Update: buttonless fobs

A variation on the concept of "car remote" is the fob. This is normally a passive device (no batteries). It contains a coil that absorbs the nearby electromagnetic field, and if it is powerful enough it awakens and is able to modulate its own absorption. As a result, whatever is transmitting the electromagnetic driving field will experience a sequence of short power losses. By representing a power loss with a 1 and normality with 0, the driver unit will receive something like 000000000...0001101010101110011. The sequence is usually always the same (very high-end units implement challenge-response) and is unique for every fob.

The same or similar technology is used in some contactless cards and keys.

Since this kind of electromagnetic coupling only happens at very short distances, the fob is considered "safe". It most definitely isn't.

Attack 1: while the driver unit is massive (and usually mounted inside a car), it is possible to install one in a suitcase. Pass within one meter from the victim (whose will awaken, believe itself near the car, and transmit the unlock code). Record the unlock code. Profit.

Attack 2: much more expensive, but effective against USD 100,000 cars, so possibly worth your while. Requires two suitcases connected via Internet (put one Android phone in "personal access point mode" inside each for USD 79,98). The first is the same suitcase as above, but it does not record the unlock code, it transmits it to the other suitcase. A coil in the second suitcase starts siphoning energy from the car transmitter. The following "dialogue" (classical man-in-the-middle attack) ensues in the next few milliseconds:

  • SUITCASE 1: (absorbing)
  • CAR: (sleepily)...is there a fob nearby? Who's drinking me energy?
  • SUITCASE 1: Yes, I'm a fob. I'm your fob.
  • CAR: let me generate a random unguessable number: 12345. Add yours. What's the answer?
  • SUITCASE 1 (to suitcase 2): get me the answer to 12345 +.
  • SUITCASE 2 (to fob): My number is 12345. Add yours. What's the answer?
  • FOB: It's 73219.
  • SUITCASE 2 (to suitcase 1): 73219
  • SUITCASE 1 (to car): It's 73219.
  • CAR: You're correct. Alarm deactivated, unlocking door. Have a nice day.

UPDATE: and now they did it on CCTV, and the "suitcase" is no larger than a box.

This kind of vulnerability requires a contactless, buttonless fob, and can be protected against by wrapping the fob in electromagnetic shielding. This is not the same thing as aluminum, which protects against radio waves. Magnetic coupling is best shielded by iron foil or special magnetic shielding. Also keeping two identical fobs (of different cars) in close contact will work, if riskier, because by transmitting different codes on the same frequency they're liable to confuse the receiver.

  • 6
    Some of the high-end manufactures also seem to have particularly vulnerable systems. BMW, for instance, has been plagued by keyless thefts. Here's one article about it.
    – Xander
    Commented Sep 27, 2013 at 16:51
  • If my car has only OEM central remote door lock and if i install some aftermarket alarm system with remote key, then probably my car central locking "procedure" comes less secure, because aftermarket key is anyway connected to OEM locking components? or i am wrong? (In viewpoint if aftermarket remote key uses poor security)
    – Guntis
    Commented Sep 30, 2013 at 8:42
  • In general, I'd say that whenever two systems interconnect, the security you get is the lesser of the two, less again any additional loss due to the interconnection itself (e.g.: if, to connect, either system has to be made less secure - some part disabled, or downgraded - to let the connection work at all). In your scenario, though, I would think that the original remote system would be completely disconnected (they won't both work in parallel), so you'll get all the security of the aftermarket system... and only that. If it's more than the original, well and good. If it's less...
    – LSerni
    Commented Sep 30, 2013 at 9:12
  • 2
    @Iserni Nice answer. Has anything changed in 2016? Have they defined any standards for door security?
    – HRJ
    Commented Oct 20, 2016 at 2:18
  • 1
    The sad thing is how damn easy it would be to implement a very secure remote key. Private ed25519 key in the key, public key in the car. Simple challenge-response to sign a secret. Or hell, it could even be made as trivial as both devices sharing a symmetric key burnt in by the OEM and used for an HMAC of the wireless commands.
    – forest
    Commented Dec 11, 2017 at 1:13

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