Why is it transmitting without the user pressing a button? Is that a feature? As you walk up to the car it automatically starts like magic? I'm not familiar with these newer cars.
In typical designs, the car continually transmits a low-frequency (e.g., 135 kHz) radio signal to wake up any wireless keys within range. When a key receives this signal, it replies with a VHF (e.g., 315 MHz) signal, and the car unlocks or starts when a door is opened or the start button is pressed.
The reply signal, at least, is uniquely coded to the car. The attack is to extend the range of the LF wake-up signal, causing a key stored away from the car to transmit a valid reply.
In some models, besides the transponder described above, the key also has a passive RFID tag, which works with a reader in the car to allow starting even if the battery in the key is dead.
(The article is wrong about the broadcasts, by the way; if the key transmitted continually, its battery wouldn’t last long.)
Could you just record the relay signal and play it back whenever, essentially replicating the key?
It is pretty much standard across all cars now days, except maybe the very bottom of the line models.
The really nice feature is when you walk away (a few seconds after you're out of range), the doors automatically lock. However, the downside of this feature is my wife's car does not have it -- and so at least half of the time when I am driving it I forget and leave it unlocked in parking lots.
This is the problem with a lot of the newer tech in cars like backup alarms. You become used to various features in your own car and when you rent a car you need to consciously remember that the vehicle doesn't have $FEATURE. Effectively, cars are becoming a lot less standardized. A car I rented a few weeks ago beeped at me a couple times and it took a while before I realized it was the lane departure warning triggering on a couple turns.
A reasonable person would probably have turned around and exchanged the car with the rental company at this point.
I am not a reasonable person.
Instead, I headed directly to a truck stop and purchased a heavy-duty power inverter, dropped the back seat, and crammed my portable PA speaker into the trunk, connected to the car's trunk-mounted battery through the inverter and to my iPhone through a shielded audio cable run from the trunk to the front seat.
The result sounded far better than it should have, and what it lacked in convenience (I had to pop the trunk to power it down) and channel separation (one speaker = mono), it more than made up for in dB SPL.
(for the record, I've also repaired eBay purchases that arrived in worse-than-advertised condition rather than returning them, for no other reason than that learning how to fix things is more fun than going through the hassle of returning them)
My brother in law did this on a ski trip with a borrowed Range Rover. It was only at the end of the week he realised he'd left his keys in a jacket pocket in the car the entire time and it had been sitting unlocked in the car park half a mile down the road from the apartment. Thankfully it was fine but stealing it would've been a case of getting in, pressing the start button and driving away.
It's not transmitting anything, it works pretty much the same way NFC works. Both the key and the car have their own public/private key pairs(which were obviously set by the manufacturer) and when you touch the handle the car transmits an unlock request to the key, encrypted with the car key's public key(this is going to get confusing lol) - when the key receives the message, it decrypts it using its own private key, if it's correct then it replies with an "ok" message encrypted with the car's public key. When the car receives that it decrypts it using its own private encryption key and opens the doors. Simple, and in theory unbreakable. The issue is that the car doesn't measure how far away from vehicle the key is - it only relies on the fact that the transmitters used by the car and the key are super-low range(like, within 50cm). Which is obviously defeated by using signal boosters.
Newer vehicles are already mitigating this attack, eg by measuring signal timings. Signal relay introduces a delay which can be identified and rejected.