Keyless entry was built so you could leave the key in your pocket. The car notices the key is close, unlocks when you touch the handle and starts at the press of a button. A relay attack turns that convenience around: the key never has to leave the house for the car to believe it is standing beside it.
How a relay attack works
The first thing to understand is who does the talking. In a keyless car, the car calls out and the key answers.
That exchange was designed to work over a short range, so that only a key next to the car could answer. A relay stretches the range.
In practice the key does not have to be anywhere near a window or a door for this to matter, but it helps the thieves when it is. A key on a hall table or a hook by the front door is often the closest thing in the house to a car parked in the driveway.
Which cars are exposed
The attack needs passive keyless entry, the kind where you never press a button on the key. A car that only unlocks when you press the key’s button is not exposed to this particular trick.
How common relay theft is in the United States is not known, because nobody counts it separately.
Newer systems are starting to close the gap by measuring the distance to the key rather than trusting that a reply means the key is near. Phone-as-key systems built on ultra-wideband radio are the clearest example.
That does not help the many keyless cars already on the road, and it does not make any system perfect. For those cars, the defense is at your end.
Why it is hard to notice
Most thefts leave something behind: a broken window, a damaged lock, a scratched column. A relay theft can leave nothing at all, and once the car is running it keeps running.
What a Faraday pouch or box does
The fix the researchers proposed is the one you can buy: put the key somewhere radio cannot reach it.
Shields made for car keys hold up well when used properly. Published tests have found pouches blocking the key, and have found the ways they let a signal through.
The failures have one cause in common: a gap. A flap not pressed shut, a pouch so full it cannot close properly, a key slipped into an outer pocket that has no lining, a lining worn through. A rigid box with a clasp avoids most of them at home, and a pouch does the job when you are out.
How to test yours
Never assume a shield works because it says so on the packet. Test it at the car, the way the car would be attacked.
- Put the key in the pouch or box and close it completely.
- Walk to the car, or sit in the driver’s seat, and try the door handle and the start button.
- If the car unlocks or starts, the shield is not working: close it again more carefully, or replace it.
- Do the same for the spare key, which is just as useful to a thief.
- Repeat the test every few months, because linings wear.
Settings that close the gap
Some cars give you a way to switch the convenience off, or to put the key to sleep when you do not need it. It is worth a few minutes with the owner’s manual to find out what yours can do.
A sleeping key, or keyless entry switched off, gives a relay nothing to carry. A driving PIN works one step later: the car may open, but it will not drive away. That is the same idea as an immobilizer, under your control.
What not to rely on
Kitchen remedies are popular and unreliable. Foil works only if it encloses the key with no gap, every single time. Appliances are worse.
Layers that still help if the signal gets through
A shield protects the key. It is worth adding a layer that protects the car if someone gets it open anyway, and a visible one does part of its work before anyone tries.
The guide to whether steering wheel locks work covers what a lock adds and what it cannot do. Beyond that, the habits are the ones any car benefits from: park in a garage or where it is lit, keep the keys away from the doors and windows of the house, and consider a tracker if your car is one thieves go looking for.
