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I have a 2006 Toyota Prius and I am worried that someone could amplify the signal of my contact-less key to unlock, start it and then drive it off.

Does the Prius stall if the car is driven away and the signal of the key gets fainter?

If not (and there is no other security feature), I think I will just convert it to use a normal key to start the car.

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No, it will not stop mid-ride. It will warn you the key is no longer in the car, but as long as you don't turn it off, it will keep running until you run out of gas or the car battery dies. It would be a safety hazard for it to randomly die, or even a slow turn off. Imagine if you were on a 65mph high way when it happens, or in the middle of an intersection.

Yes, while rare, relay attacks can happen. This article has a good breakdown of it:

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If you are paranoid about this happening to you, as the article says, stick your keys in a faraday cage, like your microwave or freezer. Or a metal tool box at home.

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  • Indeed it is a trade-off between safety and security. Since personally I don't see any advantage of keyless start of the car and that those keys cost absurdly expensive (they cost usually more than $200, some even $1000 or up), I would rather have a traditional encoded key. – Gabriel Diego Dec 9 '15 at 19:22
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    Anecdote: Someone I work with rented a car while his was in the shop. The rental had a pushbutton start. He drove to work with his wife, then his wife took the car to her office. She stopped for coffee and when she got back to the car realized her husband still had the key. I had to give him a ride to the coffee shop so he could give it to her. Unfortunately I don't recall what kind of car it was (I think it was a Nissan, but I'm not sure). – TMN Dec 9 '15 at 20:45
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Your concern is valid. This article gives a good description of the weakness of smart keys. A man in the middle attack can fool the car to think that the key is inside allowing it to start. Once the car is running it won't shut off if the key leaves the car. These keys are battery powered and if that battery dies while you are driving on the highway it may be dangerous if the car just shuts off.

Don't forget that this method takes two people and one of the people has to stand next to you for some time. It's just as easy to steal you keys or purse.

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  • With a good antenna the attack can be done a distance. My room's windows is fairly close to the street, so this concerns me. – Gabriel Diego Dec 9 '15 at 2:40
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Not a specific answer for a Toyota Prius, but:

This summer, we had an incident here in Germany, where the motor stopped after throwing the key out of the window.

There's an online police report, so this is not a hoax.

Here is my short translation:

A 33 year old mother was driving her VW Passat on the highway (German "Autobahn") A1 near Lübeck with her 1.5 year old daughter on the right rear seat. As the daughter was whining due to tiredness, the mother gave her the key ring with the ignition key to play with. As the daughter didn't calm down, the mother opened the window a little, hoping the fresh air would help.
The daughter threw the key out of the open window, causing the motor to stop. The car was still steerable and the mother managed to stop on the emergency lane. The mother called the police, who searched for the key ring and finally found it. All keys including the ignition key were damaged, but the electronics survived, so the mother was able to drive home.

So, this was a Volkswagen, not a Toyota, but it shows that bad thinks can happen.

Of course, the behavior of the car depends on what the engineers implemented and may vary from brand to brand, or from car to car.
In my personal mind, the motor should not stop while driving, but on the next halt.

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  • Any idea which model year VW this was? – Zaid Dec 9 '15 at 10:09
  • It was a newer Passat, a year is not given. Literally, it is described as "neuwertig" which means "as good as new". However, there shouldn't be so many Passat models with this keys. – sweber Dec 9 '15 at 10:33
  • I'm surprised that the steering lock did not activate in this case. – Steve Matthews Dec 9 '15 at 14:36
  • @SteveMatthews: Who knows. Depending on the model, the steering wheel may lock in any position, or only at certain angles. In the last case, driving on the highway may not have locked it due to the small movements of the steering wheel. But if it locks on the highway, you'll have a really bad day. – sweber Dec 9 '15 at 14:57
  • Whether it is better to risk being stranded with a non-running car because its key stopped working or was thrown away or being stranded and risk someone being killed because the car was stolen is a matter of personal preference. If there is a way to set this it would please both folks. – Gabriel Diego Dec 9 '15 at 19:16
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Paulster2 wrote: "Basically, this cannot happen. The Prius knows where the key is at. If the key is on the outside of the vehicle, it won't start, period."

This is not true, however.

Here we speak about thief(s) trying to steal your car. They will use the wireless amplifier to create the link between the car and your key (which is in your possession, at home). The car will unlock, thief No. 1 steps in, the car will recognize "key inside" situation and your car will start.

There is a nice video on YouTube.

Stealing the modern car packed with electronics is quite simple and fast. Prius is not an exception. These systems can be fooled too easily and cannot be trusted. Period.

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This video tells you a lot.

Basically, this cannot happen. The Prius knows where the key is at. If the key is on the outside of the vehicle, it won't start, period. If the key is too far away, it won't even unlock. If the key is left inside the vehicle and you try to lock the door, it won't let you.

Toyota engineers have thought a lot more about this than other car manufacturers have. There was an episode of Top Gear where the three guys came to the states and drove modern American muscle (Corvette, Mustang, & Challenger). Jeremy Clarkson was driving the Challenger. He parked it outside of a restaurant then went inside and sat down very near where the car was parked. One of the other two guys went outside, got in the Challenger, started it, then drove it out into the middle of the street before it died (from being too far away from the remote). Quite some hilarity. Point is, you couldn't even get that far in your Prius.

I don't believe you have anything to worry about.

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  • I saw that video of Top Gear but apparently Clarkson stopped it in the middle of the street on purpose, and actually it was staged since someone else in a forum who owns a Challenger said that the car will not start if the key is outside. Still there is nothing saying on how the car detects that the key is inside and I assume that it is by the signal strength, which can be amplified to fool the detectors. – Gabriel Diego Dec 9 '15 at 2:39
  • @gabrieldiego - It seemed from what I read and the video, the system uses triangulation to detect exactly where the key is at. Most of these systems use a key which is not active. The car sends a signal out and is picked up by the transponder in the key. The transponder is energized which sends a return signal back to the antenna in the vehicle. I don't know if it's the same way with the Prius setup, but seems logical it would continue to be used. – Pᴀᴜʟsᴛᴇʀ2♦ Dec 9 '15 at 15:30
  • +1: I don't understand the downvotes. I bought a RAV4 hybrid recently, and it beeps whenever I walk out of the vehicle with the key in my pocket whenever the engine is running. I assume these things know where the key is, and if the key is not inside the vehicle, it won't even start (although the doors may be unlocked). – juhist Dec 17 '16 at 12:17

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