Can you open your car door with a cell phone?

There's a rumor swirling around the Internet that says if someone calls your cell phone and presses the "unlock" button on your car's spare remote key as you hold your phone near the car door, the door will unlock. In other words, if you lock your keys in the car, you can call someone at home and he or she can open it remotely with another keyfob.

Sounds handy, right? But is this true? Well, unfortunately it isn't. The keyfob for your car door broadcasts on a range far above audible sound, which is encoded by the cell phone. The radio signal from the keyfob simply can't be transmitted by the mobile phone [source: Discovery Channel].

Rolling Codes and Encryption

Modern keyless entry system broadcast on a frequency between 300 and 400 MHz (megahertz). But if your keyfob sent out just a single signal, then every fob would open every car of that make and model. To ensure that no one can use his or her fob to open your car door, it's necessary produce a signal that is unique to every car.

This is where rolling codes, also known as hopping codes, come in. Whenever you press the button to unlock your car, the exact frequency transmitted by the fob is changed, and the receiver inside the car only grabs onto that particular signal. In other words, the code "rolls" or "hops" each time you use it. A controller chip inside the car receives the signal and is responsible for changing the code each time the lock or unlock button is pushed.

Before this rolling code system was developed, thieves were able to use electronic devices called "code grabbers" to lock onto your keyfob's unique signal. With rolling codes, the signal is unique every time, rendering a code grabber device useless [source: Lake].

In addition, the code is stored inside the car, not within the keyfob. A thief would need to break into the car to access the code, which defeats the purpose of getting it in the first place.

The numbers generated when the code hops is random. However, in theory, an astute hacker dead-set on stealing your car could find a way to anticipate the next code in the sequence. For this reason, the codes are encrypted as well, making each electronic keyfob have billions of possible codes.

However, no security system is totally foolproof. In 2007, a group of researchers discovered vulnerability in the algorithm used by nearly every car manufacturer to encrypt their security codes. With this vulnerability, they found they were able to unlock any car made by that automaker with the keyfob from just one of them [source: Zetter].

Do you need to worry about your vehicle being stolen from your parking lot this way? Probably not. After all, the method mentioned above is extremely high-tech and actually very complicated. That puts it out of the scope of most car thieves. It's simply easier and faster for a car thief to just try their luck by smashing a window and attempting to hot-wire the ignition.

In this next section, we'll take a look at more advanced anti-theft systems built into your keys, and analyze whether or not they're worth the cost.

So how about even more advanced anti-theft systems? Are they worth the extra cost? Read the next page to find out.