Keyless entry is a system that lets you lock, unlock, or start a car (or open a door at home) without physically inserting a key. Instead, the system uses short-range radio signals exchanged between a small transmitter you carry and a receiver built into the vehicle or lock. The technology first appeared in cars in 1982 on the Renault Fuego, and today it comes standard on most new vehicles and is increasingly common in residential smart locks.
How Car Keyless Entry Works
There are two generations of the technology in cars. The older, more familiar version is the remote fob with buttons: you press a button, the fob sends a radio signal, and the car unlocks. The newer version, called passive keyless entry, requires no button press at all. You simply walk up to the car and pull the door handle.
Passive systems rely on two different radio bands working together. The car periodically sends out a low-frequency signal at about 134.2 kHz. This signal has a deliberately short range of only 1 to 2 meters, so it can confirm that the fob is physically close to the vehicle. When you touch or pull the door handle, the car sends a challenge signal on that same low-frequency band. If your fob receives it and responds with the correct cryptographic answer (using a secret key shared between the fob and the car), the doors unlock. A higher-frequency band, typically UHF or Bluetooth Low Energy, carries the fob’s response back to the car over a range of up to about 100 meters.
Starting the engine works the same way. With the fob in your pocket or bag, you press a start button on the dashboard. The car verifies the fob is inside the cabin using the same low-frequency challenge-and-response process, then allows ignition.
Keyless Entry for Your Home
Residential keyless entry typically means a smart lock on your front door. Instead of a fob, you use a keypad code, a smartphone app, or a fingerprint reader to unlock the door. The lock communicates with your phone or smart home hub using one of several wireless protocols.
Bluetooth-based locks connect directly to your phone within about 10 meters and require an initial pairing step, usually involving a PIN. NFC-based locks work at very close range, under 4 centimeters, meaning you tap your phone or a card against the lock. Locks that connect to a broader smart home system often use Z-Wave (operating at 908.42 MHz in the U.S.) or Zigbee (at 2.4 GHz). Z-Wave tends to have longer range with less interference from Wi-Fi networks, while Zigbee supports more devices in a mesh network and responds slightly faster. Both let you lock and unlock remotely through a hub or voice assistant.
Smartphone Digital Keys
A growing number of cars and locks now let you ditch the fob entirely and use your phone as the key. These digital keys work through either NFC or Bluetooth. With NFC, you hold your phone within a few centimeters of the door handle or lock, similar to tapping a contactless payment terminal. With Bluetooth, your phone communicates from up to 10 meters away, enabling the same walk-up-and-open experience as a traditional fob.
Some newer vehicles from BMW and others use ultra-wideband (UWB) radio, the same technology inside Apple’s AirTag. UWB sends rapid bursts of radio waves and precisely measures how long each burst takes to travel between your phone and the car. This time-of-flight measurement makes it extremely difficult for anyone to intercept and relay the signal, which is a significant security advantage over older systems.
The Relay Attack Problem
The biggest security concern with passive keyless entry in cars is the relay attack. Two thieves work as a team: one stands near your front door with a device that picks up the faint low-frequency signal leaking from your key fob inside the house. That device amplifies the signal and transmits it to a second device held by the accomplice standing next to your car. The car “thinks” the fob is right there and unlocks. The whole process can take less than 60 seconds.
This works because older passive systems only check whether the fob responds correctly, not whether the signal traveled a physically plausible distance. The attack is most effective when your fob is stored near an exterior wall or front door, which puts it closer to the thief’s antenna. Weaknesses in older encryption schemes used by some manufacturers have made certain vehicles especially vulnerable.
How to Protect Yourself
The most commonly recommended defense is a signal-blocking pouch, often called a Faraday pouch, that wraps your fob in material designed to block radio waves from getting in or out. However, testing by researchers at Edith Cowan University found that performance varies dramatically by brand. Of five commercially available Faraday bags tested, only one consistently blocked all signals across cellular, Wi-Fi, and Bluetooth frequencies. Several others let signals through depending on the phone model being tested. Surprisingly, plain aluminum foil performed as well as the best bag for blocking cellular signals, though it was less reliable against Wi-Fi.
If you’re buying a Faraday pouch, the takeaway is that not all pouches are equally effective. Test yours by placing your fob inside it and trying to unlock your car from normal range. If the car still responds, the pouch isn’t doing its job. Other practical steps include storing your fob far from exterior walls and doors, and checking whether your car has a setting to disable the passive entry feature when you don’t need it.
UWB-based digital keys are the most robust long-term solution. Because UWB measures the actual travel time of radio pulses between your phone and the car, a relay attack can’t fool the system. The signal simply can’t be captured and forwarded fast enough to produce a plausible distance measurement.
What Happens When the Battery Dies
A dead fob battery doesn’t mean you’re locked out. Most cars with keyless entry hide a physical key blade inside the fob itself, usually released by a small switch or button on the fob’s casing. There’s a corresponding traditional keyhole on the car, sometimes hidden behind a cap on the driver’s door handle. This mechanical backup lets you unlock the door. To start the engine, many cars have a spot on the steering column or center console where you can hold the dead fob directly against a reader that powers it just enough to authenticate.
For home smart locks, the backup depends on the model. Most include a traditional keyhole that works even when the lock’s batteries are completely dead. Some models have an emergency override: you remove the lock’s outer cover, use a screwdriver to rotate a small slot on the mounting plate, and then use a physical key to move the deadbolt. A few models accept an external battery contact on the outside of the lock, letting you hold a 9-volt battery against it to provide just enough power to enter your code. Keeping a physical backup key in a secure location outside your home is the simplest insurance against a dead battery.