Security researcher Samy Kamkar demonstrated his "Rolljam" device at the Def Con 2015 security conference. With a component cost of around $32, this tiny gadget is designed to defeat the "rolling codes" security used in most keyless entry systems, including those for cars, garage doors, alarm systems, etc.
The device is smaller than a cell phone and is designed to be hidden on or near a target vehicle or garage. It lies in wait until someone uses his or her key fob within radio range to intercept and record your wireless key's command.
The device uses a clever jamming technique to steal one of your rolling codes - the first signal from the user's key fob is jammed by the device and will fail to unlock the door. The user will then push his key fob a second time, thinking the first push didn't work due to a technical glitch, unaware that anything is amiss.
When the key fob is pushed for the second time, RollJam jams that signal too and simultaneously broadcasts the first code. The second code is recorded and can be used by the attacker to open the victim's car or garage at will.
RollJam, as Kamkar describes it, is meant to be hidden on or near a target vehicle or garage, where it lies in wait for an unsuspecting victim to use his or her key fob within radio range. The victim will notice only that his or her key fob doesn’t work on the first try. But after a second, successful button press locks or unlocks a car or garage door, the RollJam attacker can return at any time to retrieve the device, press a small button on it, and replay an intercepted code from the victim’s fob to open that car or garage again at will. “Every garage that has a wireless remote, and virtually every car that has a wireless key can be broken into,” says Kamkar.