RollJam, which is smaller than a cellphone, is designed to defeat "rolling codes" security systems used in most modern cars keyless entry and alarm systems and modern garage door openers, said Wired.
The hacker places the device near the target car or garage and waits for the victim to use his or her key fob. The victim will notice that the key fob didn't work on the first try, so he or she will press it again. At this time, the hacking device will have cracked the code of the wireless entry system and can now access the device at any time, Wired reported.
For the full Wired article, see here.