Rolling code
Introduction


A **rolling code** (or sometimes called a **hopping code**) is used in [keyless entry](/wiki/Keyless_entry "Keyless entry") systems to prevent a simple form of [replay attack](/wiki/Replay_attack "Replay attack"), where an [eavesdropper](/wiki/Eavesdropping "Eavesdropping") records the transmission and replays it at a later time to cause the receiver to 'unlock'. Such systems are typical in [garage door openers](/wiki/Garage_door_opener "Garage door opener") and keyless car entry systems.


Why are keyless entry systems vulnerable to replay attacks?