Rolling code

A rolling code (or sometimes called a hopping code) is used in keyless entry systems to prevent replay attacks, where an eavesdropper records the transmission and replays it at a later time to cause the receiver to 'unlock'.

Such systems are typical in garage door openers and keyless car entry systems. The most widely spread algorithm for code hopping systems today is KeeLoq.

Techniques

• Common PRNG (Pseudo Random Number Generator) — preferably cryptographically secure — in both transmitter and receiver
• Transmitter sends 'next' code in sequence
• Receiver compares 'next' to its calculated 'next' code.
• A typical implementation compares within the next 256 codes in case receiver missed some transmitted keypresses.

Application in RF Remote Control

A rolling code transmitter is useful in a security system for providing secure encrypted Radio frequency (RF) transmission comprising an interleaved trinary bit fixed code and rolling code. A receiver demodulates the encrypted RF transmission and recovers the fixed code and rolling code. Upon comparision of the fixed and rolling codes with stored codes and determining that the signal has emanated from an authorized transmitter, a signal is generated to actuate an electric motor to open or close a movable component.

Rolling code vs. fixed code RF remote control

Remotes control send signals in code. When the sending code is the same as the code which is expected by the receiver, the relay will switch and the door, barrier or car will open. Remote controls with a fixed code will always send out the same code. A rolling code (or hopping code) always send out a different code. The code changes every time because of the algorithm which is part of the remote control.

See also

• Replay attack

References

http://auto.howstuffworks.com/remote-entry1.htm

• Atmel, Inc.'s application note AVR411 "describes a Secure Rolling Code Algorithm transmission protocol for use in a unidirectional wireless communication system"