A rolling code is used in keyless entry systems to prevent replay attacks, where an eavesdropper

Eavesdropping

Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

 records the transmission and replays it at a later time to cause the receiver to 'unlock'.

Such systems are typical in garage door opener

Garage door opener

A garage door opener is a motorized device that opens and closes garage doors. Most are controlled by switches on the garage wall, as well as by remote controls carried in the garage owner's cars, or more rarely, on keychains.- The electric opener :...

s and keyless car entry systems. The most widely spread algorithm for code hopping systems today is KeeLoq

KeeLoq

KeeLoq is a proprietary hardware-dedicated NLFSR-based block cipher. The uni-directional command transfer protocol was designed by Frederick Bruwer, PhD, CEO at Nanoteq Ltd and the cryptographic algorithm was created by Professor Gideon Kuhn with the silicon implementation by Willem Smit, PhD, at...

.

Techniques

• Common PRNG (Pseudo Random Number Generator) — preferably cryptographically secure
  Cryptographically secure pseudorandom number generator
  A cryptographically secure pseudo-random number generator is a pseudo-random number generator with properties that make it suitable for use in cryptography.Many aspects of cryptography require random numbers, for example:...
  
   — 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

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

(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.