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.
- Common PRNG (pseudorandom 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 comparison 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
Remote controls send signals in code. When the sending code is the same as the code which is expected by the receiver, then the receiver will actuate the relay, unlock the door, or open the barrier. Remote controls with a fixed code always send the same fixed code. Remote controls with a rolling code (or hopping code) always send out a different code from the one previously sent.
The Microchip HCS301 was once the most widely used system on garage and gate remote control and receivers. The chip uses the KeeLoq algorithm. The HCS301 KeeLoq system transmits 66 data bits.
- 34 bits are not encrypted : a 28-bit serial number, 4 bits of button information, and 2 status bits (repeat and low battery indicators).
- 32 bits are encrypted (the rolling code) : 4 bits of button information, 2 bits of OVR (used to extend counter value), 10 bits of DISC (discrimination value; often the low 10 bits of the serial number), and a 16-bit counter. In a resyncing situation, the encrypted 32 bits are replaced with a 32-bit seed value.
- Microchip (2001), HC301 KeeLoq Code Hopping Encoder (PDF), Microchip Technology Inc., DS21143B