Rolling code: Difference between revisions

Revision as of 21:53, 5 May 2014

Revision as of 03:35, 9 July 2014

Line 1:

A '''rolling code''' (or sometimes called a '''hopping code''') is used in [[keyless entry]] systems to prevent [[replay attack]]s, where an [[Eavesdropping|eavesdropper]] records the transmission and replays it at a later time to cause the receiver to 'unlock'. Such systems are typical in [[garage door opener]]s and keyless car entry systems.

Such systems are typical in [[garage door opener]]s and keyless car entry systems. The most widely spread algorithm for code hopping systems today is [[KeeLoq]].{{Citation needed|date = December 2008}}

== Techniques ==

* Common PRNG ([[Pseudopseudorandom Randomnumber Number Generatorgenerator]]) — preferably [[Cryptographically secure pseudorandom number generator|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 Remoteremote Controlcontrol ==

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 ==

[[File:Rolling-code-rf-remote-control-2.png|thumb|200px|this is a typical inner photo of audi a6 car keyless entry remote, which uses rolling-code technology chip HCS301 to enhance transmission security.]]

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.

== Typical Rolling code System - KeeLoq ==

[[File:Rolling-code-rf-remote-control-2.png|thumb|200pxupright|thisHCS301 ischip afrom typicalan inner[[Audi photo of audi a6 carA6]] keyless entry remote, which uses a rolling- code technology chip HCS301 to enhance transmission security.system]]

{{main|KeeLoq}}