Rolling code

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 latertime 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) 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.

References

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


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