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Rolling code

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A rolling code (or somtimes 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.

Techniques

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

References

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

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