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When I use the electronic key fob to lock or unlock my car, no other car parked nearby has its locks activated. This is as it should be, but how does the system work?

• In principle, electronic car locks work in much the same way as a traditional lock and key. The key fob transmits a coded binary sequence, and if this matches the code programmed into the receiver then the lock releases.

To guard against the possibility that someone might intercept and record the code for unauthorised use, the more sophisticated systems use an algorithm that allows the code sequence to change with each key press.

Tony Ellis, Titahi Bay, New Zealand

• Key fobs share an encryption key – a way of scrambling and unscrambling a message – with the car. This is learned by the car via the manufacturer’s diagnostic tools. A car will know the encryption keys for a number of key fobs.

Modern fobs have an internal electronic counter that rises each time the button is pressed (usually with a range of several billion), and it is this ever-changing counter number that is encrypted and transmitted. On receipt the car checks that the counter value is close to, and greater than, the previously received number. This protects against replay attacks, where thieves record the key’s radio transmission with the intent of replaying it to unlock the doors. Once used, a counter code won’t be valid again in the car’s lifetime. Other cars within range will try to decode the transmission using their own encryption key, but shouldn’t see a valid sequence.

To avoid the problems of people who constantly fiddle with the key fob while the car is out of range, missed transmissions are allowed (within reason).

As it isn’t possible to determine the encryption key, the system stays secure even if you know that several successive messages sent by the fob encrypt numerically adjacent values. For this reason, many thieves have now resorted to investing in diagnostic tools and their own key, which they can add to the car’s list of known keys.

John Clayton, Lincoln, UK

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