BOX 2.2
Car Key Fobs

Remote car door key fobs and garage door openers work by way of radio signals. Very early units sent a constant signal, which is clearly insecure. Later versions used an 8-bit key to distinguish among different transmitters. Again, this was inadequate; thieves learned to record and retransmit the signals. Many modern units use a so-called rolling code, which is generated from a pseudo-random number generator. Eavesdropping on a few code transmissions should not provide enough information to predict the next code. To avoid problems from the loss of synchronization, a range of codes is accepted by the receiver. A mechanism is also provided to resynchronize the transmitter and the receiver. No identification signal per se is transmitted by such devices. Many new cars use a radio transponder embedded in the keys themselves to unlock the ignition. Some of these use rolling codes; others use challenge/response technologies. Key fobs contain a modest amount of storage and computational ability. Specialized equipment is required to copy them, but they are easily stolen.