Radio Signals Are Everywhere
This seems like a simple enough problem to solve but it is really quite complex. If each key fob has a unique serial number programmed into it, then it could simply send out that serial number as part of the radio signal. The computer in the car would receive the signal, look at the serial number and unlock the doors only if the serial number is correct.

A serial number like this would work fine, except for one small problem: criminals. Criminals would be able to sit in the parking lot with a radio scanner and capture each serial number. Then, later on, they would be able to retransmit your serial number and get into your car.

The Wonder of the Rolling Code
Engineers have developed something called a rolling code transmitter to deal with this issue. The transmitter in your fob sends a different random number every time you push the button. However, the number that is transmitted today to open your car, will not work the next time that number is used. So, if criminals try to capture your fob’s transmission, the number they capture is worthless since it can only be used once.

But if your fob sends a different random number every time you push the button, how does your car know it is the correct number? The answer is synchronization. The fob and the car are synchronized and they both know how the random numbers are generated because they are both using the same random-number generator.

Therefore when the fob sends a random number, the car receives it and unlocks the doors. Continuing on, both the car and the fob pick another random number for the next time that the fob is used. Because they are both synchronized, they will both pick the same random number.

The Magic Number Is 256
Since criminals do not have any way to know what the next random number will be, capturing the code does them no good. But the rolling code system does create another problem. What if you press the button on your fob and you are nowhere near your car? Or what if your kid gets hold of your fob and presses the button 10 times for the fun of it? Now the fob’s random number will be different from the one your car expects.

Engineers solve this problem with something called looking-ahead. When your car sees a random number come in from your fob, it looks at the next 256 numbers in the random-number sequence. If it sees any of those 256 numbers, it will unlock your car.

Better than a Billion in One
Because there are all these random numbers flying around from millions of different key fobs, there is conceivably some small chance that your fob might open somebody else’s car. In older systems that used a 40-bit rolling code, the chance was about 1 in a billion that it would randomly open another car of the same make and model. In newer systems that use even more bits, the odds are so slim that they are of no significance.