An auto keyless entry system has two components: a transmitter unit and a receiver unit. Both units work in the same radio frequency. When a driver pushes the button on his transmitter fob, the transmitter sends a specific pass code with a function code to the receiver unit in the car to request an operation.
Both the transmitter and receiver units have controller chips that contain a memory location that holds a 40-bit code generated by a pseudo-random number generator. When a driver pushes the button on the fob, the transmitter sends the 40-bit code along with a function to the receiver, which matches this code with the code in its memory location. If the two codes match, it performs the function, such as unlock door, as indicated by the function code. If the transmitter codes don't match, the receiver does nothing.
When the transmitter sends out a 40-bit code, its pseudo-random number generator generates a new code and stores it in the controller chip. When the receiver receives a valid code, its pseudo-random number generator also generates a new code. Since both units use the same pseudo-random number generator, they keep the codes synchronized. When these codes no longer synchronize, most receivers accept the next 256 numbers generated by the pseudo-random number generator.