Immobiliser Introduction – Part 2

  1. Immobilizer System Overview

Four main components make up the key-based immobilizer unit. The transponder is the heart of the system. Battery-free, the transponder can be equipped with several different functionalities best suited to a vehicle’s specific needs. It sends data as a modulated RF signal. The transceiver and antenna coil provide the power for the transponder to function, generating and radiating a high frequency magnetic field. The signal, once demodulated by the transceiver, is sent to the controller, the final component, for data processing.


Two distinct RFID systems can be found on the market, set apart by their method of transmitting energy.


  • Full Duplex Systems.

In this system, the unit sends data similar to a telephone, in that the transponder can send and receive data and energy simultaneously. This is often accomplished using load modulation.


  • Half Duplex Systems.

In this method, the energy and data are sent one after the other, as done with walkie-talkies. The transponder stores the energy collected from the transceiver and sends the data once the energy is fully produced.


While these two systems obviously impact the physical design of the system, one is not superior to the other when it comes to automotive security.


  1. Cryptographic Background

When working with cryptographic immobilization, there are two tasks that the system must be able to perform: identification and authentication. The security system must be able to identify the driver and prove his identity. Cryptography provides multiple means of acquiring this information.



Authentication is achieved through the sharing of a secret. This method is incredibly common in other areas, such as passwords for email or social media accounts, or a PIN number for a debit card. This secret must be utilized as a method of proving one’s identity. In many areas, this method of authentication makes sense since keyboards and number pads are common place. The implementation of such hardware would be cumbersome and unwelcome by many motorists. This method also provides the least amount of security.



Unique biological characteristics offer a high level of security. Vehicles that require fingerprint, retinal, face, or voice scans in order to operate would make vehicles all but impossible to steal. The practicality of this method is very low, however, as hardware and technical complications are unacceptable for automotive use. Renting or borrowing cars in an emergency would also become infinitely more complicated, if not impossible.



Possession is the most prevalent method of authentication currently used in automobiles. In its simplest form, the driver possesses a key or remote that achieves authentication. While its security does not equal that of biometrics, higher security can be attained if the key contains a transponder device or a similar electronic tag that must match a tag within the vehicle in order to authenticate.


In each of these systems, authentication methods are limited in that they verify the identity of the key in order to proceed, but the key does not perform any checks in return. A mutual authentication process would allow the key to preform authentication of its communication partner in return, providing a much tighter security system for the vehicle.


Security of the vehicle can be improved even further by implementing a simple symmetrical algorithm referred to as a challenge/response protocol. The security system housed in the automobile verifies the identity of the key by communicating a challenge or question. The key would send its response, an answer to the question. A mutual secret must be known and shared by each component for a correct answer to be given. This concept has a number of advantages that increase security. During day-to-day use, the secret is not released and the responses needed to complete the exchange would vary for each use.

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