Immobiliser Introduction – Part 3

  1. Standard Security Architectures using RFID

There are several different security systems on the market which use RFID transponders, each with its own benefits.


Fixed Code Systems.

The most widely used on the market, these systems rely on the unique identification codes stored in the transponders within a vehicle. A controller learns this codes and when the key is placed in the car’s lock cylinder, the code within the transponder is analyzed and compared to the codes the controller has stored within its memory.


Depending on the type of transponder, security of this type of system varies. One type of transponder is a “write-once” unit that is purchased unprogrammed. Readers/writers that are commercially available read the transponder’s code and program the new unit. In doing this, the fixed code has been perfectly copied and is undistinguishable from its source. Another type of system, True Read Only, are factory programmed and cannot be copied. While it is technically possible to emulate the data using radio, the design knowledge and effort required is tremendous.


Rolling Code Systems

These systems are not entirely different from the fixed code system. Their main characteristic is the time limit on the key’s secret code. This time window is generally one ignition cycle to the next. The transponder, of the read/write variety, is reprogrammed with a new secret. The key code is constantly changing, but this method still falls under the umbrella of static authentication.


Sometimes transponders fail or are inadvertently reprogrammed while away from the vehicle. In these cases, a failsafe is built into the system which ensures their reliability. If the need arises, a transponder can be resynchronized so the driver can still use their vehicle. These procedures are critically important for security systems such as these.


Password Protected Transponders.

These types of transponders give an easy path to mutual authentication. Operating like many password protected systems and accounts, the transponder within the vehicle will deny a key access to its information if an incorrect password is provided. The length of a password does a lot for its security and can be customized to give higher or lower security.


Like all passwords, if the transponder is available to a thief, it can be guessed. But the amount of time that could take is anywhere from a minute to many years.


Total transaction time for password protected systems can be less than ideal for many drivers, especially when the implementation of the system requires new hardware. This element makes this system impractical for many.


Combined Rolling Code/Password Systems

These systems use read/write transponders that are password protected. Timing and resynchronization, among other critical issues, are applicable and give this method a much higher level of security than a simple password protected transponder.


Crypto Transponders

These second-generation transponders, developed by Texas Instruments, find their basis in the half-duplex RFID technology of TIRIS TM. They are compatible with all standard RF interfaces within this product line.


System Overview

This crypto device, a Digital Signature Transponder (DST), utilizes the challenge/response system. The transceiver in the vehicle sends the key a 40-bit number at random. It uses Pulse Width Modulation (PWM) to send the challenge. In the key, the transponder shifts the code to a challenge register. For a brief period, the transceiver provides energy and its encryption logic generates a 24-bit response.



Before receiving a response, the vehicle’s security system calculates its expected answer by using the same algorithm and encryption key. Once the transponder answers, the system compares the expected answer and the given one. If the two match perfectly, then the security system is satisfied and the data is sent to the engine’s computer. If time is short, then the challenge and response can be generated beforehand and stored.


This system’s advantages should be evident:

  • The authentication process is dynamic as the response can be different each time it is used, dependingon the challenge.
  • The transponder cannot be copied
  • The encryption key is never communicated once the initialization of the transponder has occurred.
  • The encryption key cannot be released
  • If needed, the encryption key can be permanently changed or locked.


The power consumption of a transponder is important. They are elaborate micro systems intended to perform using very little power. Less than 1mA is consumed during a typical energy transfer allowing a capacitor to charge over long distances very quickly. Even during encryption, the standard of power consumption for a transponder is below 16mA, making its maximum read range comparable to average Read Only systems.

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