The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 274

Figure 10.11 Size comparison: low-cost transponder chip in the eye of a needle (reproduced
by permission of Philips Electronics N.V.)
sensitive applications may be impaired by unauthorised reading or writing of data in
the transponder. Two examples of such applications are the contactless cards used as
tickets in the public transport system and transponders in vehicle keys for electronic
immobilisation systems.
There are various procedures for preventing unauthorised access to a transponder.
One of the simplest mechanisms is read and write protection by checking a password .
In this procedure, the card compares the transmitted password with a stored reference
password and permits access to the data memory if the passwords correspond.
However, if mutual authorisation is to be sought or it is necessary to check that
both components belong to the same application, then authentication procedures are
used. Fundamentally, an authentication procedure always involves a comparison of
two secret keys , which are not transmitted via the interface. (A detailed description of
such procedures can be found in Chapter 8). Cryptological authentication is usually
associated with the encryption of the data stream to be transmitted (Figure 10.12). This
provides an effective protection against attempts to eavesdrop into the data transmission
by monitoring the wireless transponder interface using a radio receiver.
In addition to the memory area allocated to application data, transponders with
cryptological functions always have an additional memory area for the storage of the
secret key and a configuration register ( access register , Acc) for selectively write
protecting selected address areas. The secret key is written to the key memory by the
manufacturer before the transponder is supplied to the user. For security reasons, the
key memory can never be read.
Hierarchical key concept Some systems provide the option of storing two sepa-
rate keys — key A and key B — that give different access rights. The authentication
between transponder and reader may take place using key A or key B. The option of