The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 275

Modulator

Crypto circuit

Coil 1

Mode register

Controller

Memory

crypto control
OP-code detect
EEPROM control
read/write control

(320 bit EEPROM)

crypto key

Coil 2

64 or 128 bit
ID code

Testlogic POR

Input register

VDD VSS

Testpads

Figure 10.12 Block diagram of a writable transponder with a cryptological function to perform
authentication between transponder and reader (reproduced by permission of TEMIC Semicon-
ductor GmbH, Heilbronn)

allocating different access rights (Acc) to the two keys may therefore be exploited in
order to define hierarchical security levels in an application.
Figure 10.13 illustrates this principle for clarification. The transponder incorporates
two key memories, which are initialised by the two keys A and B. The access rights that
the readers are allocated after successful authentication depends upon the setting that
has been selected in the transponder (access register) for the key that has been used.
Reader 1 is only in possession of key A. After successful authentication, the selected
settings in the access register (Acc) only permit it to read from the transponder memory.
Reader 2, on the other hand, is in possession of key B. After successful authentication
using key B, the settings selected in the access register (Acc) permit it to write to the
transponder memory as well as reading from it.

Sample application—hierarchical key Let us now consider the system of travel
passes used by a public transport network as an example of the practical use of hier-
archical keys . We can differentiate between two groups of readers: the 'devaluers' for
fare payments and the 'revaluers' which revalue the contactless smart cards.
The access rights to the transponder's two access registers A and B are configured
such that, after successful authentication using key A, the system only permits the
deduction of monetary amounts (the devaluation of a counter in the transponder). Only
after authentication with key B may monetary amounts be added (the revaluation of
the same counter).
In order to protect against attempted fraud, the readers in vehicles or subway
entrances, i.e. devaluers, are only provided with key A. This means that a transponder
can never be revalued using a devaluer, not even if the software of a stolen devaluer
is manipulated. The transponder itself refuses to add to the internal counter unless the
transaction has been authenticated by the correct key.