
2.1 Design Paradigms
Our assumption is that we are constructing an ad-hoc system for attack purposes, and mass production is
not involved. Therefore modular design and perfect implementation are not the main design goals. Instead,
we focused on quick, simple, and cheap methods.
There are two design paradigms that can be followed; the “normal” paradigm is to design all the sys-
tem sub-units to have a uniform 50 Ω input and output impedance. The other paradigm is to design and
implement a proprietary RF system, with non-standard characteristics.
The advantages of using standard design include the variety of ready-to-use designs, applications notes,
and test equipment. The resulting system is scalable, versatile, and modular. However, the need for accurate
design, dealing with accurate filters and semiconductor’s min-max parameters and ratings, stretches the
design and implementation time, and may cause long and tedious system testing and tuning.
In contrast, designing a proprietary, non-standard interface systems has some practical advantages. First,
accuracy is no longer mandatory. Second, the system can work in its natural output and input characteristics
without the need to adjust its interfaces to standard characteristics, that might need extra matching networks
and components. In particular, some amplifier designs have an output impedance that differs from 50 Ω, and
their designated antennas’ impedance is closer to the amplifier’s impedance than to 50 Ω. In this case, there
is no sense to adjust both amplifier output and antenna input to 50 Ω.
Since our goal was to emulate a hacker, we chose to follow the proprietary design paradigm. We used
50 Ω designs where they suited our needs, but we did not attempt to tune all the sub-units precisely. As we
shall see, the results were quite satisfactory, despite the very basic work environment and tools.
2.2 System Units
The skimmer is comprised of 5 basic units (see Figure 2): A reader, a power amplifier, a receive buffer, an
antenna and a power supply. The RFID reader generates all the necessary RF signals according to the ISO
14443 type A protocol. These signals are amplified by the power amplifier to generate the RF power which
is radiated through the loop antenna. The loop antenna performs the interaction with the ISO 14443 RFID
tag, and senses the load modulation signals. These signals are buffered by the Load Modulation Receive
Buffer and fed back to the reader detection input. The Reader communicates with a host system via an
RS232 serial interface. Typically, the host is a computer, however, it can also be a small micro-controller
based card, with some non-volatile memory that collects and stores skimmed data.
Our main objective was to increase the output power and antenna size as these two factors directly
influence the reading range.
2.3 The RFID Reader
The RFID reader module we used was the Texas Instrument (TI) S4100 Multi-Function reader module,
[TI03]. The module can be purchased alone for around $60, and the TI web site ([TI05]) contains sufficient
documentation for designing and programming this module. The S4100 module has a built in RF power
amplifier that can drive approximately 200 mW into a small antenna. The TI module supports several
RFID standards. We focused on the ISO 14443 Type A standard, that is used in contactless smartcards and
E-passports.
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