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Re: [Discuss-gnuradio] detecting covert RFID scans

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] detecting covert RFID scans
Date: Sun, 27 Dec 2015 19:44:59 +0100
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since there have been RFID reader implementations using USRPs, I do
assume it would be possible to develop such a system, albeit it might be
a little complex on the analog/discrete circuitry side of things.
I really can't tell what the quality of statements you'll be able to get
out of such a system -- False error probably / Receiver Operating
Characteristics are usually result of a system design that is based on
actual testing; since I have no experience with RFID extractable field
strengths, it's a bit hard to make a general statement here.

To be honest, for a start, since RFID isn't your normal "TX
amp->Voltage->Antenna->EM wavefront->far field->Antenna->Voltage->RX
Amp" system, this sounds like a pretty self-built hardware-involved
project. As a first step, you'd probably just build a coil antenna for
the RFID excitation / power signal, and amplify/model that to something
that you can feed into your ADC. These signals are around 30MHz, if I
remember correctly, so you cannot get the signal with RTL dongles, or
the B2x0, or the E3x0, without upconversion, which usually involves
proper preamplification. When you're there, you've actually built
something that can show you whether something transmits on the resonant
frequency of your tuned coil antenna with but a simple transistor and an
LED, without the help of SDR; of course, as soon as you're able to
listen in on the communication between an actual tag and the reader, you
might learn a lot more, but you'll need to be very careful when doing
that -- the energy absorbed by the tag is part of the way these
communicate, so a stray "measurement" coil might probably seriously
disrupt operation. Again, this is not "radio" in the sense of power
being "radiated" by an antenna and you can just pull of energy with an
antenna with an effective area much smaller than what's covered by the


On 12/27/2015 07:14 PM, Daniel Pocock wrote:
> On 27/12/15 19:03, Marcus Müller wrote:
>> The problem is that technically, the energy sent out by an RFID reader
>> isn't big enough to detect readers from afar; they are near-field
>> devices, as opposed to the typical far-field antenna based radio
>> transmitters.
> If the sniffer was carried in close proximity to the card itself (e.g.
> the card and sniffer in a backpack) and the backpack passed a hidden
> RFID scanner at the entrance to a shop, would the sniffer be able to
> detect the communication?
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