[Hackrf-dev] Help with NFC decoding.

[evilsocket]

If anyone wants to keep the conversation going ( instead of receiving
dozens of emails :P ) I've posted the question on reddit
http://www.reddit.com/r/sdr/comments/331jeq/q_how_do_i_demodulate_a_series_of_magnitudes_in/

2015-04-18 18:44 GMT+03:00 evilsocket <evilsocket at gmail.com>:

> Yeah as long as I know, NFC-A is modulated on ASK 100% from the reader
> side and ASK 10% from the tag side, so using only one type of demodulation
> I should be able to see only the reader data and wrongly demodulated tag
> data ... but once I'll be able to correctly demodulate the reader side,
> I'll "follow" the protocol and know what should be demodulated how ....
> hopefully :P
>
> I'm pretty sure it's ISO14443a, at least that's what the nfc reader app
> I'm using is saying.
>
> 2015-04-18 17:46 GMT+03:00 Martin Holst Swende <martin at swende.se>:
>
>> On 2015-04-18 16:10, evilsocket wrote:
>> >> I think this is a great goal, but have you considered using GNU Radio
>> >> to confirm that it works and then rewrite it in C/C++ piece by piece?
>> >
>> > Yep but in that case would be copying, not understanding, wouldn't it?
>> :D
>> >
>> >> Have you looked at the waveform to see if you can identify where the
>> >> data is being transmitted?
>> >
>> > Yep, I'm on the right frequency and it's clearly ASK.
>> >
>> >> I think you're on the right track.
>> >
>> > Great! :D
>>
>> One thing that will bite you with this approach, is that you'll only get
>> reader-side of the comms this way. The reader does OOK, but the tag does
>> not transmit; it modulates the reader signal. So, depending on whether
>> the tag coil is open or closed, it will dampen or not dampen the
>> reader-signal. So you'll need to have some more levels to discern these
>> pretty small variations in the signal.
>>
>>
>> >
>> >> It probably depends on what is being transferred.  A good place to
>> >> start would be these specs:
>> >> http://members.nfc-forum.org/specs/spec_list/
>> >
>> > Thanks so much :)
>>
>> Obviously, depending on what you're observing, it will vary, but I would
>> guess that the traffic you're seeing is iso14443a, afaik the most
>> commonly used. The documents you're looking for would be the ISO 14443a
>> specs. There are four of them:
>>
>> 1. Physical characteristics
>> 2. Radio frequency power and signal interface
>> 3. Initialization and anticollision
>> 4. Transmission protocol.
>>
>> Start with 2, and work your way up... I can email them to you off-list
>> if you'd like, shoot me an email in that case.
>>
>> Otherwise, it's probably iso15693 you're seeing. That's more common in
>> vicinity (as opposed to "proximity") cards which can be read over a
>> greater distance, e.g. ski passes.
>>
>> Good luck
>> /Martin
>>
>
>
>
> --
>
>
>
> _________________________
> Simone '*evilsocket*' Margaritelli
> http://www.evilsocket.net/  <http://www.evilsocket.net/>
>



-- 



_________________________
Simone '*evilsocket*' Margaritelli
http://www.evilsocket.net/  <http://www.evilsocket.net/>
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