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#1 2017-12-07 12:27:01

tim0s
Contributor
Registered: 2017-12-07
Posts: 2

Emulation of a "custom" DESFire card

Hi, I have a card which seems to be a DESFire "variant". For most use cases no encryption is used, the reader simply does anti-collision and uses the UID to "authenticate" the user. I would like to clone my card with a proxmark3.

If I query the original card with an ACR122 and nfc-list, this is what I get (replaced some bytes with X for my privacy)

1 ISO14443A passive target(s) found:
ISO/IEC 14443A (106 kbps) target:
    ATQA (SENS_RES): 00  41
    UID (NFCID1): 16  XX  XX  XX  XX  0d  3d
    SAK (SEL_RES): 20
    ATS: 77  77  81  02

If I do a hf 14a sim 3 16XXXXXXXX0d3d I get

    ATQA (SENS_RES): 03  44 
    UID (NFCID1): 16  XX  XX  XX  XX  0d  3d 
    SAK (SEL_RES): 20 
    ATS: 58  80  02 

It was quite easy to set the right ATQA value by setting (in armsrc/iso14443a.c)

     response1[0] = 0x01;                                               
     response1[1] = 0x00;

However I have no luck changing the ATS to a 4 byte sequence. What I have tried is to change (in the same file):


       uint8_t response6[] = { 0x05, 0x77, 0x77, 0x81, 0x02, 0x00, 0x00 }; // dummy ATS (pseudo-ATR), answer to RATS:
        // Format byte = 0x58: FSCI=0x08 (FSC=256), TA(1) and TC(1) present,
        // TA(1) = 0x80: different divisors not supported, DR = 1, DS = 1
        // TB(1) = not present. Defaults: FWI = 4 (FWT = 256 * 16 * 2^4 * 1/fc = 4833us), SFGI = 0 (SFG = 256 * 16 * 2^0 * 1/fc = 302us)
        // TC(1) = 0x02: CID supported, NAD not supported
        ComputeCrc14443(CRC_14443_A, response6, 5, &response6[5], &response6[6]);

However, nfc-list simply stays silent with this configuration. Since I have only one proxmark I have no idea what data is actually sent sad  Does anyone know that part of the pm3 code and can give me some hints about what might go wrong?

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