Similarly to TCP datagrams, the GDM on the NFC have this field called
'unknown' on 'proxmark3'. The value should be all zeroes then filled with a
checksum exactly how that is done on TCP networking.
The checksum on the GDM is a SipHash with the greater number of rounds
possible. The length is the length of the GDM minus one - 16 bytes or
15 length. The input is the GDM pseudogram with all zeroes. The key is the
device name.
The GDM pseudogram is introduced both on the HMAC-256 and on the SipHash.
The length on the SipHash needs to be considered.
Considering the "key B" field is 0x5A, the 'unknown' value is a CRC-16
and the third byte can be arbitrarily chosen.
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MLO