Clarification on Padding in SDMReadCtr of NTAG 424 DNA Documentation

[napangelo]

Hi,

I have been reviewing the documentation for the NTAG 424 DNA, specifically referring to the example provided on page 12 regarding the decoding of PICCData (https://www.nxp.com/docs/en/application-note/AN12196.pdf).

In the document, an example of decrypted PICCData is given as follows:

• Decrypted PICCData: c704de5f1eacc0403d0000da5cf60941
• PICCDataTag: c7
• UID: 04de5f1eacc040
• SDMReadCtr: 3d0000

My inquiry pertains to the SDMReadCtr's padding. Specifically, I am trying to understand how to accurately identify where the padding of zeros begins for the SDMReadCtr, as converting from hex to decimal yields various results based on the assumed start of padding (e.g., 3d = 61, 3d0 = 976, etc.).

Could you please provide guidance on how to differentiate the actual counter value from the padding within the SDMReadCtr? For example, in "SDMReadCtr: 3d0000", how can I distinguish between the hex counter value and the padding to accurately interpret the counter's value?

Thanks,

Angelo Napolano

[danielchen]

Hi Angelo Napolano:

SDMReadCtr is a 24-bit unsigned integer.  The SDMReadCtr is reset to 000000h when enabling SDM with ChangeFileSettings.  In cryptographic calculations and represented with binary encoding on the external interface, the SDMReadCtr is represented LSB first.
When represented with ASCII encoding on the contactless interface, it is represented
MSB first.

Regards

Daniel