NTAG 424 DNA: Truncated MAC Calculation

[phillipn]

In the Application Note (AN12196), it defines the calculation of MACt in section 3 as below:

phillipn_0-1700453559637.png

In section 4.4.4.2, where CMACInputOffset == CMACOffset, in the SDMMAC calculation, it shows the example,

phillipn_1-1700453613694.png

You will notice that the SDMMAC (94EED9EE65377086) doesn't contain the even bytes of the session key K (3FB5F6E3A807A03D5E3570ACE393776F).

However, the defined pattern holds for section 4.4.4.2.2, where CMACInputOffset != CMACOffset,

phillipn_2-1700453724071.png

And for 5.3 CommMode.MAC,

phillipn_3-1700453826039.png

And for 5.4 CommMode.Full,

phillipn_4-1700453867653.png

And for 6.8.2 Write NDEF File - using Cmd.WriteData, CommMode.FULL

phillipn_5-1700453940701.png

And others. 

Why is it the case that the Truncated MAC calculation for the example in 4.4.4.2 is different than the others? 

Thanks!

[danielchen]

Hi @phillipn 

please note,  4.4.4.2,  zero length input

danielchen_0-1701072748616.png

Regards

Daniel