NTAG 424 DNA: Truncated MAC Calculation

phillipn — Mon, 20 Nov 2023 04:23:25 GMT

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

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

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,

And for 5.3 CommMode.MAC,

And for 5.4 CommMode.Full,

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

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!

Re: NTAG 424 DNA: Truncated MAC Calculation

danielchen — Mon, 27 Nov 2023 08:13:10 GMT

please note, 4.4.4.2, zero length input

Regards

Daniel