Intro/Goal:
Hi, my name is Manuel and I am software engineer. Recently I bought some of your NTAG 424 DNA, that I want to use for a product authenticity solution.
Documents/Material:
I read through some documents but mainly the following two:
https://www.nxp.com/docs/en/data-sheet/NT4H2421Gx.pdf
https://www.nxp.com/docs/en/application-note/AN12196.pdf
Hardware:
I bought a Identiv u Trust 3700 F CL reader that is connected to my MacBook Pro and i am running the NXP TagXplorer.
Configuration:
Using this as an example URL, where I have 3 custom params at the end, "n", "t", "p":
my-test-for-tags.com/scan?uid=00000000000000&ctr=000000&cmac=0000000000000000&n=2&t=1&p=3
I activated "ADD TAG UID", "ADD INTERACTION COUNTER" and "ENABLE SUN MESSAGE". I also set the calculated offset Index at 33 which is right after the questionmark in the URL (i hope this is correct?!)
after writing this information to the tag i am going to NTAG 424 DNA Secruity settings where i hit "Authenticate First"
(for this first try i dont want to change any of the default keys on the chip, that would be my next step once i get this simple example working)
After being successfully authenticated
I am going to the "File Management" options and set the following data:
Problem:
When i go now to NDEF Operations and click on READ NDEF button, it will give me for example this URI here:
https://my-test-for-tags.com/scan?uid=044B6A4A4E6880&ctr=000021&cmac=3E12626CBBFB3FB9&n=002&t=1&p=3
if i click the Read from Tag button again it gives me:
https://my-test-for-tags.com/scan?uid=044B6A4A4E6880&ctr=000022&cmac=211167FD30F03BEA&n=002&t=1&p=3
which gives me the same uid, but an incremented counter and a new cmac which i guess is correct.
This data is now being sent to the backend, but how can i now recalculate the cmac(3E12626CBBFB3FB9) given in the URI from the two params uid(044B6A4A4E6880) and ctr(000021) in my backend?
I guess I also need to know the key that was used to encrypt it also on the backend, correct?
I guess in this default case the key is just 16bits of zero like this 0000000000000000, correct?
But how exactly would this calculation now work? I cant figure it out with the two documents above.
Can you please give me a step by step example for this cmac calculation on my backend with my values given here?
Thanks for your help in advance.
Hi Manuel Mertl,
Hope you are doing great, I believe the issue you are having is with the "Zero length input" Mac, for this I believe there is a pseudo code in the AN mentioned before, please check the following:
this step
translates to the following.
in this case you first need to calculate the mac on only 00's. so first you are going to do the following:
once you have this value you need to get the K1 and K2 values:
this are going to be used in the next step which is getting the M1 Value,
then you are going to calculate the MAC I used AES CBC 128,
Hope this is better, and help you understand this process, some calculators and software for this support the zero length input Mac so you only need to set some flag or just send the mac in 00s and they do it but it all depends on the sw used.
BR
Jonathan
Hi Manuel Mertl,
Hope you are doing great, as we discussed in our private messages please check this :
in the application note you can check 4.4.4.2 this should work for you let me explain how, there are different ways to do the SUN I will show the first two figures showed in AN.
A.- is the way you are working
B.- is explained in the 4.4.4.2 example.
in this case the UID and CTR are encrypted and sent as "e" Variable.
since you are sending the information in plain you can skip some steps of table 5.
so basically you can go directly into step 11 because the previous steps Decrypt the information and get the UID, CTR, etc. in this case you already have that data because is in plain, so first you will do the SV2, as explained in step 11
the you will need to create the session key for MACing like in step 12
which translates to this
then you will proceed to do the step 13
which I explained in the previous post.
please let me know if this is clear to you or if you have any questions.
Have a nice day !
BR
Jonathan
Thanks for the answer, but i have some follow up questions to that:
1) is the start of the SV2 value always: 3CC3 0001 0080 no matter what UID and Counter etc. i use? (so can i hard code this part?)
2) When calculating the KsesSdmFileReadMAC ... is this using the calculateMFMAC Java Function?
If so then the key-paramter would be 00000000000000000000000000000000 and the valueToMac-paramter would be 3CC30001008004DE5F1EACC0403D0000 ?
I tried it like this, but it gives me a different value:
I get B5E3073D35AC936F instead of 3FB5F6E3A807A03D5E3570ACE393776F
Hi Manuel Mertl,
for your first question, the basic answer is yes, this is defined in the DS section 9.3.9.1
this is the previous step to session keys calculation, the SV1 is used for the session encryption and the SV2 is used for session MACing, this is not the MAC this is used to generate the key to MAC data during a session. for example next time a customer taps on the tag the SV will change due to the ctr changes, so each tap will generate a different key therefore a different key for encryption and MAC.
for your second question the calculation should be correct, as you can see in my procedure for step 12 I see your calculations are correct,
as you can see here you split the key into odd bytes, the odd bytes are correct so the entire key should be correct, please dont do this here:
that is done at the end of step 13, not here, because you need the 16 bytes here for the step 13 calculations as you can see the values on step 12 are used as key on step 13. then the result of 13 is split into odd bytes to have 8 bytes of data.
Hope I am being clear here if you still have questions let me know. I would highly recommend checking the process on the DS and AN.
Hope you are doing great.
BR
Jonathan
Thank you so much for your explanation jonathaniglesias, we are nearly there :smileygrin:
I was finally able to recalculate the values from the document using this code below (for anyone interested):
receiving the folllowing output:
-------------------------------------------------
but I tried now to change my value for SV2 according to my first post all the way up here:
so instead of using the following value from the example:
i used the uid(044B6A4A4E6880) and ctr(000021) from my example up there, leaving the other code the same:
which gave me the following result when i scanned the tag itself:
https://my-test-for-tags.com/scan?uid=044B6A4A4E6880&ctr=000021&cmac=3E12626CBBFB3FB9&n=002&t=1&p=3
https://my-test-for-tags.com/scan?uid=044B6A4A4E6880&ctr=000021&cmac=3E12626CBBFB3FB9&n=002&t=1&p=3
but my java app outputs the following:
so 3E12626CBBFB3FB9 != 704A92FA6DBBDB35
:smileysad:
Do you see any mistakes in my configuration screenshots from the first post? Like Offset numbers, wrong keys, etc?
Please find here again the screenshots:
Hi Manuel,
Hope you are doing great, well I think the problem is you are using the CTR in the wrong way I just followed your data like this
After Step 13 I get :
683e8e12a962a26c5ebb0afb343f90b9
68 3e 8e 12 a9 62 a2 6c 5e bb 0a fb 34 3f 90 b9
cmac = 3e 12 62 6c bb fb 3f b9
https://my-test-for-tags.com/scan?uid=044B6A4A4E6880&ctr=000021&cmac=3E12626CBBFB3FB9&n=002&t=1&p=3
Which is equal to your calculation.
BR
Jonathan
