How does the NDEF Header work when the Payload is bigger than 1 Byte?

Question

I am trying to understand how a Mifare Ultralight Chip, I've got, works. This page pretty much explains it for normal circumstances: https://learn.adafruit.com/adafruit-pn532-rfid-nfc/ndef (At the very end of the page)

But now I have the problem that I want to write data on the chip that is bigger than 254 Byte. As an example, I am writing 493 'A's on it (With the 7 Byte Header, the Data will be 500 Byte overall). Here's what I get from reading the chip (the numbers are in decimal, not hex, so instead of '0xFF' it will say '255'):

0:________  4  34  42 132 144   0 

1:________186 144 107 129 144   0 

2:________192  72   0   0 144   0 

3:________225  16 109   0 144   0 

4:________  3 255   1 247 144   0 

5:________193   1   0   0 144   0 

6:________  1 240  84   2 144   0 

7:________101 110  65  65 144   0 

8:________ 65  65  65  65 144   0 ...`

What I understand is, the first 4 blocks are the Header, which contains serial number, etc. Then the fifth block starts with a 3, which means "NDEF Message", and then I get the size.

For example, if I just write one 'A' on the chip, the given size will be 1 Byte payload + 7 Byte header = 8 Byte overall. So the 5th Block would look like this:

4:________  3   8 209   1 144   0
(As the size doesn't exceed 254 byte, the 209 and 1 are not part of that size byte anymore)

but when I am writing 248 'A's, the overall size will be 255 bytes, which changes the 5th block to this:

4:________  3 255   0 255 144   0

First I thought that this would indicate that I need to calculate x * y + z where in this example, x = 255, y = 0 and z = 255, so 255 * 0 + 255 cause that would give me 255, the overall size, but when I add one 'A', so the size is 256 Bytes, the header changes to:

4:________  3 255   1   0 144   0

This again would indicate that the calculation is x * y + y + z, 255*1+1+0 = 256.

But then again, when I use 659 'A's to get 666 Byte, I get this:

4:________  3 255   2 157 144   0

This again doesn't work for either of these algorithms:

255*2+157 = 667
255*2+2+157 = 669

And as a last example: 476 Bytes:

4:________  3 255   1 223 144   0

255 * 1 + 223 = 478
255 * 1 + 1 + 223 = 479

Can anyone explain to me what I am doing wrong or why I just can't get the correct algorithm that leads me back to the Overall size?

score 0 · Answer 1 · answered Sep 16 '20 at 12:56

This is difficult to answer without the exact chip type and then examining it's datasheet.

But I think you are probably getting confused by reading the information from Adafruit as is not the full specification of all the possibilities and it is for a Mifare Classic 1K Card which is not a NFC Standard Type chip and not the same as an Ultralight . How a NDEF message is put on this chip is propitiatory for Classic cards and is fully detailed at https://www.nxp.com/docs/en/application-note/AN1304.pdf

For an Utralight chip are compatible with NFC Type 2 specification, details how it is compatible is at https://www.nxp.com/docs/en/application-note/AN1303.pdf

For the full NFC Type 2 Spec is available at http://apps4android.org/nfc-specifications/NFCForum-TS-Type-2-Tag_1.1.pdf

The full NDEF spec is also available at https://github.com/haldean/ndef/blob/master/docs/NFCForum-TS-NDEF_1.0.pdf

So there are number of factors that will affect the size calculation including Short of Standard Records, Chunking, etc.

Hopefully if you read all the correct documentation you will be able get an answer as that Adafruit link is not right for your chip and a very simplified explanation.

Also note that writing the numbers in Decimal not the standard Hex make it a very difficult read as most specs are detailed in Hex.

score 0 · Accepted Answer · answered Sep 17 '20 at 05:24

I found it out. It's not about multiplying and adding, it's about looking at it in binary.

First, the first full byte isn't directly included in the calculation. What counts are the other two bytes.

In the 476 Byte Example, I got

4:________  3 255   1 223 144   0

This means that I have to look at 1 and 223 in binary:

  1 = 0000 0001
223 = 1101 1111

Now I have to merge them together:

0000 0001 1101 1111 = 479

If the overall size is over 259 Byte, 3 Extra bytes get added (idk why, but it's how my chip works. It uses 1 or 4 bytes, depending on size)

The other example were 666 + 3 Byte = 669:

4:________  3 255   2 157 144   0

  2 = 0000 0010
157 = 1001 1101

Total: 0000 0010 1001 1101 = 669

How does the NDEF Header work when the Payload is bigger than 1 Byte?

2 Answers2