NRF.nfcRaw (and nfcUrl) limited to payload/message length 255

[fanoush]

see also conversation https://forum.espruino.com/conversations/354213/#17099032

https://github.com/espruino/Espruino/blob/master/libs/bluetooth/jswrap_bluetooth.c#L2638 flatStrPtr is uint8_t * so the dataLen is truncated even if data buffer is allocated with full length here https://github.com/espruino/Espruino/blob/master/libs/bluetooth/jswrap_bluetooth.c#L2626 and also terminated with full length here https://github.com/espruino/Espruino/blob/master/libs/bluetooth/jswrap_bluetooth.c#L2641

same bug/feature is there for nfcUrl - two lines here truncate length to uint8_t https://github.com/espruino/Espruino/blob/master/libs/bluetooth/jswrap_bluetooth.c#L2456-L2457

If this should not be supported then at least the length should be checked and rejected to not to allocate big buffers and terminate NDEF at wrong place.

[RonnyNewco]

The Pucket.js with the Nordic Semiconductor chip set supports up to 992 bytes. Limiting the usable size to 246 bytes should be avoided.

[fanoush]

When checking the source and nfc tag 2 type an NDEF structures the header here https://github.com/espruino/Espruino/blob/master/libs/bluetooth/bluetooth.h#L319 is somewhat mixed bag regarding the length.

At position 0x10 there is 0x03 0x00 with comments TT - tag type (03), ML message len (0) The ML byte is set from code as message length. So this allows only 1 byte.

And then there is TF = TNF and Flags = 0xC1. In this the SR bit in not set so the payload length then takes 4 bytes.

So the first part of the header does not allow size over 254 and then the NDEF payload is in long format to allow more, but only one byte is written in code anyway.

However If I understand it correctly the TT+ML is in fact something called TLV block described here so the ML byte should be only up to value 254 since value 255 has special meaning of TLV_L_FORMAT_FLAG which says there are next two bytes having 16 bit length (?)

So my guess is to fix that

• the payload is ok but we need to write long value (in big endian format?) there
• the NFC 2 tag structure 0x03 0x00 could also be 0x03 0xff 0x00 0x00 and then the length could be 16 bit value (?)

[RonnyNewco]

This s a dump from a tag I did successfully wrote to. Hopefully helpful on those questions...609 bytes payload..

[fanoush]

also found this https://stackoverflow.com/a/42108779 where it confirms my guess

The tag for this TLV is 0x03. The length can be either in one-byte format (for NDEF messages with a length between 0 and 254 bytes) or in three-byte format (for NDEF messages with a length of 255 or more bytes).

If you have longer NDEF message (e.g. one with 259 bytes), you would use the three-byte length format: 03 FF0103

So it should work, however I am not sure what happens when for simplicity we will encode length <255 in longer format i.e. 254 as 0x03 0xff 0x00 0xFE - should be valid but maybe it may confuse some implementations?

[RonnyNewco]

If it works with IOS/Android which can be tested my assumption is that it should be fine...

[fanoush]

found NFCForum-TS-Type-2-Tag_1.1.pdf specification here https://github.com/somq/nfccard-tool/blob/master/docs/NFCForum-TS-Type-2-Tag_1.1.pdf and it says so using longer format for smaller size is not as described there and may break some validation

[RonnyNewco]

Tried to work around that by right padding with space so the long format could allways be used but that hits browser specfic behaviour. Works with chrome but not with Safari/Firefox that sends this along and them som sites will crash. So this workaround is not viable...

[fanoush]

@RonnyNewco if you want you can try PUCKJS build from https://github.com/espruino/Espruino/pull/2407/checks - download PUCKJS artifact here it is dfu zip inside another zip

This code works for me in build for AMIIBOLINK NFC tag mentioned in the forum. I tried urls with ndef record length of 254,255 and up like this

>NRF.nfcURL("http://espruino.com?param="+'x'.repeat(254-22-5)+'z')
=undefined
>global['\xff'].NfcData
="_\xF2\xC8\xEDe\xE2\x86ed\3\xFF\xFF\xE1\x11|\x0F\3" ... "xxxxxxxxxxxxxxxz\xFE"
>NRF.nfcURL("http://espruino.com?param="+'x'.repeat(254-22-5-1)+'z')
=undefined
> 

my phone shows it just fine in NXP TagInfo app

[RonnyNewco]

I am pretty much a novice in this domain. But when trying to update I get the following error message:

No manifest.json file found when selecting the zip file.

solved This can be fixed by extracting and repackaging as the firmware loader assumes a flat structure and not a sub catalogue...

[fanoush]

Yes, sorry, as mentioned it is zip inside zip, unpack the first one and use the one inside for DFU. Do not unpack the second one. EDIT: oh you already solved it, great.

[RonnyNewco]

Perfect. Works as intended. Thank you very much for your effort. Much appropriated.