Data representation on NFC tag when writing with Android

Question

MIFARE Classic 1K memory cards have multiple sectors to store data, so I want to know how the data is allocated and stored into the sectors of MIFARE Classic 1k when writing NDEF data to the tag in Android unsing the following command:

Ndef ndef = Ndef.get(tag);
ndef.writeNdefMessage(message);

Answer 1

As MIFARE Classic is no standard NFC tag, there is no standardized way for storing NDEF formatted data on such cards. However, NXP provides two datasheets that explains NXP's proprietary NDEF mapping for MIFARE Classic tags:

• NFC Type MIFARE Classic Tag Operation
• MIFARE Classic as NFC Type MIFARE Classic Tag

The mapping descibed in these datasheet is widely used with devices that are capable of reading/storing NDEF messages on MIFARE Classic and are also used in Android devices with NXP's NFC chipset.

The basic principles of the NDEF mapping are:

1. Create a MIFARE application directory (in block 0 (and 16 for 4K cards), readable with key A = 0xA0A1A2A3A4A5).
2. Within the MAD, mark all sectors that you want to be capable of storing NDEF data as belonging to application 0xE103.
3. All NDEF sectors must be readable with key A 0xD3F7D3F7D3F7 and must have their GPB set to 0x40 (if read/write access is permitted) or 0x43 (if only read is permitted).
4. The blocks 0-2 of all NDEF sectors represent the storage for TLV structures (similar to those on NFC Forum compliant NFC tags).
5. The TLV structure for storing an NDEF message has a tag of 0x03, so an NDEF message would be embedded as <0x03> <LEN (1 Byte)> <NDEF MESSAGE> (or <0x03> <0xFF> <LEN (2 Bytes)> <NDEF MESSAGE>.
6. The last TLV structure is followed by a Terminator TLV, a single byte 0xFE.