如何通过Android NFC API在MifareClassic上构建一个价值块

Question

我的问题是Android NFC API已经提供了“增量”和“递减”值块，但如果我有一个新的MifareClassic标签（它内部没有任何值块），我如何使用Android NFC API来构建这个新标签的价值块？

Answer 1

You should just write properly formatted data to the tag. See section 8.6.2 of the MIFARE Classic datasheet for an example.

Example Android code to store an integer value as value block in block blockIndex :

// connect to the tag using a Tag object from an NFC intent
MifareClassic mifare = MifareClassic.get(tag);
mifare.connect();

// need to authenticate first to get access
int sector = blockToSector(blockIndex);
mifare.authenticateSectorWithKeyA(sector, keyA); // you need to know key A
// mifare.authenticateSectorWithKeyB(sector, keyB); // in case you know key B

// construct value block of value zero; "address" byte is set to 0 in this example
byte[] zeroValue = {0, 0, 0, 0, 255, 255, 255, 255, 0, 0, 0, 0, 0, 255, 0, 255 };
mifare.writeBlock(blockIndex, zeroValue);

// increase the value block by some amount
mifare.increment(blockIndex, value);
// result is stored in scratch register inside tag; now write result to block
mifare.transfer(blockIndex);

Keep in mind that the access control bits for the block need to be set correctly to allow the increment operation for the key you use to authenticate.