Peer-to-peer communication using NFC in Android 4.4

With the new Host-based card emulation Android opened up a possibility for bidirectional peer-to-peer communication using NFC. Since I now have access to a Nexus 5 and Nexus 7 both running Android Version 4.4 I gave it a try.

Based on my previous example using the ACR122 as the card reader I was able to quickly modify it to adapt to this new functionality.

EnableReaderMode

There are basically two things you have to do to be able to establish an bidirectional NFC connection. Firstly, configure the host-based card emulation mode like described in one of my previous blog posts. Secondly, create an Activity that will enable the new Reader-Mode in Android 4.4 and create a reader callback that will handle the discovered tag as an IsoDep tag.

The code is really simple. For a full example, please have a look at the github project.

@Override
public void onResume() {
	super.onResume();
	nfcAdapter.enableReaderMode(this, this, NfcAdapter.FLAG_READER_NFC_A | NfcAdapter.FLAG_READER_SKIP_NDEF_CHECK,
			null);
}

@Override
public void onPause() {
	super.onPause();
	nfcAdapter.disableReaderMode(this);
}

@Override
public void onTagDiscovered(Tag tag) {
	IsoDep isoDep = IsoDep.get(tag);
	IsoDepTranceiver transceiver = new IsoDepTransceiver(isoDep, this);
	Thread thread = new Thread(transceiver);
	thread.start();
}

In your transceiver thread you first connect to the tag and then issue a SELECT AID APDU using your Application ID. This looks like this:

private static final byte[] CLA_INS_P1_P2 = { 0x00, (byte)0xA4, 0x04, 0x00 };
private static final byte[] AID_ANDROID = { (byte)0xF0, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 };

private byte[] createSelectAidApdu(byte[] aid) {
	byte[] result = new byte[6 + aid.length];
	System.arraycopy(CLA_INS_P1_P2, 0, result, 0, CLA_INS_P1_P2.length);
	result[4] = (byte)aid.length;
	System.arraycopy(aid, 0, result, 5, aid.length);
	result[result.length - 1] = 0;
	return result;
}

@Override
public void run() {
	int messageCounter = 0;
	try {
		isoDep.connect();
		byte[] response = isoDep.transceive(createSelectAidApdu(AID_ANDROID));
		while (isoDep.isConnected() && !Thread.interrupted()) {
			String message = "Message from IsoDep " + messageCounter++;
			response = isoDep.transceive(message.getBytes());
			onMessageReceived.onMessage(response);
		}
		isoDep.close();
	}
	catch (IOException e) {
		onMessageReceived.onError(e);
	}
}

Conclusion

I hope this example will allow you to write some awesome NFC apps. Please check the full example at my GitHub repo and comment if you have any questions.