I want get the UID of the Mifare Ultralight NFC tag. In Java I have this code:
TerminalFactory factory = TerminalFactory.getDefault();
List<CardTerminal> terminals = factory.terminals().list();
System.out.println("Terminals: " + terminals);
CardTerminal terminal = terminals.get(0);
Card card = terminal.connect("*");
System.out.println("card: " + card);
CardChannel channel = card.getBasicChannel();
ResponseAPDU answer = channel.transmit(new CommandAPDU(0xFF, 0xCA, 0x00, 0x00, 0x00));
byte[] uid = answer.getBytes();
The problem is that I receive two bytes and not the UID. What's the problem? Is the APDU correct?
The command you are actually using is not what you might have expected.
The correct command APDU to get the UID/serial number/enumeration identifier with this reader is:
+------+------+------+------+------+
| CLA | INS | P1 | P2 | Le |
+------+------+------+------+------+
| 0xFF | 0xCA | 0x00 | 0x00 | 0x00 |
+------+------+------+------+------+
However, the constructor you are using is defined as:
public CommandAPDU(int cla, int ins, int p1, int p2, int ne);
So with
new CommandAPDU(0xFF, 0xCA, 0x00, 0x00, 0x00)
you are creating a C-APDU with the following parameters CLA = 0xFF, INS = 0xCA, P1 = 0x00, P2 = 0x00. So far this is the same as the above APDU. But the last parameter is Ne = 0x00. Ne = 0 means that the number of expected response bytes is zero (whereas Le = 0 would mean that the number of expected response bytes is (up to) 256).
This results in effectively creating the following Case-1 APDU:
+------+------+------+------+
| CLA | INS | P1 | P2 |
+------+------+------+------+
| 0xFF | 0xCA | 0x00 | 0x00 |
+------+------+------+------+
So at most you will get the 2-byte status word as a response (either indicating success with 0x90 0x00 or indicating an error with a status code like 0x6X 0xXX).
So you can either use a byte array to form your APDU:
new CommandAPDU(new byte[] { (byte)0xFF, (byte)0xCA, (byte)0x00, (byte)0x00, (byte)0x00 } )
Or you can specify a proper value for Ne:
new CommandAPDU(0xFF, 0xCA, 0x00, 0x00, 256)