Interpreting a java card HelloWorld applet

Question

Below, you see a java card program that returns "Hello Word" when it receive APDU Command = 8000000000 ( its source )

package helloWorldPackage;

import javacard.framework.APDU;
import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;
import javacard.framework.Util;

public class HelloWorldApplet extends Applet 
{
    private static final byte[] helloWorld = {(byte)'H',(byte)'e',(byte)'l',(byte)'l',(byte)'o',(byte)' ',(byte)'W',(byte)'o',(byte)'r',(byte)'l',(byte)'d',};
    private static final byte HW_CLA = (byte)0x80;
    private static final byte HW_INS = (byte)0x00;

    public static void install(byte[] bArray, short bOffset, byte bLength) 
        {
        new HelloWorldApplet().register(bArray, (short) (bOffset + 1), bArray[bOffset]);
        }

    public void process(APDU apdu) 
        {
        if (selectingApplet()) 
            {
            return;
            }

        byte[] buffer = apdu.getBuffer();
        byte CLA = (byte) (buffer[ISO7816.OFFSET_CLA] & 0xFF);
        byte INS = (byte) (buffer[ISO7816.OFFSET_INS] & 0xFF);

        if (CLA != HW_CLA)
            {
            ISOException.throwIt(ISO7816.SW_CLA_NOT_SUPPORTED);
            }

        switch ( INS ) 
            {
            case HW_INS:
                getHelloWorld( apdu );
                break;
            default:
                ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
            }
        }

    private void getHelloWorld( APDU apdu)
        {
        byte[] buffer = apdu.getBuffer();
        short length = (short) helloWorld.length;
        Util.arrayCopyNonAtomic(helloWorld, (short)0, buffer, (short)0, (short) length);
        apdu.setOutgoingAndSend((short)0, length);
        }
}

I understand it, but I can't understand why the programmer used &0XFF in the lines :

        byte CLA = (byte) (buffer[ISO7816.OFFSET_CLA] & 0xFF);
        byte INS = (byte) (buffer[ISO7816.OFFSET_INS] & 0xFF);

Why he normally doesn't use the below line instead?

        byte CLA = (byte) (buffer[ISO7816.OFFSET_CLA]);
        byte INS = (byte) (buffer[ISO7816.OFFSET_INS]);

And also in the line :

ew HelloWorldApplet().register(bArray, (short) (bOffset + 1), bArray[bOffset]);

What does he mean by the +1 ?

Answer 1

Although we cannot see intent of an author, the line:

    byte CLA = (byte) (buffer[ISO7816.OFFSET_CLA] & 0xFF);

is 100% equivalent to:

    byte CLA = buffer[ISO7816.OFFSET_CLA];

Java does often use integers as result of operations, and because Java Card usually does not support int values, it is often needed to cast to byte or short . I can only guess that the & 0xFF and cast are present because of an over-enthusiastic attempt to get rid of intermediate int values. It could also be a bad attempt to get Java to support unsigned bytes.

---

The register method expects the instance AID. That AID is within the user parameters given during the Global Platform INSTALL for INSTALL , but it is preceded by a byte containing the length of the AID (between 5 and 15, inclusive). So the + 1 is to skip that length byte - which is in turn present at the last argument of the register method: bArray[bOffset] .