Java Card

Hi all,
perhaps someone can help me with the following problem:
I wrote an applet that does mutual challenge-Response. Everything works, although I get the response "6F00".
I also used a try-catch-block for Security Exceptions, with no sucess.

package challengeresponse;

import javacard.framework.APDU;
import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;
import javacard.framework.JCSystem;
import javacard.framework.Util;
import javacard.security.AESKey;
import javacard.security.KeyBuilder;
import javacard.security.RandomData;
import javacardx.crypto.Cipher;


public class ChallengeResponse extends Applet {
	
	// Instruction- and class-bytes
	final static byte CLASS = (byte) 0x80;
	final static byte INS_START = (byte) 0x01;
	final static byte INS_CHAL_READER = (byte) 0x02;
	
	// bArray contains (length of AID, AID, length of App. priv., App. priv., installation parameters)
	final static byte INST_PARAM_OFFSET = (byte) 0x0F;
	final static byte[] PADDING = {0x0F, 0x0F, 0x0F, 0x0F, 0x0F, 0x0F, 0x0F, 0x0F};
    
    private AESKey aeskey;
    private byte[] id;
    private byte[] challenge_card_stored;
    private RandomData challenge_card_sent;
    private Cipher cipher;
    
	private ChallengeResponse(byte bArray[], short bOffset, byte bLength) {
       
		// Extract AES key form installation parameters and build it
		aeskey = (AESKey) KeyBuilder.buildKey(KeyBuilder.TYPE_AES, KeyBuilder.LENGTH_AES_128, false);
		aeskey.setKey(bArray,(short) INST_PARAM_OFFSET);
		
		// Extract ID from installation parameters
		id = new byte[5];
		Util.arrayCopy(bArray, (short) (INST_PARAM_OFFSET + 16), id, (short) 0, (short) 5);
		
		// Use transient arrays for security reasons
		challenge_card_stored = JCSystem.makeTransientByteArray((short)8, JCSystem.CLEAR_ON_DESELECT);
		challenge_card_sent = RandomData.getInstance(RandomData.ALG_SECURE_RANDOM);
		cipher = Cipher.getInstance(Cipher.ALG_AES_BLOCK_128_ECB_NOPAD,false);
		
		// Register in card manager 
		register();
	}

	public static void install(byte bArray[], short bOffset, byte bLength)
			throws ISOException {
		
		// Install method has arguments (AES key, ID) in bArray
		new ChallengeResponse(bArray,bOffset,bLength);
	}

	public void process(APDU apdu) throws ISOException {
		
		byte[] apdu_buffer = apdu.getBuffer();

		// return if APDU is SELECT command
		if (selectingApplet())
			return;
		
        switch(apdu_buffer[ISO7816.OFFSET_INS]) {
			
        	case INS_START:
				
				// When start command is received do:
				
				// Create challenge and write it into apdu buffer and challenge_card_stored array as reference
				challenge_card_sent.generateData(apdu_buffer,(short)(ISO7816.OFFSET_CDATA), (short)8); 
				Util.arrayCopy(apdu_buffer,(short)(ISO7816.OFFSET_CDATA),challenge_card_stored,(short)0,(short)8); 
				           
				// Write padding
	            Util.arrayCopy(PADDING, (short)0, apdu_buffer, (short)(ISO7816.OFFSET_CDATA+8), (short)8);
				
				// Encrypt message
	            cipher.init(aeskey, Cipher.MODE_ENCRYPT);
	            cipher.doFinal(apdu_buffer, (short)ISO7816.OFFSET_CDATA, (short)16, apdu_buffer, (short)ISO7816.OFFSET_CDATA); 
	                        
				// Send message AES{challenge_card_stored,PADDING}
				apdu.setOutgoingAndSend(ISO7816.OFFSET_CDATA, (short)16);
        		
        	case INS_CHAL_READER:
(...)

I get the 6f00 response when I send the INS_START Command, the other commands (which I omitted here) work.

Thanks in advance!