Creating an Applet That Can Send and Receive Extended Length APDUs

To create an applet that can send and receive extended length APDUs:

1. Implement the javacardx.apdu.ExtendedLength interface in your applet:

   ...
   import javacard.framework.*;
   import javacardx.apdu.ExtendedLength;
   ...
   public MyApplet extends Applet implements 
   ExtendedLength {
   ...
   }
   
   

2. Write your applet and Applet.process(..) method as you would with any other applets. For consistency, it is advisable that your process(..) code begin like the one below:

   public void process(APDU apdu) {
      byte[] buffer = apdu.getBuffer();
    
      if (apdu.isISOInterindustryCLA()) {
          if (this.selectingApplet()) {
              return;
          } else {
              ISOException.throwIt (ISO7816.SW_CLA_NOT_SUPPORTED);
          }
      }
    
      switch (buffer[ISO7816.OFFSET_INS]) {
      case CHOICE_1: 
          ...
          return;
      case CHOICE_2: 
          ...
          ...
      default: 
          ISOException.throwIt (ISO7816.SW_INS_NOT_SUPPORTED);
      }
   }
   
   
   

3. For cases 3S, 4S, 3E and 4E, write the method to handle incoming data. Use the API so that your applet properly handles extended, as well as non-extended, cases.

   void receiveData(APDU apdu) {
       byte[] buffer = apdu.getBuffer();
       short LC = apdu.getIncomingLength();
    
       short recvLen = apdu.setIncomingAndReceive();
       short dataOffset = apdu.getOffsetCdata();
    
       while (recvLen > 0) {
           ...
           [process data in buffer[dataOffset]...]
                   ...
                   recvLen = apdu.receiveBytes(dataOffset);
       }
       // Done 
   }
   
   

4. For case 2S, 2E, write the method handling data output. A method could look something like this:

   void sendData(APDU apdu) {
      byte[] buffer = apdu.getBuffer();
    
      short LE = apdu.setOutgoing();
      short toSend = ...
    
      if (LE != toSend) {
          apdu.setOutgoingLength(toSend);
      } 
    
      while (toSend > 0) {
          ...
          [prepare data to send in APDU buffer] 
                  ...
                  apdu.sendBytes(dataOffset, sentLen);
                  toSend -= sentLen;
     }
     // Done
   }