APDU Case 2 Response too short

I'm using a ACR39T-A3 Smart Card Reader connected over USB to an Android device.

First I'm sending a APDU Case 2 command like this.

                int offset = 0;
                List<byte[]> dataList = new ArrayList<>(8);

             while (true) {



                    if (!responseString.contains("90 00")) {
                        break;
                    }


                    dataList.add(progress[0].response);




    int offsetStartInt = 0x7F & (offset >> 8);
    int offsetEndInt =  offset & 0xFF ;

    //SmartcardOS
    //T1
    byte[] apdu = new byte[]{
            (byte) 0x00, //CLA
            (byte) 0xB0, //INS
            (byte) offsetStartInt, //P1
            (byte) offsetEndInt, //P2
            (byte) 0xff, //LE

    };

     transmitApdu(apdu);
     offset += 256;

     }

Im trying to get a certificate from the Smartcard. The certificate can be bigger than 256 so i need send multiple. But as Response i just get 90 00 instead of the certificate in mulitple.

Can anybody help me? I don´t understand how does it work with the offset.

Solution

While I don't recognize your API (and responseString is somewhat too magically filled in my opinion), in principle it should work that way.

Since you specify LE as 0xFF, I recommend either

to set LE to zero instead or
to increment offset only by 255.

If your EF was not yet written to at all, some cards may return nothing but 90 00.