Send more than 261 bytes to Java Card with Android NFC

Question

I want to send an APDU with a length of 1699 bytes via NFC (class IsoDep) to a Java Card smartcard. I get the error

java.io.IOException: Transceive length exceeds supported maximum

My phone is a Samsung Galaxy S7.

I use extended-length in my applet on the card. I have verified that the card supports extended length. I tested this via pyapdutool by sending an APDU with 4000 bytes to the card.

I found that when I write this code, the result is false:

final Tag t = (Tag) tag;
myTag = IsoDep.get(t);
boolean result = myTag.isExtendedLengthApduSupported();

I have this in my manifest:

<activity
        android:name=".test"
        android:label="@string/title_test"
        android:launchMode="singleTop"
        android:theme="@style/AppTheme.NoActionBar" >
    <action android:name="android.nfc.action.TAG_DISCOVERED" />
    <category android:name="android.intent.category.DEFAULT" />
    <intent-filter>
        <action android:name="android.nfc.action.NDEF_DISCOVERED" />
    </intent-filter>
</activity>

How can I send an APDU with 1699 bytes (or any length greater than 261 bytes) through Android NFC?

Answer 1

Short answer: You can't easily do that.

You already found out that IsoDep does not "support" extended-length APDUs on your device (i.e. isoDep.isExtendedLengthApduSupported() returns false). In fact this does not mean that you can't send extended-length APDUs through the IsoDep object. It actually only means that the IsoDep object won't properly split extended-length APDUs across more than two ISO-DEP blocks and consequently an APDU that is longer than 261 bytes will be assumed to exceed the transmit buffer size. You should still be able to send extended-length APDUs with sizes <= 261 bytes.

So isoDep.isExtendedLengthApduSupported() actually indicates if you can send more than 261 bytes in one ISO-DEP transceive.

What you could do to overcome this problem is to not use the IsoDep object at all and, instead, manually implement ISO-DEP (the ISO/IEC 14443-4 transmission protocol) on top of the NfcA object (if your card is based on NFC-A / ISO/IEC 14443 Type A) or the NfcB object (if your card is based on NFC-B / ISO/IEC 14443 Type B and if your device supports exchanging data over the NfcB object). You could then split your extended-length APDUs into ISO-DEP blocks that are small enough for the transceive buffer of the NFC controller (typically 253 bytes including the header bytes, excluding the CRC bytes). However, handling the ISO-DEP protocol yourself also means that you have to take care of proper ISO-DEP activation, of handling block numbering, block acknowledgements, timeouts, waiting-time extension, etc. Which is rather complicated, and particularly timeouts are not easy to observe due to the delays through the Android NFC stack.