Java Card Read Data from Array

Question

I have created an Smartcard application where i can store data up to 60KB in an Byte Array. But when i read the Array multiple Times i get an error and i cant access the Data anymore.

Code create Array:

public void createFile(short fileID, short fileSize) {
    short index = getFileIndex(fileID);

    if(listFiles[index] == null) {
        listFiles[index] = new byte[fileSize];
    }

    listfileSizes[index] = fileSize;
}

Code Read Data:

public byte[] readDataFromFile(short fileID, short fileOffset, short length) {

    short selFileSize = getFileSize(fileID);
    byte[] data = new byte[length];

    if (selFileSize < (short)(fileOffset + length)) {
        ISOException.throwIt(ISO7816.SW_FILE_FULL); 
    }

    Util.arrayCopy(getFile(fileID), fileOffset, data, (short)0, length);

    return (byte[])data;
}

Code access Read:

short data_length = Util.getShort(buf, (short)(offset_cdata + 2));
    short file_offset = Util.getShort(buf, offset_cdata);

    if(p2 == (byte)0x01) {

        Util.arrayCopy(myfile.readDataFromFile(myfile.keepassData1, file_offset, data_length), (short)0, buf, (short)0, data_length);

    } else if (p2 == (byte)0x02) {

        Util.arrayCopy(myfile.readDataFromFile(myfile.keepassData2, file_offset, data_length), (short)0, buf, (short)0, data_length);

    }

when i reinstall the app i can read and write, but only a few time until the data is blocked. I get the error 6f00.

Answer 1

Your applet is running out of the persistent memory, hence the error.

This line

byte[] data = new byte[length];

allocates a new persistent byte array each time the method is called! This object is never deallocated, because Java Card does not support any automatic garbage collector.

You should copy the data directly into the APDU buffer:

private final byte[] readDataFromFile(short fileID, short fileOffset, short length, byte[] outBuffer, short outOffset) {
    final short selFileSize = getFileSize(fileID);
    if (selFileSize < (short)(fileOffset + length)) {
        ISOException.throwIt(ISO7816.SW_FILE_FULL); 
    }
    Util.arrayCopyNonAtomic(getFile(fileID), fileOffset, outBuffer, outOffset, length);
}