Question

I am developping an Android app, and I need to encode and decode a bytes array in a QRCode generated with the ZXing app. My problem is that my message decoded does not exactly match the generated byte array.
I tried to create a QRCode based on a byte array containing incrementing indexes, i.e.

input = [0, 1, 2, ..., 124, 125, 126, 127, -128, -127,... -3, -2, -1, 0, 1, 2, ...]

And after encoding the message in the QRCode and decoding it on the responder side, I obtain the following byte array output:

output = [0, 1, 2, ..., 124, 125, 126, 127, 63, 63,... 63, 63, 63, 0, 1, 2, ...]

All the “negative” byte values are turned to ASCII char 63: ‘?’ question mark characters.
I assume that something is going wrong with the encoding charset, but since I am using ISO-8859-1 which everyone claims to be the solution of such kind of issue (other topic treating the same kind of issue or here), I don’t see where is my mistake, or if I am skipping a step during the instanciation of the encoding or the decoding.
Here is the code that I execute to encode a given byte array:

String text = "";
byte[] res = new byte[272];
for (int i = 0; i < res.length; i++) {
    res[i] = (byte) (i%256);
}
try {
    text = new String(res, "ISO8859_1");
} catch (UnsupportedEncodingException e) {
    // TODO
}
Intent intent = new Intent(Intents.Encode.ACTION);
Intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET);
intent.putExtra(Intents.Encode.TYPE, Contents.Type.TEXT);
intent.putExtra(Intents.Encode.FORMAT, "ISO8859_1");
intent.putExtra(Intents.Encode.DATA, text);
intent.putExtra(Intents.Encode.FORMAT, BarcodeFormat.QR_CODE.toString());

boolean useVCard = intent.getBooleanExtra(USE_VCARD_KEY, false);
QRCodeEncoder qrCodeEncoder = new QRCodeEncoder(activity, intent, dimension, useVCard);
Bitmap bitmap = qrCodeEncoder.encodeAsBitmap();

And to decode a QRCode, I send the following Intent

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.qrcodeDecoding);

    Intent intent = new Intent(Intents.Scan.ACTION);
    intent.putExtra(Intents.Scan.MODE, Intents.Scan.QR_CODE_MODE);
    startActivityForResult(intent, 0);
}

And wait for Result:

@Override
protected void onActivityResult(int request, int result, Intent data)
{
    if(request == 0)
    {
        //action
        if(result == RESULT_OK)
        {
            String res = data.getStringExtra(Intents.Scan.RESULT);
            byte[] dat = null;

            try{
                    dat = res.getBytes("ISO8859_1");
            } catch(UnsopportedEncodingException e) {
                    //TODO
            }
        }
        else if(result == RESULT_CANCELED)
        {
            //TODO
        }
    }

}

Could you please tell me where are my mistakes, or where should I look at?

Thank you a lot,

Franck

Answer 1

You are making the mistake of thinking that you can turn arbitrary binary data into a valid string without using some kind of armouring. It doesn’t work. Binary -> text -> binary is lossy using any of the standard character sets / encoding. (Hint: using UTF-8 won’t work either.)

You should use something like base64 encoding or hexadecimal encoding to ensure that the binary data doesn’t get mangled.