Question

Abstract:

• reading images from file
• with toggled bits to make unusable for preview tools
• cant use encryption, to much power needed
• can I either optimize the code below, or is there a better approach

Longer description:

I am trying to improve my code, maybe you got some ideas or improvements for the following situation. Please be aware that I neither try to beat the CIA, nor care much if somebody “brakes” the encryption.

The background is simple: My app loads a bunch of images from a server into a folder on the SD card. I do NOT want the images to be simple JPG files, because in this case the media indexer would list them in the library, and a user could simply copy the whole folder to his harddrive.

The obvious way to go is encryption. But a full blown AES or other encryption does not make sense, for two reasons: I would have to store the passkey in the app, so anyone could get the key with some effort anyway. And the price for decrypting images on the fly is way too high (we are talking about e.g. a gallery with 30 200kB pictures).

So I decided to toggle some bits in the image. This makes the format unreadable for image tools (or previews), but is pretty easy undone when reading the images. For “encrypting” I use some C# tool, the “decrypt” lines are the following ones:

public class CustomInputStream extends InputStream {
    private String _fileName;
    private BufferedInputStream _stream;


    public CustomInputStream(String fileName) {
        _fileName = fileName;
    }

    public void Open() throws IOException {
        int len = (int) new File(_fileName).length();
        _stream = new BufferedInputStream(new FileInputStream(_fileName), len);
    }


    @Override
    public int read() throws IOException {
        int value = _stream.read() ^ (1 << 7);
        return value;
    }


    @Override
    public void close() throws IOException {
        _stream.close();

    }

}

I tried overwriting the other methods (read with more then one byte) too, but this kills the BitmapFactory – not sure why, maybe I did something wrong. Here is the code for the image bitmap creation:

Bitmap bitmap = null;
try {
    InputStream i = CryptoProvider.GetInstance().GetDecoderStream(path);
    bitmap = BitmapFactory.decodeStream(i);
    i.close();
} catch (Exception e1) {
    _logger.Error("Cant load image " + path + " ERROR " + e1);
}
if (bitmap == null) {
    _logger.Error("Image is NULL for path " + path);
}
return bitmap;

Do you have any feedback on the chosen approach? Any way to optimize it, or a completely different approach for Android devices?

Answer 1

You could try XORing the bytestream with the output of a fast PRNG. Just use a different seed for each file and you’re done.

note: As already noted in the question, such methods are trivial to bypass.