Steganography, only jpg as input works, when using png the resulting image looks strange

Question

I built a little java program that hides messages in an image using the least significant bit method. It works fine when inputting a jpg file. The output may be png or jpg. When inputting a png though, the result looks very stange.

Here are the original and the result images respectively:

public abstract class Builder{

public static void leastSignificantBitEncryption(String imageSource, String message, String newPath) {
    BufferedImage image = returnImage(imageSource);
    //prepare variables
    String[] messageBinString = null;
    String[] pixelBinString = null;
    final byte[] messageBin = message.getBytes(StandardCharsets.UTF_8);
    final byte[] pixelsBin = ((DataBufferByte) image.getRaster().getDataBuffer()).getData();
    //convert message and image to binary string array
    try {
        messageBinString = stringToBinaryStrings(messageBin);
        pixelBinString = stringToBinaryStrings(pixelsBin);
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
    String[] messageBinStringCut = splitIn2Bit(messageBinString);   //split message binary into 2 bit strings
    String[] pixelBinStringNew = pixelBinString.clone();    //insert 2 bit strings in last 2 bits of bytes from bitmap
    insert2Bit(messageBinStringCut, pixelBinStringNew);
    byte[] pixelsBinNew = stringArrayToByteArray(pixelBinStringNew);    //Convert string array to byte array
    try {   //Create new image out of bitmap
        int w = image.getWidth();
        int h = image.getHeight();
        BufferedImage imageNew = new BufferedImage(w, h, BufferedImage.TYPE_3BYTE_BGR);
        imageNew.setData(Raster.createRaster(imageNew.getSampleModel(), new DataBufferByte(pixelsBinNew, pixelsBinNew.length), new Point()));
        File imageFile = new File(newPath);
        ImageIO.write(imageNew, "png", imageFile);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

private static String[] stringToBinaryStrings(byte[] messageBin) throws UnsupportedEncodingException{
    String[] bytes = new String[messageBin.length];
    int i = 0;
    for(byte b : messageBin) {
        bytes[i] = String.format("%8s", Integer.toBinaryString(b & 0xFF)).replace(' ', '0');
        i++;
    }
    return bytes;
}

private static String binaryStringsToString(String[] messageBin) throws UnsupportedEncodingException{
    StringBuilder stringBuilder = new StringBuilder();
    int i = 0;
    while(messageBin[i] != null) {
        stringBuilder.append((char) Integer.parseInt(messageBin[i], 2));
        i++;
    }
    return stringBuilder.toString();
}

private static BufferedImage returnImage(String imageSource) {
    try{
        try {
            return ImageIO.read(new URL(imageSource));
        } catch (MalformedURLException e) {
            return ImageIO.read(new File(imageSource));
        }
    } catch (IOException ioe) {
        ioe.printStackTrace();
        return null;
    }
}

private static byte[] stringArrayToByteArray(String[] stringArray) {
    byte[] byteArray = new byte[stringArray.length];
    for(int i = 0; i < stringArray.length; i++) {
        byteArray[i] = (byte) Integer.parseInt(stringArray[i], 2);
    }
    return byteArray;
}

private static String[] splitIn2Bit(String[] inputArray) {
    String[] outputArray = new String[inputArray.length * 4];
    for(int i = 0; i < outputArray.length; i += 4) {
        String[] splitByte = inputArray[i / 4].split("(?<=\\G..)");
        outputArray[i] = splitByte[0];
        outputArray[i + 1] = splitByte[1];
        outputArray[i + 2] = splitByte[2];
        outputArray[i + 3] = splitByte[3];
    }
    return outputArray;
}

private static String[] insert2Bit(String[] twoBitArray, String[] insertArray) {
    for(int i = 0; i < twoBitArray.length; i++) {
        insertArray[i] = insertArray[i].substring(0, 6) + twoBitArray[i];
    }
    return insertArray;
}

}

Also, the testclass:

public class Test {

    public static void main(String[] args) {
        Builder.leastSignificantBitEncryption("IMAGEPATH OR URL", "MESSAGE", "PATH FOR IMAGE CONTAINING MESSAGE");
        Builder.leastSignificantBitDecryption("PATH OF IMAGE CONTAINING MESSAGE", "PATH FOR TXT CONTAINING OUTPUT");
    }
}

Answer 1

The error originates from the fact that the png image has an extra channel for transparency. System.out.println(pixelsBin.length); returns 338355 bytes for the jpg and 451140 bytes for the png.

The simplest solution would be to create the appropriate imageNew depending on the format file. For example,

int w = image.getWidth();
int h = image.getHeight();
BufferedImage imageNew = null;
if (imageSource.matches(".*jpg$")) {
    imageNew = new BufferedImage(w, h, BufferedImage.TYPE_3BYTE_BGR);
} else if (imageSource.matches(".*png$")) {
    imageNew = new BufferedImage(w, h, BufferedImage.TYPE_4BYTE_ABGR);
} else {
    // whatever
}
imageNew.setData(Raster.createRaster(imageNew.getSampleModel(), new DataBufferByte(pixelsBinNew, pixelsBinNew.length), new Point()));

However, you have to be aware that the message is not embedded in both types in the same pixels. The byte array of a 3-channel image (no transparency) goes like this

first-pixel-BLUE, first-pixel-GREEN, first-pixel-RED, second-pixel-BLUE, etc

while for a 4-channel image

first-pixel-ALPHA, first-pixel-BLUE, first-pixel-GREEN, first-pixel-RED, second-pixel-ALPHA, etc

If you care about that detail, you might be interested in removing the alpha channel from the png first, so you're always working with 3-channel images.