17

17 - New Egg Design

Description

Level: medium
Author: darkice

Thumper is looking for a new design for his eggs. He tried several filters with his graphics program, but unfortunately the QR codes got unreadable. Can you help his?!

Solution

The hint from the challenge description lead me to a writeup of a similar challenge. PNG images contain information about which filter was used before the image was compressed. The filter types can be used to hide information inside an image.

With this in mind I wrote my own program that extracts the PNG filter information. To make this task a bit easier I used PNGJ, a Java library for PNG encoding.

public static void main(String[] args) throws Exception {
    PngReader pngReader = new PngReaderByte(new File("eggdesign.png"));
    StringBuilder binary = new StringBuilder();

    for (int row = 0; row < pngReader.imgInfo.rows; row++) {
        IImageLine line = pngReader.readRow();
        FilterType filterType = ((ImageLineByte) line).getFilterType();

        if (filterType == FilterType.FILTER_NONE) {
            binary.append("0");
        } else if (filterType == FilterType.FILTER_SUB) {
            binary.append("1");
        } else {
            throw new Exception("Unknown filter type");
        }

    }

    pngReader.end();

    StringBuilder flag = new StringBuilder();

    Arrays.stream(binary.toString().split("(?<=\\G.{8})"))
            .forEach(chunk -> flag.append((char) Integer.parseInt(chunk, 2)));

    System.out.println(flag.toString());
}

The text hidden inside the image was:

Congratulation, here is your flag: he19-TKii-2aVa-cKJo-9QCj