Level: Medium
Author: brp64 feat. M.
Visiting the following railway station has left lasting memories.
Santas brand new gifts distribution system is heavily inspired by it. Here is your personal gift, can you extract the destination path of it?
First, I did an image reverse-search to find out more about the railway station. This lead to the Wikipedia article about rule 30. From there one it was obvious what I had to do: I took the "tree" structure generated by rule 30 and XORed it with the QR Code. At first this did not give a valid QR Code because the rule 30 structure was misaligned by one pixel. By running the following code I got the new QR code:
public class Rule30 {
public static void main(String[] args) throws IOException {
BufferedImage image = ImageIO.read(new File("original.png"));
byte[][] rule30 = rule30();
for (int i = 0; i < 33; i++) {
for (int j = 0; j < 33; j++) {
Color color = new Color(image.getRGB(i, j));
byte value = toByte(color);
byte newValue = (byte) (value ^ rule30[j][i + 84]); // Off by one!
image.setRGB(i, j, fromByte(newValue).getRGB());
}
}
ImageIO.write(image, "PNG", new File("solution.png"));
}
private static byte[][] rule30() {
byte[][] cells = new byte[33][200]; // Greater
cells[0][cells[0].length / 2 + 1] = 1;
for (int r = 0; r < 32; r++) {
for (int c = 1; c < 199; c++) {
byte lhs = cells[r][c - 1];
byte mid = cells[r][c];
byte rhs = cells[r][c + 1];
cells[r + 1][c] = (byte) (lhs ^ (mid | rhs));
}
}
return cells;
}
private static byte toByte(Color color) {
return (byte) (Color.BLACK.equals(color) ? 1 : 0);
}
private static Color fromByte(byte value) {
return value == 1 ? Color.BLACK : Color.WHITE;
}
}
Scanning the code gave the flag HV19{Cha0tic_yet-0rdered}.