NETCOMP 3.0 - X11 Protocol

noticed the packet is transmitted using X11 Protocol

https://www.x.org/releases/X11R7.7/doc/xproto/x11protocol.html

we can use X11 dissector in wireshark

in this protocol there is an extension named PutImage used to visualize an image file

https://tronche.com/gui/x/xlib/utilities/manipulating-images.html

https://xmonad.github.io/xmonad-docs/X11-1.10.3.10/Graphics-X11-Xlib-Image.html

it displayed the image, opcode used for the extension is 72. we can use it as filter

the image is rendered as ZPixMap, we can create a script to render the image using PIL

from PIL import Image

hex_data = """
example 
"""

raw_data = bytes.fromhex(hex_data)
raw_data = raw_data[24:] #first 24 bytes is an identity of the packets, we dont need it to render the image
width, height = 250, 250  

expected_size = width * height * 4  # 4 bytes per pixel (RGBA)

if len(raw_data) != expected_size:
    print(f"Warning: Data size mismatch. Expected {expected_size} bytes, got {len(raw_data)} bytes.")
else:
    image = Image.frombytes("RGBA", (width, height), raw_data)

    image.save("output_image.png")
    image.show()

originally it transmitted so many images so i need manually try it until i noticed that there are some “nice” image with size 250x250 that we might be the flag

so we will gonna use it as filter again

because the raw data is too big, i cant dump it using tshark, i dont know why so i manually dumped it.

convert the dumped data into images

from PIL import Image
import os

raw_folder = "raw"
width, height = 250, 250  

expected_size = width * height * 4  # 4 bytes per pixel (RGBA)

output_folder = "result"
os.makedirs(output_folder, exist_ok=True)  # Create folder if it doesn't exist

image_count = 0

# Loop through all files in the "raw" folder
for hex_file in os.listdir(raw_folder):
    hex_file_path = os.path.join(raw_folder, hex_file)

    if not os.path.isfile(hex_file_path):
        continue  # Skip if it's not a file

    with open(hex_file_path, "r") as file:
        hex_data = file.read().strip().replace("\n", "")  # Remove newlines and whitespace

    raw_data = bytes.fromhex(hex_data)
    raw_data = raw_data[24:]

    if len(raw_data) != expected_size:
        print(f"Warning: Data size mismatch in {hex_file}. Expected {expected_size} bytes, got {len(raw_data)} bytes.")
        continue

    try:
        image = Image.frombytes("RGBA", (width, height), raw_data)

        output_path = os.path.join(output_folder, f"image_{image_count}.png")
        image.save(output_path)
        print(f"Saved: {output_path}")

        image_count += 1
    except Exception as e:
        print(f"Error processing {hex_file}: {e}")

print(f"Total images saved: {image_count}")

But the results are still in a random order, so we need to sort them

when analyzing the raw data noticed the pcap contains so many png that didnt detected with the x11 dissector.

so i got an assumption that the file names is the order (also after consulting to the author hahaha) so i will create a script to reorder it based on what i’ve found.

import os
import shutil
import re

new_order = [
    27, 34, 6, 13, 28, 19, 12, 9, 40, 8, 37, 39, 4, 15, 23, 3, 33, 25, 10, 16,
    7, 32, 31, 1, 38, 20, 26, 5, 24, 29, 14, 30, 17, 0, 11, 18, 2, 21, 22, 35, 36
]

sourceFolder = "result"
desFolder = "flag"

os.makedirs(desFolder, exist_ok=True)

def extract_number(filename):
    match = re.search(r'\d+', filename)  
    return int(match.group()) if match else float('inf') 

files = sorted(os.listdir(sourceFolder), key=extract_number)

if len(files) != len(new_order):
    print(f"Error: Number of files in '{sourceFolder}' ({len(files)}) does not match the expected order ({len(new_order)}).")
    exit(1)

for i, new_name in enumerate(new_order):
    old_file_path = os.path.join(sourceFolder, files[i])
    new_file_name = f"{new_name}" + os.path.splitext(files[i])[1]  
    new_file_path = os.path.join(desFolder, new_file_name)

    shutil.copy(old_file_path, new_file_path)

print(f"Files have been successfully renamed and saved to '{desFolder}'.")

Final flag: Netcomp{742cb56806e9fb74441367d7eab9c258}