Matrix Challenge — Steganography¶

Challenge summary¶

Image: The Matrix “bullet time” scene with a cyan waveform overlay and a SHA1 of 4bbab076a0aa488761cd216a82bf4e508a2953ab to verify the file.
Hints referenced the “red pill,” “red signals,” LSBs, regrouping bits into bytes, and the Answer to the Ultimate Question (42).
My key observation was to focus on the red channel’s least significant bits, rebuild bytes, then XOR with 0x42 to undo the distortion.

Solution outline¶

I extracted the red channel from the PNG and kept only the least significant bit of every pixel (LSB steganography).
I flattened the LSB bitstream and grouped it into bytes (8 bits, MSB first) to reconstruct the hidden data.
I XORed every byte with 0x42 (the hexadecimal representation of “42” from Hitchhiker’s Guide) to remove the obfuscation.
I searched the result for HACKDAY{...} and printed the flag.

Key implementation¶

from PIL import Image
import numpy as np

img = Image.open('Matrix_challenge.png')
red = np.array(img)[:, :, 0]
flat_lsb = (red & 1).flatten()

bytes_out = []
for i in range(0, len(flat_lsb) - 8, 8):
    byte = 0
    for bit in range(8):
        byte = (byte << 1) | flat_lsb[i + bit]
    bytes_out.append(byte)

decoded = bytes(b ^ 0x42 for b in bytes_out)
flag_start = decoded.find(b'HACKDAY{')
flag_end = decoded.find(b'}', flag_start)
print(decoded[flag_start:flag_end + 1].decode())

Result¶

Flag: HACKDAY{e3a12b9383038b0c6d755bcb39d3bf879cac3750588226ba1c52d64fde0a7c96}