UMassCTF 2026 - Deep Down

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There’s something in the water…

Initial Analysis

The challenge provides a GIF file (CHALL.gif). Inspecting the file with standard tools reveals a 12-frame animation.

  • File Analysis: Running identify -verbose CHALL.gif shows that the image uses an 8-color Global Color Table (3-bit).
  • Palette Anomaly: The palette contains redundant entries for almost identical colors. Specifically, the dark blue color of the water (11, 41, 71) is mapped to both index 1 and index 3.

Solution

In many steganography challenges involving GIFs or indexed PNGs, redundant palette indices are used to hide data. I wrote a script to isolate these two specific indices by extracting the frames and visualizing the pixels.

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import PIL.Image
import numpy as np

# 'P' 代表 Palette（调色板）模式
img = PIL.Image.open('frame-0.gif').convert('P')
data = np.array(img)

# Visualize index 1 as '#' and everything else as '.'
for r in range(100):
    row_str = "".join(["#" if x == 1 else "." for x in data[r]])
    print(row_str)

By printing the pixel grid of the water area and specifically looking for the “hidden” index (Index 1 vs Index 3), a clear ASCII art representation of the flag appeared:

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.....................................#.....................................................#........
..................................####.....................................................####.....
..................................#...........................................................#.....
..................................#...#..#...#......#####......#####...#.######.######...##...#.....
...#..#.#####.#####.#####.#####...#..##..#...#......#...#......#......##.....#......#...#.#...#.....
...#..#.#.#.#.#...#.#.....#......##.#.#..##..#......#####......#.....#.#.....#......#..#..#...##....
...#..#.#.#.#.#####.###...###...##....#..#.#.#......#...#......#..##...#....#......#..#######..##...
...#..#.#...#.#...#...###...###..##...#..#.#.#......#...#......#..###..#...#......#.......#...##....
...#..#.#...#.#...#.....#.....#...#...#..#..##......#...#......#....#..#...#......#.......#...#.....
...####.#...#.#...#.#####.#####...#..###.#...#.####.#...#.####.######.###..#......#.......#...#.....
..................................#...........................................................#.....
..................................####.....................................................####.....
.....................................#.....................................................#........

Flag

UMASS{1N_A_G1774}