BYU CTF 2026 - Chromatic

Challenge Description

Red.

We get chromatic.mp4 and a one-word hint.

Initial Analysis

The file is a genuine H.264/MP4 video (the x264 string is in the header), about 1620 frames. The hint “Red.” points straight at the red channel.

Extract the frames and sample a pixel from one:

ffmpeg -i chromatic.mp4 frames/f_%04d.png

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from PIL import Image
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im = Image.open("frames/f_0001.png").convert("RGB")
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print(im.getpixel((w // 2, h // 2)))   # (98, 0, 0)  -> R=98 == 'b'

Each frame is a flat color whose red value is an ASCII code (98 is b, 121 is y, and so on); green and blue stay at zero. The same byte repeats for a run of frames.

[!WARNING] The obvious shortcut — collapse consecutive duplicate red values into one character — is wrong. It silently eats doubled letters: ll becomes l, so all reads as al and really as realy. Decode by fixed-size frame blocks instead.

A run-length pass shows the encoding is regular: every character occupies exactly 30 frames (1620 / 30 = 54 characters), so we slice the frame list into 30-frame blocks and read one byte per block.

Decode

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from PIL import Image
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import glob
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reds = []
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for f in sorted(glob.glob("frames/*.png")):
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    im = Image.open(f).convert("RGB")
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    w, h = im.size
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    reds.append(im.getpixel((w // 2, h // 2))[0])   # red channel
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N = 30   # frames per character (from run-length analysis)
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flag = "".join(chr(reds[i + N // 2]) for i in range(0, len(reds), N))
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print(flag)

Output:

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byuctf{It's_all_red_I_really_thought_it_would_be_more}

The text is a pun on “it is all red” — the flag really is all red, encoded one character per 30 frames.

Flag

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byuctf{It's_all_red_I_really_thought_it_would_be_more}