BYU CTF 2026 - Inception

Challenge Description

I found this weird file on my computer. I tried opening it, but there were some problems.

We get a single file named inception.

Initial Analysis

file calls it a PNG, but strings and a hexdump show several file formats stacked into one. A file within a file within a file, hence Inception. Locating the signatures:

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import re
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data = open("inception", "rb").read()
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# IEND (PNG end)   @ 815
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# PK\x03\x04 (ZIP) @ 823   -> contains data.bin
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# %PDF             @ 964
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# %%EOF            @ 1617

So there are three payloads in one blob: a PNG, a ZIP appended after it, and a PDF after that. Each holds one part of the flag.

Extraction

Part 1 - the PNG itself. Rendering the image shows text:

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flag part 1
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================================
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byuctf{wh4t_

Part 2 - the appended ZIP. Carve from the PK offset and read data.bin:

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import zipfile, io
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pk = data.find(b"PK\x03\x04")
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z = zipfile.ZipFile(io.BytesIO(data[pk:]))
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print(z.read("data.bin").decode())
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# flag part 2
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# ================================
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#   th3

Part 3 - the trailing PDF. Decode its FlateDecode content stream:

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import zlib, re
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m = re.search(rb"stream\r?\n(.*?)\r?\nendstream", data, re.S)
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print(zlib.decompress(m.group(1)).decode())
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# ... (flag part 3) ... (_fr3ak}) ...

Assemble

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part1: byuctf{wh4t_
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part2: th3
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part3: _fr3ak}

Flag

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byuctf{wh4t_th3_fr3ak}