│ Category: Cryptography
│ Author: Imattas aka Zemi
│ Flag: byuctf{cuz_st3r30typ3s_hurt_92de04}
│ Source Path: crypto/hurtful
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--[ Challenge Description ]--
-- text --
This one is for the nerds
Files:
- [enc.py](https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/enc.py)
- [output.txt](https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/output.txt)
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--[ Provided Materials ]--
- Repository folder
https://github.com/BYU-CSA/BYUCTF-2026/tree/main/crypto/hurtful
- crypto/hurtful/README.md
https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/README.md
- crypto/hurtful/solve.sage
https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/solve.sage
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--[ Recon / Initial Analysis ]--
This page imports the BYUCTF 2026 source material for the Cryptography challenge
Hurtful. The prompt is kept separate from the solve notes, and upstream
artifacts are linked so the challenge can be replayed from the original
repository.
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--[ Vulnerability / Observation ]--
The useful observation comes from the imported solve notes below. I kept the
original technical path intact while normalizing the page metadata, challenge
grouping, and author attribution for the Volume 2 writeup archive.
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--[ Exploitation / Solution ]--
Source: crypto/hurtful/README.md
https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/README.md
This is something called a Stereotyped Message
https://gsdt.github.io/blog/2018/07/20/stereotyped-message-attack/ attack,
meaning if you know a part of the message being encrypted and the public key.
You do need to know the length of the unknown field of the message so that you
can set up the right polynomial to attack.
The solve script brute forces the flag length to set up the polynomial to solve.
Starting around a good minimum for length of the flag, you can get the full
thing in less than a minute iirc.
Flag: byuctf{cuz_st3r30typ3s_hurt_92de04}
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--[ Full Exploit Script ]--
- crypto/hurtful/solve.sage
https://github.com/BYU-CSA/BYUCTF-2026/blob/main/crypto/hurtful/solve.sage
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--[ Key Takeaways ]--
- The BYUCTF 2026 challenge material is preserved with local archive formatting.
- The page author is normalized to Imattas aka Zemi.
- The source repository remains linked for handouts, services, and solve
artifacts.