Skip to main content

Brainiac

Challenge​

Imported from local notes.md.

Solution​

Original Notes​

Brainiac

Challenge Summary​

  • Given: a single attached file named challenge.txt and a description containing SHA1: 33746b3052f748a9d41f030d2be4f196d02453cb.
  • Goal: recover a flag in the format CIT{string}.
  • Constraints: no remote service or input channel, just static analysis of the attachment.

Initial Recon / Triage​

  • Observations: the attached file contains only the characters +, -, <, >, [, ], and ..
  • File identification: that character set is Brainfuck source code.
  • Entry points: verify the provided SHA1 against the downloaded file and decode the Brainfuck program.

Hypotheses & Approach​

  • Hypothesis 1: the given SHA1 is an integrity check for the attachment, not the hash of the flag.
  • Hypothesis 2: decoding the Brainfuck source should print the flag directly.

Execution Steps (Reproducible)​

Stage 1​

Commands:

cd /root/cit2026CTF/Brainiac
sha1sum starting_files/challenge.txt
python3 - <<'PY'
from pathlib import Path
code = Path('starting_files/challenge.txt').read_text().strip()
print('unique chars:', ''.join(sorted(set(code))))
PY

Results:

  • The file hash matched the challenge description exactly: 33746b3052f748a9d41f030d2be4f196d02453cb.
  • The source uses the exact instruction set expected for Brainfuck.

Stage 2​

Commands:

cd /root/cit2026CTF/Brainiac/artifacts
python3 solve_brainiac.py
cat decoded_output.txt

Results:

  • Decoding the Brainfuck program produced the flag directly.
  • The printed output was CIT{Wh@t_in_th3_w0rld_i$_th1s_l@ngu@g3}.

Artifacts Produced​

  • artifacts/solve_brainiac.py: reusable Brainfuck decoder for this attachment.
  • artifacts/decoded_output.txt: plaintext emitted by the program.

Flag​

CIT{Wh@t_in_th3_w0rld_i$_th1s_l@ngu@g3}