cryptoPromedium

Happy Meal

VolgaCTF 2026

Task: Recover a 12-word mnemonic seed phrase generated by an FCSR, given the first 3 words and source code. Solution: Used 2-adic rational approximation (extended GCD) on 21 known output bits to recover the connection integer q=845, then generated the full phrase.

$ ls tags/ techniques/

brute_force known_plaintext prng mnemonic shift_register fcsr 2-adic state_recovery

2-adic_rational_approximationfcsr_state_recoveryextended_gcd_lattice_reductionknown_plaintext_prng_attack

Analysis

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$ python3 exploit.py --target 192.168.1.1 --port 8080
[*] Connecting to target...
[+] Shell obtained!

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