"""
EXAMPLE: Hack.lu 2020 — "Russian threesome" (RE, 500p, 5 solves)
https://github.com/p4-team/ctf/tree/master/2020-10-23-hacklu/russian_threesome

VULN: a balanced-ternary drum-machine dumps two permutation sectors `s7` (40
bytes of ciphertext) and `s17` (a 256-byte S-box). The flag is recovered by
repeatedly applying the INVERSE permutation to each `s7` byte until the sum of
the resulting bytes equals the number of applications (a fixed-point/self-
consistency check). Final bytes decode as CP1251 (Russian proverb).

Run:
    cd /home/code/ctfkit
    python3 examples/russian_threesome/solve.py
Expected flag: "Кто хочет много знать, тому мало спать."
"""
import os

HERE = os.path.dirname(os.path.abspath(__file__))


def main():
    s7 = [int(c, 16) for c in open(f"{HERE}/s7").read().split()]
    s17 = [int(c, 16) for c in open(f"{HERE}/s17").read().split()]
    s17 = s17[1::2]
    inverse = [0] * 256
    for i, j in enumerate(s17):
        inverse[j] = i

    for potential_sum in range(256 * len(s7)):
        flag = []
        for ch in s7:
            c = ch
            for _ in range(potential_sum):
                c = inverse[c]
            flag.append(c)
        if sum(flag) == potential_sum:
            b = bytes(flag)
            try:
                flag_str = b.decode("cp1251")
            except Exception:
                flag_str = b.decode("latin1")
            print("FLAG =", flag_str)
            return flag_str
    print("no flag found")
    return None


if __name__ == "__main__":
    main()