server.py

import ctypes
import os
from hashlib import sha512
import signal

# libcsidh interop start
# modified from DiceCTF 2023 - seaside

PRIVATE_KEY_SIZE = 74
PUBLIC_KEY_SIZE = 64
MAX_EXPONENT = 5

# libcsidh: https://yx7.cc/code/csidh/csidh-latest.tar.xz
# Run this to build libcsidh: `(cd csidh-20210627; make; mv libcsidh.so ..)`
libcsidh = ctypes.CDLL("./libcsidh.so")


def apply_iso(start, iso):
    end = ctypes.create_string_buffer(PUBLIC_KEY_SIZE)
    ret = libcsidh.csidh(end, start, iso)
    if not ret:
        raise RuntimeError("csidh failed")
    return end


def pub_from(buf):
    return ctypes.create_string_buffer(buf, PUBLIC_KEY_SIZE)


def priv_from(es):
    return ctypes.create_string_buffer(bytes([e & 0xFF for e in es]), PRIVATE_KEY_SIZE)


def rand_priv():
    priv = ctypes.create_string_buffer(PRIVATE_KEY_SIZE)
    libcsidh.csidh_private(priv)
    return priv


def hash_to_priv(buf):
    v = int(sha512(bytes(buf)).hexdigest(), 16)
    es = []
    M = 2 * MAX_EXPONENT + 1
    for _ in range(PRIVATE_KEY_SIZE):
        es.append(v % M - MAX_EXPONENT)
        v //= M
    return priv_from(es)


# libcsidh interop end


c1 = libcsidh.base
leet = [1, 3, 3, 7]
leet = (leet * (PRIVATE_KEY_SIZE // len(leet) + 1))[:PRIVATE_KEY_SIZE]
c2 = apply_iso(libcsidh.base, priv_from(leet))


class RNG:
    def __init__(self, c1, c2, l, r):
        self.c1 = c1
        self.c2 = c2
        self.l = l
        self.r = r
        self.c = 0
        self.v = 0
        self.trunc = 12

    def next(self):
        self.l, self.r = self.r, hash_to_priv(
            apply_iso(apply_iso(self.c1, self.l), self.r)
        )
        return int.from_bytes(
            bytes(apply_iso(apply_iso(self.c2, self.r), self.l)), "little"
        )

    def bit(self):
        if self.c == 0:
            self.v = self.next()
            self.c = PUBLIC_KEY_SIZE * 8 - self.trunc
        self.c -= 1
        r = self.v & 1
        self.v >>= 1
        return r


class Game:
    def __init__(self, rng):
        self.rng = rng
        self.money = 1337
        self.win = 13337

    def round(self):
        bit = int(input("guess > "))
        if bit == self.rng.bit():
            self.money += 1
            print("You win!")
        else:
            self.money -= 3
            print("You lose!")

    def play(self):
        print("Welcome to Imaginary Casino!")
        print(
            "Immerse yourself in the captivating simplicity of Imaginary Casino, where a single game of coin flip becomes a thrilling quantum experience. Powered by a quantum-secure random number generator, every flip is truly unpredictable, ensuring an unparalleled level of fairness and excitement."
        )
        print(
            f"You begin your journey at Imaginary Casino with an initial balance of ${self.money}, and as you progress, the grand flag awaits you once you reach the milestone of ${self.win}."
        )
        print("(Generated by ChatGPT)")
        print()
        while self.money > 0:
            self.round()
            print(f"You have ${self.money} left.")
            if self.money >= self.win:
                print("Here's your flag:")
                with open("flag.txt") as f:
                    print(f.read().strip())
                return
        print("You're broke!")


if __name__ == "__main__":
    signal.alarm(30)
    rng = RNG(c1, c2, rand_priv(), rand_priv())
    Game(rng).play()