Assume

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import random
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import ast
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import sys
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p = random_prime(2^64-1, False, 2^63)
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print(p)
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R = Integers(p)
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def gen_rand_string(n):
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    return "".join([chr(random.randint(65, 90)) for _ in range(n)])
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g = mod(primitive_root(p), p)
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target_str = open("flag.txt").readline()
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#print(target_str)
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open("target.txt","w+").write(target_str)
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def send_msg(sender, recipient, content):
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    print(f"{sender} {recipient} {content}")
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for pos in range(len(target_str)):
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    fixed_eve = gen_rand_string(1)
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    for iter in range(20):
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        a = random.randint(1, p-1)
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        b = random.randint(1, p-1)
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        send_msg("Alice", "Bob", g^a)
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        send_msg("Bob", "Alice", b)
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        send_msg("Bob", "Alice", g^b)
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        if random.randint(1, 2) == 1:
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            # interception occurs
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            c = random.randint(1, p-1)
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            send_msg("Alice", "Bob", fixed_eve)
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            send_msg("Alice", "Bob", g^c)
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        else:
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            send_msg("Alice", "Bob", target_str[pos])
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            send_msg("Alice", "Bob", g^(a * b))
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        print()
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def input_with_timeout(prompt, timeout):
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    sys.stdout.write(prompt)
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    sys.stdout.flush()
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    ready, _, _ = select.select([sys.stdin], [], [], timeout)
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    if ready:
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        return sys.stdin.readline().rstrip('\n')
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    raise Exception
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try:
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    answer = input_with_timeout('', 20)
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    try:
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        answer = ast.literal_eval(answer)
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        if target_str == answer:
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            print(":o")
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            print(flag)
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        else:
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            print("im upset")
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    except Exception as e:
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        print("im very upset")
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except Exception as e:
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    print("\nyou've let me down :(")

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for pos in range(len(target_str)):
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    fixed_eve = gen_rand_string(1)
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    for iter in range(20):
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        a = random.randint(1, p-1)
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        b = random.randint(1, p-1)
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        send_msg("Alice", "Bob", g^a)
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        send_msg("Bob", "Alice", b)
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        send_msg("Bob", "Alice", g^b)
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        if random.randint(1, 2) == 1:
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            # interception occurs
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            c = random.randint(1, p-1)
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            send_msg("Alice", "Bob", fixed_eve)
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            send_msg("Alice", "Bob", g^c)
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        else:
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            send_msg("Alice", "Bob", target_str[pos])
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            send_msg("Alice", "Bob", g^(a * b))
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        print()

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tjctf{legendary_legendre0xd5109ab3}