Problem description

Janken is an easy rated Misc challenge.

As you approach an ancient tomb, you're met with a wise guru who guards its entrance. In order to proceed, he challenges you to a game of Janken, a variation of rock paper scissors with a unique twist. But there's a catch: you must win 100 rounds in a row to pass. Fail to do so, and you'll be denied entry.

Analysis

Analyzing the decompilation of the binary (janken), we find a loop for the 100 rounds mentioned in the prompt running the following game function.

Solution

Here's an excerpt of the pwntools script used to solve the challenge.

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io = start()
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info(io.recvuntil(b'>> ')) # prompt
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io.sendline(b'1')
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rps = ['rock', 'scissors', 'paper']
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rps_win = {
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    'rock': 'paper',
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    'paper': 'scissors',
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    'scissors': 'rock',
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}
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def wait_for_next_second(epoch_time):
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    while int(time.time()) <= epoch_time:
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        time.sleep(0.05)
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for _i in range(0, 99):
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    epoch_time = epoch_time = time.time() # time before asking for prompt
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    if epoch_time - int(epoch_time) > 0.8:
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        wait_for_next_second(epoch_time)
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    info(io.recvuntil(b'>> ')) # wait for prompt
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    time2 = time.time()
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    rand_io = process(['./rand', f"{epoch_time}"])
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    r = int(rand_io.readline().strip())
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    enemy_answer = rps[r % 3]
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    answer = rps_win[enemy_answer]
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    info(f"Sending result: {answer}")
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    time3 = time.time() # time before sending response
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    io.sendline(bytes(answer, encoding="utf-8"))
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    time4 = time.time() # time after sending response
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    print(f"{_i}: Time delta: {epoch_time:.3f}, {time2:.3f}, {(time2 - epoch_time):.3f}, {(time4-time3):3f}")
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io.interactive()

Flag

We have our flag! HTB{r0ck_p4p3R_5tr5tr_l0g1c_buG}