[CSAW CTF Qualification Round 2017] - Serial (Misc 50)

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題目資訊

nc misc.chal.csaw.io 4239

解法

首先連上去會看到這個：

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8-1-1 even parity. Respond with '1' if you got the byte, '0' to retransmit.
00110111001

用關鍵字 serial parity 查了一下可以找到 serial communication data format（參考：這張圖）

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+-------+-------+-------+-------+-------+-------+-------+-------+-------+--------+------+
| start | bit 0 | bit 1 | bit 2 | bit 3 | bit 4 | bit 5 | bit 6 | bit 7 | parity | stop |
+-------+-------+-------+-------+-------+-------+-------+-------+-------+--------+------+

去頭去尾，中間看加總是不是偶數，是就記錄下來並送出 1 收下一段資料，不是就代表資料有問題，送出 0 來重新收取這段資料。

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from pwn import *
context.log_level = logging.DEBUG
r = remote('misc.chal.csaw.io', 4239)
series = []
r.recvline()
try:
    while True:
        data = r.recvline().strip()[1:-1]
        if sum(map(int, list(data))) % 2 == 0:
            series.append(data[:-1])
            r.sendline('1')
        else:
            r.sendline('0')
except Exception as e:
    pass
print ''.join([chr(int(x, 2)) for x in series])

Flag:flag{@n_int3rface_betw33n_data_term1nal_3quipment_and_d@t@_circuit-term1nating_3quipment}