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2022 | Hack-A-Sat | Crypto Category Placeholder Name

Black Hole

题目

We found a satellite but we can't speak its language. It changes its encryption encoding every time we open a connection...

We've got an open connection to the satellite.

It sent us this encoded message. Decode it and send it back to get the flag.

Connecting

nc black_hole.satellitesabove.me 5300

解题思路

  • 没有给源码，直接连服务器分析 密文 编码结果 ΣΣΣ(Φ ωΦ||¡)

  • 既然是编码，应该有一定规律可循。就先试试最小的两个吧...等等！它们有什么区别么？！-ω- 十六进制字符串差了一个字符

    Generating black hole...

Encoded msg is: e7a1977d3bce40b06475a6f44ca13535a345397224a927fc2f59633f66d7272bd45c1c8c9030755cf4e05fbd20ed995480e198b52498b0dcdfde8f027dd58c0c836f50045463ee0846df632a4c4b8bc7a7978290820441649b13760ee645d2f36af7571206d66b45b5eea1e0b13de2b84505a1b85c09d032f206f12b1179ca347eaecc6344af06e8c34bfe93072a5c0c6587a7e1c6c6bd1cd37a8868da78ecd103ce8d4f8e48d38369f669feb2ebf8ec9a74f70dbf22e84028b9290665ff2822fc4311047e68f02b24a5661cea07b42ec2dbfed36c964785cee1d70818aa2bdeece7680ef144c6e1797f695d2681a1210be06cd9e9d65487bbda4c5bba402030
We can stream 18909 bytes of data before the sat kills the connection. Please help. (Send your message in hex.)
(18909) Msg: 00
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
# 9844ce7e63ee946ac1b2572627e46fa2309f6bc2a10dd9f0062f23d826884f20ab56eeaec99490ff8694fc8b8e5097177a0deb5345212e9eb52ac7e90edfecafedc8e1ef9b3c7c9018581ca238ea246705b9185af5d748df08563e45d0e64b642743d826bcd7257c8e126193f0bc15682027a2c7d7611d9327c0779808e2c82f3f0cea10192127ff0ce72091f9d42b576180e18a0e3b5d159df3e4c30c0_7_a59d98756abc68a009cdf125282757a1e7a9d1792cb0d8664007af41ba1fc625562ebbf2601018899aa22a1c27e30fd43e368d40d39f4e8b962c42e24c9d0ecedf0681bdab587f5c7a2950658e141175b5c1e358fdc765b6d6ff187f7fc8a3757eb9
(18908) Msg: 01
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
# 9844ce7e63ee946ac1b2572627e46fa2309f6bc2a10dd9f0062f23d826884f20ab56eeaec99490ff8694fc8b8e5097177a0deb5345212e9eb52ac7e90edfecafedc8e1ef9b3c7c9018581ca238ea246705b9185af5d748df08563e45d0e64b642743d826bcd7257c8e126193f0bc15682027a2c7d7611d9327c0779808e2c82f3f0cea10192127ff0ce72091f9d42b576180e18a0e3b5d159df3e4c30c0_6_a59d98756abc68a009cdf125282757a1e7a9d1792cb0d8664007af41ba1fc625562ebbf2601018899aa22a1c27e30fd43e368d40d39f4e8b962c42e24c9d0ecedf0681bdab587f5c7a2950658e141175b5c1e358fdc765b6d6ff187f7fc8a3757eb9

  • 随后试了一些 00，发现长度大于等于 \(2\) 的消息不能包含 00（看上去 0100 是个例外）

    (18909) Msg: 0000
Must provide message with no NULL bytes (00)
(18907) Msg: 0001
Must provide message with no NULL bytes (00)
(18905) Msg: 0100
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
(18903) Msg: 010000
Must provide message with no NULL bytes (00)
(18900) Msg: 000001
Must provide message with no NULL bytes (00)
(18897) Msg: 010001
Must provide message with no NULL bytes (00)

  • （省略打表找规律的过程）通过观察可以确定

    • 相同长度的消息，编码结果相差的字节数不会超过消息长度
    • 在一次通信中，对于相同长度的消息，明文的每个位置与编码结果有固定且唯一的映射
    • 对于明文的每一个位置可以分别考虑值的高八位和低八位，其编码结果由高八位与低八位的映射结果组合而成，如明文 10 对应编码结果 45，明文 0a 对应编码结果 e3，那么可推出明文 1a 对应编码结果 43
    • 明文不同位置与编码结果值的映射不同

  • 因为还限制了发送消息字节的总数，所以一次查询应尽可能多获得信息

    from pwn import *
from Crypto.Util.number import long_to_bytes

def compare(a, b):
    a, b = bytes.fromhex(a), bytes.fromhex(b)
    diff = 0
    for i, j in zip(a, b):
        if i != j:
            diff += 1
    return diff

def index_diff(a, b):
    a, b = bytes.fromhex(a), bytes.fromhex(b)
    pos = []
    for i in range(len(a)):
        if a[i] != b[i]:
            pos.append(i)
    return pos

ticket = 'ticket{foxtrot294921delta3:GD1KWFvlJJN0ge3qaEddP9Olmir30Q5z7V67AmZ_e1b8RtSz61E8uIlvDpuCSGS6pw}'

conn = connect('black_hole.satellitesabove.me', 5300)
conn.sendafter('Ticket please:' , f'{ticket}\n')

enc = conn.recvline_contains('Encoded msg is: ').decode().split(' ')[-1]

# 获取编码消息的长度
enc_len = 0
for i in range(140, 0x100): # 经过测试，待求消息的长度基本在 140 及以上
    conn.sendafter('Msg:', f'{"01" * i}\n')
    res = conn.recvline().decode().strip()
    if compare(enc, res) <= i:
        enc_len = i
        print(f'Length: {i}')
        break

# pre: 消息 b'\x01' * enc_len 的编码结果，便于确定明文各个位在编码结果的位置
# pos_map: 明文各个位置与十六进制字符串的位置映射
# first_map: 明文各个位置高八位的编码映射表 0x0_ - 0xf_
# second_map: 明文各个位置低八位的编码映射表 0x_0 - 0x_f
pre, pos_map, first_map, second_map = res, [-1] * enc_len, {}, {}
for i in [0x02, 0x10, 0x21, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff]:
    s = long_to_bytes(i) * enc_len
    conn.sendafter('Msg:', f'{s.hex()}\n')
    res = conn.recvline().decode().strip()
    pd = index_diff(res, pre)
    for j in pd:
        p = j * 2
        if p in first_map:
            first_map[p][res[p]] = (i & 0xf0) // 0x10
        else:
            first_map[p] = {res[p]: 0}
        if p in second_map:
            second_map[p][res[p + 1]] = i & 0xf
        else:
            second_map[p] = {res[p + 1]: 2}

# 获得编码结果各个位置与明文值的映射关系后，就可以构造明文来确定位置的映射关系
for i in range(0, enc_len, 0x10):
    s = bytearray(b'\x01') * enc_len
    for j in range(min(0x10, enc_len - i)):
        s[i + j] = 0x10 + j
    conn.sendafter('Msg:', f'{s.hex()}\n')
    res = conn.recvline().decode().strip()
    pd = index_diff(res, pre)
    pos, sm = [], []
    for j in pd:
        p = j * 2
        pos.append(p)
        sm.append(second_map[p][res[p + 1]])
    for j, k in zip(sm, pos):
        pos_map[i + j] = k

s = bytearray(b'\x00') * enc_len
for i in range(enc_len):
    s[i] = first_map[pos_map[i]][enc[pos_map[i]]] * 0x10 + second_map[pos_map[i]][enc[pos_map[i] + 1]]
conn.sendafter('Msg:', f'{s.hex()}\n')
conn.interactive()
# Satellite-link synced! Flag: flag{foxtrot294921delta3:GEPzPQGVu-6MW0Ly8t6rSDotRhMZUOVCgnp-lcMPJbIiuvNwfH2MeDjkChz6vPvg8Hn6sGWG2i_8XroCUhRsIE4}

Flag

flag{foxtrot294921delta3:GEPzPQGVu-6MW0Ly8t6rSDotRhMZUOVCgnp-lcMPJbIiuvNwfH2MeDjkChz6vPvg8Hn6sGWG2i_8XroCUhRsIE4}

最后更新: 2022年5月30日 09:20:20
Contributors: YanhuiJessica

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