DASCTF X GFCTF2022十月赛Re赛后复现

0x00 前言

又是get新知识的一天呢

0x01 pycode

拿到题目附件

是个txt文件看到代码第一眼愣了一下没见过

  1           0 LOAD_CONST               0 (0)
              2 LOAD_CONST               1 (('number',))
              4 IMPORT_NAME              0 (Crypto.Util)
              6 IMPORT_FROM              1 (number)
              8 STORE_NAME               1 (number)
             10 POP_TOP

  3          12 LOAD_CONST               2 ('8b2e4e858126bc8478d6a6a485215f03')
             14 STORE_NAME               2 (enc)

  5          16 LOAD_CONST               3 (<code object extract_number at 0x0000020F3AE81DB0, file "1.py", line 5>)
             18 LOAD_CONST               4 ('extract_number')
             20 MAKE_FUNCTION            0
             22 STORE_NAME               3 (extract_number)

 13          24 LOAD_CONST               5 (<code object transform at 0x0000020F3AE8F0C0, file "1.py", line 13>)
             26 LOAD_CONST               6 ('transform')
             28 MAKE_FUNCTION            0
             30 STORE_NAME               4 (transform)

 24          32 LOAD_NAME                5 (__name__)
             34 LOAD_CONST               7 ('__main__')
             36 COMPARE_OP               2 (==)
             38 POP_JUMP_IF_FALSE      104

 26          40 LOAD_NAME                6 (input)
             42 LOAD_CONST               8 ('input your number:')
             44 CALL_FUNCTION            1
             46 STORE_NAME               7 (num)

 27          48 LOAD_NAME                8 (bytes)
             50 LOAD_METHOD              9 (fromhex)
             52 LOAD_NAME                7 (num)
             54 CALL_METHOD              1
             56 STORE_NAME              10 (tmp)

 28          58 LOAD_NAME                4 (transform)
             60 LOAD_NAME               10 (tmp)
             62 CALL_FUNCTION            1
             64 LOAD_METHOD             11 (hex)
             66 CALL_METHOD              0
             68 STORE_NAME              12 (res)

 29          70 LOAD_NAME                2 (enc)
             72 LOAD_NAME               12 (res)
             74 COMPARE_OP               2 (==)
             76 POP_JUMP_IF_FALSE       96

 30          78 LOAD_NAME               13 (print)
             80 LOAD_CONST               9 ('ok,your flag : DASCTF{{{name}}}')
             82 LOAD_ATTR               14 (format)
             84 LOAD_NAME                7 (num)
             86 LOAD_CONST              10 (('name',))
             88 CALL_FUNCTION_KW         1
             90 CALL_FUNCTION            1
             92 POP_TOP
             94 JUMP_FORWARD             8 (to 104)

 32     >>   96 LOAD_NAME               13 (print)
             98 LOAD_CONST              11 ('wrong')
            100 CALL_FUNCTION            1
            102 POP_TOP
        >>  104 LOAD_CONST              12 (None)
            106 RETURN_VALUE

Disassembly of <code object extract_number at 0x0000020F3AE81DB0, file "1.py", line 5>:
  6           0 LOAD_FAST                0 (x)
              2 LOAD_FAST                0 (x)
              4 LOAD_CONST               1 (11)
              6 BINARY_RSHIFT
              8 BINARY_XOR
             10 STORE_FAST               0 (x)

  7          12 LOAD_FAST                0 (x)
             14 LOAD_FAST                0 (x)
             16 LOAD_CONST               2 (7)
             18 BINARY_LSHIFT
             20 LOAD_CONST               3 (2022072721)
             22 BINARY_AND
             24 BINARY_XOR
             26 STORE_FAST               0 (x)

  8          28 LOAD_FAST                0 (x)
             30 LOAD_FAST                0 (x)
             32 LOAD_CONST               4 (15)
             34 BINARY_LSHIFT
             36 LOAD_CONST               5 (2323163360)
             38 BINARY_AND
             40 BINARY_XOR
             42 STORE_FAST               0 (x)

  9          44 LOAD_FAST                0 (x)
             46 LOAD_FAST                0 (x)
             48 LOAD_CONST               6 (18)
             50 BINARY_RSHIFT
             52 BINARY_XOR
             54 STORE_FAST               0 (x)

 10          56 LOAD_FAST                0 (x)
             58 RETURN_VALUE

Disassembly of <code object transform at 0x0000020F3AE8F0C0, file "1.py", line 13>:
 14           0 LOAD_CONST               1 (b'')
              2 STORE_FAST               1 (new_message)

 15           4 LOAD_GLOBAL              0 (len)
              6 LOAD_FAST                0 (m)
              8 CALL_FUNCTION            1
             10 STORE_FAST               2 (l)

 16          12 SETUP_LOOP              82 (to 96)
             14 LOAD_GLOBAL              1 (range)
             16 LOAD_FAST                2 (l)
             18 LOAD_CONST               2 (4)
             20 BINARY_FLOOR_DIVIDE
             22 CALL_FUNCTION            1
             24 GET_ITER
        >>   26 FOR_ITER                66 (to 94)
             28 STORE_FAST               3 (i)

 17          30 LOAD_FAST                0 (m)
             32 LOAD_FAST                3 (i)
             34 LOAD_CONST               2 (4)
             36 BINARY_MULTIPLY
             38 LOAD_FAST                3 (i)
             40 LOAD_CONST               2 (4)
             42 BINARY_MULTIPLY
             44 LOAD_CONST               2 (4)
             46 BINARY_ADD
             48 BUILD_SLICE              2
             50 BINARY_SUBSCR
             52 STORE_FAST               4 (enc)

 18          54 LOAD_GLOBAL              2 (number)
             56 LOAD_METHOD              3 (bytes_to_long)
             58 LOAD_FAST                4 (enc)
             60 CALL_METHOD              1
             62 STORE_FAST               4 (enc)

 19          64 LOAD_GLOBAL              4 (extract_number)
             66 LOAD_FAST                4 (enc)
             68 CALL_FUNCTION            1
             70 STORE_FAST               4 (enc)

 20          72 LOAD_GLOBAL              2 (number)
             74 LOAD_METHOD              5 (long_to_bytes)
             76 LOAD_FAST                4 (enc)
             78 LOAD_CONST               2 (4)
             80 CALL_METHOD              2
             82 STORE_FAST               4 (enc)

 21          84 LOAD_FAST                1 (new_message)
             86 LOAD_FAST                4 (enc)
             88 INPLACE_ADD
             90 STORE_FAST               1 (new_message)
             92 JUMP_ABSOLUTE           26
        >>   94 POP_BLOCK

 22     >>   96 LOAD_FAST                1 (new_message)
             98 RETURN_VALUE

百度了解过后知道了是python 的字节码，类似汇编的一种东西

对照python官方文档对字节码进行还原，即可拿到python代码

还原出的python代码如下:

from Crypto.Util import number

enc = '8b2e4e858126bc8478d6a6a485215f03'

def extract_number(x)        :
    x = x ^ x >> 11
    x = x ^ x << 7 & 2022072721
    x = x ^ x << 15 & 2323163360
    x = x ^ x >> 18
    return x

def transform(m):
    new_message = b''
    l = len(m)
    for i in range(l // 4):
        enc = m[i * 4: i * 4 + 4]
        enc = number.bytes_to_long············(enc)
        enc = extract_number(enc)
        enc = number.long_to_bytes(enc, 4)
        new_message += enc
    return new_message

if __name__ == '__main__':
    num = input("input your number:")
    tmp = bytes.fromhex(num)
    res = transform(tmp).hex()
    if enc == res:
         print('ok,your flag : DASCTF{{{name}}}'.format(name=num))
    else:
         print("wrong")

加密算法是mt算法中提取伪随机数的过程，其中的常数有些许更改

我们从最后一步开始看

1	x1 = x ^ x >> 18 // >> 位运算符

算式中，x的(32-18)位以上异或0即不改变，所以我们就可以知道 x1的高18位（14-32）

x1的15-32位不变
x1的1-14位由x的1-14位与x的18-32位异或所得

官方exp如下:

from Crypto.Util import number

def right_back(e,shift,a):            
    tmp = e
    tmp1 = e
    for i in range(32//shift):
         t = (tmp1 >> shift) & a
         tmp = t ^ tmp
         tmp1 = t
     return tmp

def left_back(e,shift,a):
     tmp = e
     tmp1 =e
     for i in range(32//shift):
         t = (tmp1 << shift) & a
         tmp = t ^ tmp
         tmp1 = t
     return tmp

def convert(e):
     e = right_back(e,18,0xffffffff)
     e = left_back(e,15,2323163360)
     e = left_back(e,7,2022072721)
     e = right_back(e,11,0xffffffff)
     return e

def transform(message):
     assert len(message) % 4 == 0
     new_message = b''
     for i in range(len(message) // 4):
         block = message[i * 4 : i * 4 +4]
         block = number.bytes_to_long(block)
         block = convert(block)
         block = number.long_to_bytes(block)
         new_message += block
     return new_message

enc = '8b2e4e858126bc8478d6a6a485215f03'
enc = bytes.fromhex(enc)
flag=transform(enc)
print(flag)
a = flag
print(a.hex())