syntax invalid while translating C code to Python

Question

I have C snippet(decompiled from IDA) to be translated to Python:

  # v29 = 0;
  # v30 = -1342924972;
  # while ( v29 < v62 ) // v62 is length of string to be decoded
  # {
  #   v31 = (int *)v60;
  #   v32 = __ROL4__(v30, 3);
  #   v33 = *((_BYTE *)v31 + v29) - v29 - v32;
  #   v34 = (int *)v60;
  #   *((_BYTE *)v34 + v29) = v33;
  #   v35 = __ROR4__(v33, 11);
  #   v36 = __ROL4__(v30, 5);
  #   v30 = v30 + (v29++ ^ v36 ^ v35) - 1204489519;
  # }


def decode_msg(dstr, str_len):
  bstr = list(dstr)
  v29 = 0
  v32 = 0
  v33=0
  v35=0
  v30 = -1342924972
  while(v29 < str_len):
    v32 = ((v30 & 0xffffffff) << 3) & 0xffffffff
    v33 = ((hex(ord(bstr[v29])) & 0xff) - v32) & 0xff
    bstr[v29] = v33 & 0xff
    v35 = ((v33 & 0xffffffff) >> 11) & 0xffffffff
    v36 = ((v30 & 0xffffffff) << 5) & 0xffffffff
    v29 = v29 + 1
    v30 = (v30 & 0xffffffff) + (v29 ^ v36 ^ v35) - 1204489519
  return ''.join(bstr)

C code is in comments. The C code decodes a byte array, v60 is the array. I have error:

v33 = ((hex(ord(bstr[v29])) & 0xff) - v32) & 0xff
TypeError: unsupported operand type(s) for &: 'str' and 'int'

I am totally Python noob. I think hex() converts each item in dstr to a number. So why is it still str?

No, hex() converts a number to a string containing the hexadecimal representation of that number. — Daniel Roseman
– Daniel Roseman, Commented Nov 30, 2016 at 15:51
from IDA to Python: nice touch! you just don't have to use hex, ord is already a number. — Jean-François Fabre
– Jean-François Fabre ♦, Commented Nov 30, 2016 at 15:54

brianpck · Accepted Answer · 2016-11-30 16:00:05Z

2

As mentioned, hex returns a string, which obviously does not support bitwise operations like & with a numeric type:

>>> type(hex(3))
<class 'str'>
>>> hex(3) & 0xf
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for &: 'str' and 'int'

The ord function already returns an int, so you can just remove the hex function altogether:

>>> ord('c') & 0xff
99

answered Nov 30, 2016 at 16:00

brianpck

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Comments

yLaguardia · Accepted Answer · 2016-11-30 16:02:09Z

2

ord is already an int, you don't need the hex, which returns a string.

And I may be wrong, but this line may give you problems

bstr[v29] = v33 & 0xff

You'd need to cast it to string again:

bstr[v29] = chr(v33 & 0xff)

answered Nov 30, 2016 at 16:02

yLaguardia

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