Last year, an elf made a perfect program for Santa. Now he has to change it but he lost the source code. Please help him to reverse this compiled program.
IMPORTANT: The flag for this challenge is not in the standard competition format.
Not sure if it was a part of challenge, but the actual PYO file was archived with 7z compressor. Again, we have a compiled Python code:
$ file 1.pyo
1.pyo: python 3.6 byte-compiled
However, unlike Day 5, we cannot use decompyle3 because it only supports Python 3.7+, so I had to use a different Python decompiler:
$ sudo apt install python3.6
$ python3.6 -m pip install uncompyle6
$ uncompyle6 1.pyo > 1.py
See decompiled code.
So, the flag consists of characters that are encoded either with Base64, hexlify, or CRC. With the first two it is more or less obvious what to do. See first part of the solution:
$ ./solution1.py
bytearray(b'flag{h3x_b4se64_cRc_?he?3w?r}?')
As of CRC, it is a relatively fast algorithm, so we can try to bruteforce the required condition. With something like this:
$ ./solution2.py
0
1
2
...
82
R w t E
Putting the pieces together, we get the flag:
flag{h3x_b4se64_cRc_whet3wEr}