PCC '23 - Rev - [etyBtloB]

Challenge Description

Solution

This binary was a per-user binary; meaning each user had a different flag in their binaries. Starting the instance; we're greeted with a simple web-page from where we can download the binary

Now, this challenge was uploaded as part of the PCC qualifiers but no one was able to solve it. However it was pretty easy. Running the binary:

Well, let's try and look at the strings of the binary:

One thing that is weird; everything is rotated, one line looked like it made sense; reversing it:

Well, it matches GCC, but HCD. Oh, It looks like each byte is shifted-by-1 and the entire binary is currently reversed. Let's firstly un-reverse the binary and then look at the bytes to actually confirm our theory.

#!/usr/bin/env python3

with open('boltbyte', 'rb') as f:
    _in = f.read()

_in = _in[::-1] # Reversing

with open('final', 'wb') as f:
    f.write(_in)

Now, let's look the binary's bytes:

Well, this confirmed our theory, every even byte is shifted by-1, so let's write a simple script to fix the binary:

#!/usr/bin/env python3

with open('boltbyte', 'rb') as f:
    _in = f.read()

data = _in[::-1] # Reversing

buf = b""
for i in range(0, len(data)):
    buf += ((data[i] - 1) % 256 if i % 2 == 0 else data[i]).to_bytes(1, 'little')

with open('final', 'wb') as f:
    f.write(buf)

After running the script and running file on file:

Overall, I know it was a bit of a guessy challenge, but ;-;. I hope you enjoyed the writeup.