# GreyCTF'23 Background: Grey Cat The Flag CTF, or GreyCTF is hosted by [NUS Greyhats](https://nusgreyhats.org/) in collaboration with [National Cybersecurity R\&D Labs](https://ncl.sg/). I joined this CTF as a [team](https://ctfd.nusgreyhats.org/teams/621) with my friends, Domi and Bowen. Altogether, we solved 6 challenges, placing us 57th place in the Singapore category and 147th place in the open category out of 454 teams. ## Crypto ### 1. Baby Crypto ``` whuo{squi4h_sifx3h_v0h_co_i4b4T} ```

Pretty simple challenge, it is a cipher. Caesar cipher, to be exact. So I just used dcode.fr for this.

The flag was found! "**grey{caes4r\_csph3r\_f0r\_my\_s4l4D}**" ## Misc ### 1. CrashPython

I didn't get the screenshot of the full page unfortunately, but the gist of it was that there was a placeholder to input python code to run to cause a crash (through segmentation fault), but the code had to fulfil the requirements of the blacklist. The blacklist included keywords that couldn't be used in the code. Initially, I tested with a stack overflow error with infinite recursion as such: ```python def recursion(): recursion() recursion() ``` But it didn't work. My teammate then told me (Eureka moment...) that it didn't meet the requirements of a segmentation fault. I then looked around for solutions to cause a segmentation fault. I ended up with this simple code:

import ctypes
def segfault():
    # Access an invalid memory location
    ctypes.string_at(0)

segfault()

And that worked!