UIUCTF 2023 - At Home
At Home - almost
At Home is an easy crypto challenge for the UIUCTF2023. It’s a challenge I spent some time on, but did not get the flag.
This challenge provides a text file and python file. The text file has your e, n, and c variables for the python file.
Inside the chal.py file, we see where is, in the flag variable as a binary. Also a lot of skip-able lines until the c= equation with flag.
The approach I tried was a type of bruteforce, for getting the flag in bytes. Started with a different while loop looking for flag size, by multiplying by 10, until the while loop ended.
Once I had the size, I could then narrow down the value from checking large ballpark numbers to then narrowing it down to more fine grain numbers. Starting a new counter for each digit from left to right.
Finally in the code, printing out what it has found and converting back to ascii.
Running this script, the c value I get from brute-forcing is the same as the challenge’s c value. Although the flag value doesn’t seem right. Unsure if I decoded this wrong or if there is another half of flag in the this challenge. Not sure. Haven’t seen anyone else post a writeup for this challenge yet. Would like to see what step I’ve missed.
This was an fun challenge. Haven’t done many cryto challenges yet. There may have been an algorithm to reverse the initial flag encoding program. But, I like that there is a way to get the answer with quick brute-forcing that doesn’t take hours or days.