This is from the 2021 Vishwa CTF
For this challenge, we are given the following clue and a ELF binary:
After some preliminary poking at the file, I execute it in the terminal to see what it does. I see that it waits for input from the user and replies with “EWWWW DUMBBB” and exits:
Next I load it into my debugger and inspect the code:
I see that there is cmp performed and it results in a jmp to the failure message:
I modify the jmp and fill it with NOPs:
I then provide some random input and watch for its response:
It looks like a scrambled flag. Most likely a simple shift cipher… maybe a ROT (rotation). I head over to rot13.com and decode it:
Now I have the flag!
