Maelstrom

This is a crypto problem that provides a decrypt.py file. If you try running it, it prints part of the flag and hangs.

There is a function x that is just checking whether or not a value is prime:

def x(num):
    if num > 1:
        for i in range(2, num):
            if (num % i) == 0:
                return False
                break
        return True
    else:
        return False

You can write a better prime checker yourself, but I just used the isprime function in sympy and kept track of primes that have already been checked:

python3 -m venv venv
. venv/bin/activate
pip install sympy

from sympy.ntheory import isprime

primes = set()

def x(num):
    if num in primes:
        return True
    else:
        n_prime = isprime(num)
        if n_prime:
            primes.add(num)
        return n_prime

I included my edited decrypt.py file.

Flag: flag{more_primes_more_good}

Original writeup (https://github.com/roberson-io/bsides_boston_2020/tree/master/maelstrom).