UMassCTF 2021 - Chains: solution script

solve.py

import itertools
import sys
import subprocess
import shlex
from functools import lru_cache

# Array of values used, as an array of bytes
from arr import arr as ar

def grouper(n, iterable, fillvalue=None):
	args = [iter(iterable)] * n
	return itertools.zip_longest(*args, fillvalue=fillvalue)

@lru_cache(maxsize=4096*2)
def get_pos(v1, v2):

	cmd = "sed -n '{} p' vals/{}.out".format(v2, v1)
	s = subprocess.run(shlex.split(cmd), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	o, _ = map(int, s.stdout.decode().split())
	return o

if __name__ == '__main__':

	# Convert the array of bytes into an array of ints
	arr = []
	for grp in grouper(4, ar):
		if None in grp:
			arr.append(0)
		else:
			arr.append(int.from_bytes(grp, "little"))

	i = 0
	flag = ""
	while True:

		if i > 0x29f7:
			flag += ord(10)
			print(ord(10))
			break

		print(i, end="\r")

		try:
			v1 = arr[i]
			v2 = arr[i+1]
		except:
			print(flag)
			sys.exit()

		j = get_pos(v1, v2)
		flag += chr((j + 0xca5b17ff) % 256)

		i += 2

	print("")
	print(flag)