solve_follow_the_current.py

import os
import zlib
def keystream(key):
	index = 0
	while 1:
		index+=1
		if index >= len(key):
			key += zlib.crc32(key).to_bytes(4,'big')
		yield key[index]
		
def decrypt(enc,k):
	arr=[]
	for i in enc:
		arr.append(i ^ next(k))
	return bytes(arr)
with open('enc','rb') as enc:
	enc=enc.read()
while True:	
	key=os.urandom(2)
	key=keystream(key)
	flag=decrypt(enc,key)
	if b'actf' in flag:
		print(flag)
		break