Tenable CTF 2021 - Hacker Manifesto

solve.py

# Takes a group of 3 bytes and parses it into an offset, length and character
class Group:
	def __init__(self, b):
		self.off = ((b[1] & 0b11) << 8) | b[0]
		self.num = b[1] >> 2
		self.ch = chr(b[2])

# Yields lst in chunks of size n
def get_chunks(lst, n):
	for i in range(0, len(lst), n):
		yield lst[i:i + n]

def out_to_string(data):
	return "".join(o.ch for o in data)

if __name__ == '__main__':
	
	with open("hacker_manifesto.txt", "rb") as f:
		data = f.read()

	# Parse the bytes into groups of 3
	data = [Group(chunk) for chunk in get_chunks(data, 3)]

	# Iterate through the groups
	out = []
	for i, group in enumerate(data):
		
		if group.off == 0 and group.num == 0:
			out.append(group)

		else:

			start = -group.off
			end = start + group.num

			# We need this check because out[-n:0] does not work as expected (out[-n:0] != out[-n:])
			if end == 0:
				out += out[start:]
			else:
				out += out[start:end]

			out.append(group)

	print(out_to_string(out))