waiter.py

"""
Solution for RCTF 2017 challenge "Need a waiter"

714 points, 9 solves
"""

import codecs

rearranger = [11,15,14,13,7,9,6,3,0,1,2,10,5,12,8,4]

ciphertext = "fc55e53d4bee0c96a781e1cffe7f3dce433242df298f2a97edc235eb191c9da6d22116cdba99beb14ca6d4bd77098e0797"

ciphertext = [x for x in bytes.fromhex(ciphertext)]

def rng(seed):
	return (123 * seed + 59) % 65536

seed = ciphertext.pop() ^ 66

randomised = []

for i in range(len(ciphertext)):
	seed = rng(seed)
	ciphertext[i] ^= seed % 256

reordered = []

for startpoint in range(0,len(ciphertext),len(rearranger)):
	chunk = ciphertext[startpoint:startpoint+len(rearranger)]
	for i in range(len(rearranger)):
		reordered.append(chunk[rearranger.index(i)])

rot13 = "".join(chr(x) for x in reordered if x != 0)

print(codecs.encode(rot13, 'rot_13'))