Hackover CTF rollthedice solution

rollthedice.py

from pwn import *
from Crypto.Cipher import AES

r = remote("challenges.hackover.h4q.it", 1415)

def decr(key, roll):
	return u16(AES.new(key).decrypt(roll)[:2], endian="big")

for i in range(32):
	r.recvuntil("My dice roll: ")
	roll = b64d(r.recvline())
	r.recvuntil("Your dice roll: ")
	r.sendline(b64e(roll))
	r.recvuntil("My key: ")
	key = b64d(r.recvline())
	dice = decr(key, roll)
	log.info("Got dice roll: %d" % dice)

	# only the first 2 bytes of the AES block are relevant
	# we can bruteforce a key which decrypts to the bytes we need
	i = 0
	while True:
		trykey = p64(0)+p64(i)
		if decr(trykey, roll) == 7-dice:
			break
		i+=1

	r.recvuntil("Your key: ")
	r.sendline(b64e(trykey))

r.interactive()