UIUCTF X Marked the Spot writeup

Given:

For the challenge we were given and encrypted file and the encryption algorithm

from itertools import cycle

flag = b"uiuctf{????????????????????????????????????????}"
# len(flag) = 48
key  = b"????????"
# len(key) = 8
ct = bytes(x ^ y for x, y in zip(flag, cycle(key)))

with open("ct", "wb") as ct_file:
    ct_file.write(ct)

This is simple XOR, and we can retrieve the key using the association property of the XOR operation as we know some part of the plaintext, which is uiuctf and key is ok 8 charaters, to get the full key we only need to bruteforce one character

Solve Script:

from itertools import cycle
from pwn import xor 


f = open('ct','rb')
flag = f.read()

pt = bytes.fromhex("uiuctf{".encode().hex())

key = xor(flag[:7],pt)

for i in range(0xff):
    key = key + bytes([i])
    pt = xor(flag,key)
    if b'uiuctf{' in pt:
        if b'}' in pt[len(pt)-1:]:
            print(pt)
    key = key[:-1]

flag: uiuctf{n0t_ju5t_th3_st4rt_but_4l50_th3_3nd!!!!!}