xorxorxor.py

from binascii import unhexlify
import math

# STEP 1 => Get the bytes of the flag
encrypted_flag = unhexlify(open('output.txt','r').read().split(' ')[1].strip('\n'))
part_flag = b'HTB{'
# STEP 2 => Get the encryption key by xoring the known string with the similar placed output bytes
key = [chr(b ^ encrypted_flag[a]) for a,b in enumerate(part_flag)]

messages = []

#STEP 3 => decrypt the flag now that we have the key
for i in range(int(len(encrypted_flag) / len(key)) + 1):
  k = math.floor(len(encrypted_flag)/len(key)) * key if len(encrypted_flag) % len(key) == 0 else math.floor(len(encrypted_flag)/len(key)) * key + key[:len(encrypted_flag)%len(key)]
  message = b''
  size = len(key)
  d = encrypted_flag[size*i:size*(i+1)]
  for a,b in enumerate(d):
    message += bytes(chr(b ^ ord(key[a])),'utf-8')
  messages.append(message)

  print(b''.join(messages))