babyencryption.py

import string
from binascii import unhexlify

encrypted = unhexlify(open('msg.enc','r').read())
plaintext = []
#brute force the encryption logic
for b in encrypted:
  for ch in string.printable:
    if ((123 * ord(ch) + 18) % 256 == b):
      plaintext.append(ch)
      break

print(''.join(plaintext))