BBCTF 2023 Visionary Cipher solver

brute.py

#!/usr/bin/python3
# BBCTF 2023 Visionary Cipher solver by Gynvael Coldwind
from string import ascii_lowercase, digits
from random import choices
from hashlib import md5
import sys

alphabets = ascii_lowercase + digits + "_{}"
def pos(ch):
  return alphabets.find(ch)

hash = '17382b1a9caad37bd127f2a7984ccbb9'
C = 'h50k8kbq{u}1qb_z1apvnfxqboh}vp30qrbbxs'

def known(idx, plaintext):
  p1 = pos(C[idx])
  p2 = pos(plaintext)
  return (p1 - p2) % len(alphabets)

key = [
  known(0, 'f'),
  known(1, 'l'),
  known(2, 'a'),
  known(3, 'g'),
  known(4, '{'),
  0,
  0,
  known(-1, '}'),
  0,
  0
]

def decode(text, key):
  k = len(key)
  n = len(text)
  l = len(alphabets)
  return "".join([alphabets[(pos(text[i]) - key[i % k]) % l] for i in range(n)])

for a in range(len(alphabets)):
  key[5] = a
  print('.', end='')
  sys.stdout.flush()
  for b in range(len(alphabets)):
    key[6] = b
    for c in range(len(alphabets)):
      key[8] = c
      for d in range(len(alphabets)):
        key[9] = d
        res = decode(C, key)
        m = md5(res.encode("ascii")).hexdigest()
        if m == hash:
          print(res)


#print(key)