ctf-1-cipher.py

import os
import random
import string

ALPHABET = string.ascii_lowercase + string.digits
def encrypt(password, text):
  message   = "".join([c.lower() if c.isalnum() else "" for c in text])
  plaintext = [ALPHABET.find(c.lower()) for c in message]
  keystream = [ALPHABET.find(c.lower()) for c in password]
  n = len(plaintext)
  m = len(keystream)
  ciphertext = [(plaintext[i] + keystream[i % m]) % len(ALPHABET) for i in range(n)]
  return "".join([ALPHABET[c] for c in ciphertext])

def decrypt(password, text):
  message   = "".join([c.lower() if c.isalnum() else "" for c in text])
  ciphertxt = [ALPHABET.find(c.lower()) for c in message]
  keystream = [ALPHABET.find(c.lower()) for c in password]
  n = len(ciphertxt)
  m = len(keystream)
  plaintext = [(ciphertxt[i] - keystream[i % m]) % len(ALPHABET) for i in range(n)]
  return "".join([ALPHABET[c] for c in plaintext])

flag = "fa5ef1a9" # przykładowa flaga
text = "hello world" # przykładowa treść
key1 = "ab" # przykładowy klucz nr 1
key2 = "def" # przykładowy klucz nr 2
key3 = "uvxyz" # przykładowy klucz nr 3
text0 = text + ("pw" + "open" + flag + "close") + text
text1 = encrypt(key1, text0)
text2 = encrypt(key2, text1)
text3 = encrypt(key3, text2)
ciphertext = text3

print(decrypt(key1, decrypt(key2, decrypt(key3, ciphertext))))