new_contents: LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
  
  
  def translate_message(key, message, mode):
      """
      >>> translate_message("QWERTYUIOPASDFGHJKLZXCVBNM","Hello World","encrypt")
      'Pcssi Bidsm'
      """
      chars_a = LETTERS if mode == "decrypt" else key
      chars_b = key if mode == "decrypt" else LETTERS
      translated = ""
      # loop through each symbol in the message
      for symbol in message:
          if symbol.upper() in chars_a:
              # encrypt/decrypt the symbol
              sym_index = chars_a.find(symbol.upper())
              if symbol.isupper():
                  translated += chars_b[sym_index].upper()
              else:
                  translated += chars_b[sym_index].lower()
          else:
              # symbol is not in LETTERS, just add it
              translated += symbol
      return translated
  
  
  def encrypt_message(key: str, message: str) -> str:
      """
      >>> encrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
      'Pcssi Bidsm'
      """
      return translate_message(key, message, "encrypt")
  
  
  def decrypt_message(key: str, message: str) -> str:
      """
      >>> decrypt_message("QWERTYUIOPASDFGHJKLZXCVBNM", "Hello World")
      'Itssg Vgksr'
      """
      return translate_message(key, message, "decrypt")
  
  
  def main():
      message = "Hello World"
      key = "QWERTYUIOPASDFGHJKLZXCVBNM"
      mode = "decrypt"  # set to 'encrypt' or 'decrypt'
  
      if mode == "encrypt":
          translated = encrypt_message(key, message)
      elif mode == "decrypt":
          translated = decrypt_message(key, message)
      print(f"Using the key {key}, the {mode}ed message is: {translated}")
  
  
  if __name__ == "__main__":
      import doctest
  
      doctest.testmod()
      main()
  
subject: create monoalphabetic cipher (#3449)