default_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

def masc(k : str, plain : str, alphabet : str = default_alphabet):
  # make sure that k only contains characters from alphabet
  for c in k:
    if c not in alphabet:
      raise ValueError("key contains character thats not in alphabet, namely: " + c)

  # build key
  ## make unique
  k_unique = []
  for c in k:
    if not (c in k_unique):
      k_unique.append(c)

  ## append alphabet
  last = k_unique[len(k_unique) - 1]
  alphabet_pos = alphabet.index(last)
  for i in list(range(alphabet_pos + 1, len(alphabet))) + list(range(0, alphabet_pos)):
    if alphabet[i] not in k_unique:
      k_unique.append(alphabet[i])

  return k_unique