OTP Super Secure Secret Messaging

from random import randint

ALPHABET = 'abcdefghijklmnopqrstuvwxyz'


def generate_otp(sheets, length):
  for sheet in range(sheets):
    with open("otp" + str(sheet) + ".txt", "w") as f:
      for i in range(length):
        f.write(str(randint(0, 26)) + "\n")


def load_sheet(filename):
  with open(filename, "r") as f:
    contents = f.read().splitlines()
  return contents


def get_plaintext():
  plaintext = input('Please type your message ')
  return plaintext.lower()


def load_file(filename):
  with open(filename, "r") as f:
    contents = f.read()
  return contents


def save_file(filename, data):
  with open(filename, 'w') as f:
    f.write(data)


def encrypt(plaintext, sheet):
  ciphertext = ''
  for position, character in enumerate(plaintext):
    if character not in ALPHABET:
      ciphertext += character
    else:
      encrypted = (ALPHABET.index(character) + int(sheet[position])) % 26
      ciphertext += ALPHABET[encrypted]
  return ciphertext


def decrypt(ciphertext, sheet):
  plaintext = ''
  for position, character in enumerate(ciphertext):
    if character not in ALPHABET:
      plaintext += character
    else:
      decrypted = (ALPHABET.index(character) - int(sheet[position])) % 26
      plaintext += ALPHABET[decrypted]
  return plaintext


def menu():
  choices = ['1', '2', '3', '4']
  choice = '0'
  while True:
    while choice not in choices:
      print('What would you like to do?')
      print('1. Generate one-time pads')
      print('2. Encrypt a message')
      print('3. Decrypt a message')
      print('4. Quit the program')
      choice = input('Please type 1, 2, 3 or 4 and press Enter⇴ ')
      if choice == '1':
        sheets = int(
          input('How many one-time pads would you like to generate?⇒ '))
        length = int(input('What will be your maximum message length?⇝ '))
        generate_otp(sheets, length)
      elif choice == '2':
        filename = input('Type in the filename of the OTP you want to use➠ ')
        sheet = load_sheet(filename)
        plaintext = get_plaintext()
        ciphertext = encrypt(plaintext, sheet)
        filename = input('What will be the name of the encrypted file?➾ ')
        save_file(filename, ciphertext)
      elif choice == '3':
        filename = input('Type in the filename of the OTP you want to use➳ ')
        sheet = load_sheet(filename)
        filename = input('Type in the name of the file to be decrypted➻ ')
        ciphertext = load_file(filename)
        plaintext = decrypt(ciphertext, sheet)
        print('The message reads:')
        print('')
        print(plaintext)
      elif choice == '4':
        exit()
      choice = '0'


menu()