import string
alpha = string.ascii_lowercase

plainText = input("Enter your plaintext to be encrypted: ")
userKey = input("Enter your alphabetical key; exits on invalid character: ")
cipherText = ''

# Check validity of key; for demonstration purposes I only accept alphabet characters

for char in userKey:
 if char.lower() not in alpha:
  print("Invalid key; quitting.")
  quit()
 else:
  rot = alpha.index(char)
  print(rot)