Python Simple Cypher

from random import shuffle

from tkinter.filedialog import askopenfilename

# Read the enciphered message

with open('encipher.txt', 'r') as infile:

    cipher_text = infile.read().rstrip().replace('\n', ' ').lower()

# Read the key

with open('key.txt', 'r') as keyfile:

    key = list(keyfile.read().rstrip())

# Standard alphabet plus space

alph = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z', ' ']

if len(key) < 27:

    key.append(' ')

# Decipher the message

plain_text = ''

for c in cipher_text:

    if c in key:

        idx = key.index(c)

        plain_text += alph[idx]

    else:

        plain_text += c

# Write the deciphered message to file

with open('decipher.txt', 'w') as outfile:

    outfile.write(plain_text)

print(plain_text)