Example of Known Plain-text Attack (KPA) to break Vigenère's Cipher

kpa.py

"""
Using Known Plain-text Attack (KPA) to break Vigenère's Cipher

Check the following link to know more about the Vigenère's Cipher, including how to use KPA to break the encryption.
https://www.commonlounge.com/vigeneres-cipher-b27ffb470c3a4d2b83378edee0a55dab/
"""

import re
import string

plain_text = "This is a sample text. Or is it?!?!"
cyphered_text = "ffswgeycekbjoxcjryvgegd"

def remove_gunk(text):
    """
    Remove any none alphabetical characters
    """
    return re.sub(r"[^a-z]", "", text.lower(), 0, re.MULTILINE)

def convert_to_numbers(text):
    """
    Converts character to number. a -> 1, b -> 2, ... z -> 26
    """
    return list(map(lambda x: string.ascii_lowercase.index(x) + 1, text))

def KPA(plain_text, cyphered_text):
    """
    Known Plain-text Attack
    """
    pt = convert_to_numbers(remove_gunk(plain_text))
    ct = convert_to_numbers(remove_gunk(cyphered_text))
    result = []
    for i in range(len(ct)):
        letter_location = (ct[i] - pt[i]) % 26
        ascii = chr(letter_location + 1 + 96)
        result.append(ascii)
    print(''.join(result))

KPA(plain_text, cyphered_text)
# This will print 'mykeymykeymykeymykeymyk'. You can deduce from this that the key is 'mykey'