Vigenère cipher. Ref: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

vigenere_cipher.py

#!/usr/bin/env python

import string
import textwrap
import functools


def filter_not_upper(text: str) -> str:
    return "".join(filter(lambda c: c in string.ascii_uppercase, text))


def make_vigenere_table(inverse: bool = False) -> dict[str, dict[str, str]]:
    vigenere_table = {}
    base: int = ord("A")
    for r in string.ascii_uppercase:
        vigenere_table[r] = {}
        for c in string.ascii_uppercase:
            char = chr((ord(r) + ord(c)) % 26 + base)
            if inverse:
                vigenere_table[r][char] = c
            else:
                vigenere_table[r][c] = char
    return vigenere_table


_encrypt_table = make_vigenere_table()
_decrypt_table = make_vigenere_table(True)


def encode(text: str, key: str, decrypt: bool = False) -> str:
    result = ""
    text = filter_not_upper(text.upper())
    key = filter_not_upper(key.upper())
    for word in textwrap.wrap(text, len(key)):
        for r, c in zip(key, word):
            if decrypt:
                result += _decrypt_table[r][c]
            else:
                result += _encrypt_table[r][c]
    return result


encrypt = functools.partial(encode, decrypt=False)
decrypt = functools.partial(encode, decrypt=True)


if __name__ == "__main__":
    key = "lemon"
    plaintext = "attackatdawn"
    ciphertext = "lxfopvefrnhr"
    assert encrypt(plaintext, key) == ciphertext.upper()
    assert decrypt(ciphertext, key) == plaintext.upper()