playfair_cipher.py

import itertools
import string
from collections.abc import Generator, Iterable


def chunker(seq: Iterable[str], size: int) -> Generator[tuple[str, ...], None, None]:
    it = iter(seq)
    while True:
        chunk = tuple(itertools.islice(it, size))
        if not chunk:
            return
        yield chunk


def prepare_input(dirty: str) -> str:
    """
    Prepare the plaintext by up-casing it
    and separating repeated letters with X's
    """

    dirty = "".join([c.upper() for c in dirty if c in string.ascii_letters])
    clean = ""

    if len(dirty) < 2:
        return dirty

    for i in range(len(dirty) - 1):
        clean += dirty[i]

        if dirty[i] == dirty[i + 1]:
            clean += "X"

    clean += dirty[-1]

    if len(clean) & 1:
        clean += "X"

    return clean


def generate_table(key: str) -> list[str]:

    # I and J are used interchangeably to allow
    # us to use a 5x5 table (25 letters)
    alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
    # we're using a list instead of a '2d' array because it makes the math
    # for setting up the table and doing the actual encoding/decoding simpler
    table = []

    # copy key chars into the table if they are in `alphabet` ignoring duplicates
    for char in key.upper():
        if char not in table and char in alphabet:
            table.append(char)

    # fill the rest of the table in with the remaining alphabet chars
    for char in alphabet:
        if char not in table:
            table.append(char)

    return table


def encode(plaintext: str, key: str) -> str:
    table = generate_table(key)
    plaintext = prepare_input(plaintext)
    ciphertext = ""

    # https://en.wikipedia.org/wiki/Playfair_cipher#Description
    for char1, char2 in chunker(plaintext, 2):
        row1, col1 = divmod(table.index(char1), 5)
        row2, col2 = divmod(table.index(char2), 5)

        if row1 == row2:
            ciphertext += table[row1 * 5 + (col1 + 1) % 5]
            ciphertext += table[row2 * 5 + (col2 + 1) % 5]
        elif col1 == col2:
            ciphertext += table[((row1 + 1) % 5) * 5 + col1]
            ciphertext += table[((row2 + 1) % 5) * 5 + col2]
        else:  # rectangle
            ciphertext += table[row1 * 5 + col2]
            ciphertext += table[row2 * 5 + col1]

    return ciphertext


def decode(ciphertext: str, key: str) -> str:
    table = generate_table(key)
    plaintext = ""

    # https://en.wikipedia.org/wiki/Playfair_cipher#Description
    for char1, char2 in chunker(ciphertext, 2):
        row1, col1 = divmod(table.index(char1), 5)
        row2, col2 = divmod(table.index(char2), 5)

        if row1 == row2:
            plaintext += table[row1 * 5 + (col1 - 1) % 5]
            plaintext += table[row2 * 5 + (col2 - 1) % 5]
        elif col1 == col2:
            plaintext += table[((row1 - 1) % 5) * 5 + col1]
            plaintext += table[((row2 - 1) % 5) * 5 + col2]
        else:  # rectangle
            plaintext += table[row1 * 5 + col2]
            plaintext += table[row2 * 5 + col1]

    return plaintext