## Python, , playfair_cipher.py

``````import itertools
import string
from typing 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
``````