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"""

Each character on a computer is assigned a unique code and the preferred standard is

ASCII (American Standard Code for Information Interchange).

For example, uppercase A = 65, asterisk (*) = 42, and lowercase k = 107.

A modern encryption method is to take a text file, convert the bytes to ASCII, then

XOR each byte with a given value, taken from a secret key. The advantage with the

XOR function is that using the same encryption key on the cipher text, restores

the plain text; for example, 65 XOR 42 = 107, then 107 XOR 42 = 65.

For unbreakable encryption, the key is the same length as the plain text message, and

the key is made up of random bytes. The user would keep the encrypted message and the

encryption key in different locations, and without both "halves", it is impossible to

decrypt the message.

Unfortunately, this method is impractical for most users, so the modified method is

to use a password as a key. If the password is shorter than the message, which is

likely, the key is repeated cyclically throughout the message. The balance for this

method is using a sufficiently long password key for security, but short enough to

be memorable.

Your task has been made easy, as the encryption key consists of three lower case

characters. Using p059_cipher.txt (right click and 'Save Link/Target As...'), a

file containing the encrypted ASCII codes, and the knowledge that the plain text

must contain common English words, decrypt the message and find the sum of the ASCII

values in the original text.

"""

from __future__ import annotations

import string

from itertools import cycle, product

from pathlib import Path

VALID_CHARS: str = (

string.ascii_letters + string.digits + string.punctuation + string.whitespace

)

LOWERCASE_INTS: list[int] = [ord(letter) for letter in string.ascii_lowercase]

VALID_INTS: set[int] = {ord(char) for char in VALID_CHARS}

COMMON_WORDS: list[str] = ["the", "be", "to", "of", "and", "in", "that", "have"]

def try_key(ciphertext: list[int], key: tuple[int, ...]) -> str | None:

"""

Given an encrypted message and a possible 3-character key, decrypt the message.

If the decrypted message contains a invalid character, i.e. not an ASCII letter,

a digit, punctuation or whitespace, then we know the key is incorrect, so return

None.

>>> try_key([0, 17, 20, 4, 27], (104, 116, 120))

'hello'

>>> try_key([68, 10, 300, 4, 27], (104, 116, 120)) is None

True

"""

decoded: str = ""

keychar: int

cipherchar: int

decodedchar: int

for keychar, cipherchar in zip(cycle(key), ciphertext):

decodedchar = cipherchar ^ keychar

if decodedchar not in VALID_INTS:

return None

decoded += chr(decodedchar)

return decoded

def filter_valid_chars(ciphertext: list[int]) -> list[str]:

"""

Given an encrypted message, test all 3-character strings to try and find the

key. Return a list of the possible decrypted messages.

>>> from itertools import cycle

>>> text = "The enemy's gate is down"

>>> key = "end"

>>> encoded = [ord(k) ^ ord(c) for k,c in zip(cycle(key), text)]

>>> text in filter_valid_chars(encoded)

True

"""

possibles: list[str] = []

for key in product(LOWERCASE_INTS, repeat=3):

encoded = try_key(ciphertext, key)

if encoded is not None:

possibles.append(encoded)

return possibles

def filter_common_word(possibles: list[str], common_word: str) -> list[str]:

"""

Given a list of possible decoded messages, narrow down the possibilities

for checking for the presence of a specified common word. Only decoded messages

containing common_word will be returned.

>>> filter_common_word(['asfla adf', 'I am here', ' !?! #a'], 'am')

['I am here']

>>> filter_common_word(['athla amf', 'I am here', ' !?! #a'], 'am')

['athla amf', 'I am here']

"""

return [possible for possible in possibles if common_word in possible.lower()]

def solution(filename: str = "p059_cipher.txt") -> int:

"""

Test the ciphertext against all possible 3-character keys, then narrow down the

possibilities by filtering using common words until there's only one possible

decoded message.

>>> solution("test_cipher.txt")

3000

"""

ciphertext: list[int]

possibles: list[str]

common_word: str

decoded_text: str

data: str = Path(__file__).parent.joinpath(filename).read_text(encoding="utf-8")

ciphertext = [int(number) for number in data.strip().split(",")]

possibles = filter_valid_chars(ciphertext)

for common_word in COMMON_WORDS:

possibles = filter_common_word(possibles, common_word)

if len(possibles) == 1:

break

decoded_text = possibles[0]

return sum(ord(char) for char in decoded_text)

if __name__ == "__main__":

print(f"{solution() = }")

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