project_euler.problem_059.sol1

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.

Attributes

COMMON_WORDS
LOWERCASE_INTS
VALID_CHARS
VALID_INTS

Functions

filter_common_word(→ list[str])	Given a list of possible decoded messages, narrow down the possibilities
filter_valid_chars(→ list[str])	Given an encrypted message, test all 3-character strings to try and find the
solution(→ int)	Test the ciphertext against all possible 3-character keys, then narrow down the
try_key(→ str | None)	Given an encrypted message and a possible 3-character key, decrypt the message.

Module Contents

project_euler.problem_059.sol1.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’]

project_euler.problem_059.sol1.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

project_euler.problem_059.sol1.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

project_euler.problem_059.sol1.try_key(ciphertext: list[int], key: tuple[int, Ellipsis]) → 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

project_euler.problem_059.sol1.COMMON_WORDS: list[str] = ['the', 'be', 'to', 'of', 'and', 'in', 'that', 'have']

project_euler.problem_059.sol1.LOWERCASE_INTS: list[int]

project_euler.problem_059.sol1.VALID_CHARS: str

project_euler.problem_059.sol1.VALID_INTS: set[int]