cipher_solver.py

from collections import defaultdict, Counter
import pprint


def encode(x, known):
    seen_vowels = {}
    seen_other = {}

    def check(seen, x, m):
        if x in known:
            return x
        if x not in seen:
            seen[x] = (len(seen) + 1) * m
        return seen[x]

    return tuple(
        check(seen_vowels, i, -1) if i in "AEIOU" else check(seen_other, i, 1)
        for i in x
    )


def decode(text, cipher):
    return "".join(cipher.get(i, i) for i in text)


def most_common():
    # Read the cipher text.
    lines = open("cipher19.txt").readlines()
    cleaned = "".join(x.replace(" ", "").strip() for x in lines[1::2])
    text = [cleaned[i : i + 3] for i in range(0, len(cleaned), 3)]

    # Read the code book.
    code_to_word = dict(
        tuple(x.strip("\n").split(" : "))
        for x in open("codeBook_19.txt", encoding="latin")
    )

    # Assume THE is the most common word.
    cipher = {}
    most_common_codeword = Counter(text).most_common(1)[0][0]
    the_codeword = next(x for x, y in code_to_word.items() if y == "THE")
    for l, r in zip(most_common_codeword, the_codeword):
        cipher[l] = r

    # Solve the rest.
    word_set = set(text)
    while len(cipher) < 26:
        encoding_to_possible_pairs = defaultdict(lambda: set())
        for code, word in code_to_word.items():
            encoding_to_possible_pairs[encode(code, cipher.values())].add((code, word))

        for word in word_set:
            encoding = tuple(cipher.get(i, i) for i in encode(word, cipher.keys()))
            possible_pairs = encoding_to_possible_pairs[encoding]
            if len(possible_pairs) > 1 or all(i in cipher for i in word):
                continue
            for l, r in zip(word, possible_pairs.pop()[0]):
                if l not in cipher:
                    cipher[l] = r
            break
    print(
        " ".join(
            code_to_word[decode(i, cipher)] if set(i) < set(cipher) else "?"
            for i in text
        )
    )


if __name__ == "__main__":
    most_common()