Brute forcing some vigenere cipher

brute.py

#!/usr/bin/env python3

def main():
    words = {}
    with open("words", "r") as wordfile:
        for line in wordfile:
            if len(line.rstrip()) in [2, 3, 4, 5]:
                words[line.rstrip()] = True

    cipher = [[7, 16], [24, 26], [3, 8, 19, 5], [5, 24, 25, 13],
              [24, 23, 7, 18], [2, 8, 12], [13, 21, 2, 8, 19],
              [18, 2, 23], [26, 26, 8, 26], [26, 25, 9, 11],
              [2, 15, 25], [21, 20, 7, 9, 13]]
    key_length = 1
    best = 0
    num_words = len(cipher)
    key = [0] * key_length
    while True:
        key_index = 0
        plain = 0
        plain_words = []
        for word in cipher:
            plain_word = ""
            for letter in word:
                new_letter = chr(97+((letter + key[key_index]) % 26))
                plain_word += new_letter
                key_index = (key_index + 1) % key_length
            plain_words.append(plain_word)
            if plain_word in words.keys():
                plain += 1
        ratio = plain/num_words
        if ratio > best:
            print("".join([chr(97+l) for l in key]),
                  " | ", " ".join(plain_words), ratio)
            best = ratio
        updated = key_length - 1
        while True:
            key[updated] += 1
            key[updated] = key[updated] % 26
            if key[updated] != 0:
                break
            updated -= 1
            if updated == -1:
                key_length += 1
                key = [1] * key_length
                print("Key length is %d" % key_length)
                break


if __name__ == "__main__":
    main()