cryptology25_frequencyanalysis.py

# Turing Team
# https://t.me/turing_team
# Cryptology #25
# Frequency Cryptanalysis

from string import ascii_uppercase

ETAOIN = 'ETAOINSHRDLCUMWFGYPBVKJXQZ'

def get_letter_count(text):
    letter_count = {
        'A': 0, 'B': 0, 'C': 0, 'D': 0, 'E': 0,
        'F': 0, 'G': 0, 'H': 0, 'I': 0, 'J': 0,
        'K': 0, 'L': 0, 'M': 0, 'N': 0, 'O': 0,
        'P': 0, 'Q': 0, 'R': 0, 'S': 0, 'T': 0,
        'U': 0, 'V': 0, 'W': 0, 'X': 0, 'Y': 0,
        'Z': 0
    }

    for letter in text.upper():
        if letter in LETTERS:
            letter_count[letter] += 1

    return letter_count

def get_zero_item(items):
    return items[0]

def get_freq_order(text):
    letter_to_freq = get_letter_count(text)
    freq_to_letter = {}

    for letter in LETTERS:
        if letter_to_freq[letter] not in freq_to_letter:
            freq_to_letter[letter_to_freq[letter]] = [letter]
        else:
            freq_to_letter[letter_to_freq[letter]].append(letter)

    for freq in freq_to_letter:
        freq_to_letter[freq].sort(key=ETAOIN.find, reverse=True)
        freq_to_letter[freq] = ''.join(freq_to_letter[freq])

    freq_pairs = list(freq_to_letter.items())
    freq_pairs.sort(key=get_zero_item, reverse=True)

    freq_order = []
    for freq_pair in freq_pairs:
        freq_order.append(freq_pair[1])

    return ''.join(freq_order)


def get_freq_score(text):
    freq_order = get_freq_order(text)
    score = 0

    for common_letter in ETAOIN[:6]:
        if common_letter in freq_order[:6]:
            score += 1

    for uncommon_letter in ETAOIN[-6:]:
        if uncommon_letter in freq_order[-6:]:
            score += 1

    return score