『Pythonでいかにして暗号を破るか』5章 アレンジ+練習問題回答

cracking_codes_chpt5_practice.py

# 『Pythonでいかにして暗号を破るか』5章 シーザー暗号
# caesarCipher.pyを参考にしたシーザー暗号プログラム


class CaesarCipher:
    def __init__(self, key: int, symbols: str):
        self.key = key
        self.symbols = symbols

    def encrypt(self, message: str) -> str:
        translated = ''
        symbol_len = len(self.symbols)
        for symbol in message:
            if symbol in self.symbols:
                symbol_index = self.symbols.find(symbol)
                translated_index = self.key + symbol_index
                if translated_index >= symbol_len:
                    # ラップアラウンド処理
                    translated_index -= symbol_len
                translated += self.symbols[translated_index]
            else:
                translated += symbol
        return translated

    def decrypt(self, cipher: str) -> str:
        translated = ''
        symbol_len = len(self.symbols)
        for symbol in cipher:
            if symbol in self.symbols:
                symbol_index = self.symbols.find(symbol)
                translated_index = symbol_index - self.key
                if translated_index < 0:
                    # ラップアラウンド処理
                    translated_index += symbol_len
                translated += self.symbols[translated_index]
            else:
                translated += symbol
        return translated


def make_symbols() -> str:
    upper = ''
    a_ord = ord('A')
    for i in range(0, 26):
        upper += chr(a_ord+i)
    lower = upper.lower()
    symbols = upper + lower + '1234567890' + ' !?.'
    return symbols

symbols = make_symbols()
message = 'This is my secret message.'
key = 13
caesar_cipher = CaesarCipher(key=key, symbols=symbols)
cipher = caesar_cipher.encrypt(message=message)
print(message)
print(cipher)
plain = caesar_cipher.decrypt(cipher=cipher)
print(plain)

# 1
caesar = CaesarCipher(key=8, symbols=symbols)

message = '"You can show black is white by argument," said Filby, "but you will never convince me".'
cipher = caesar.encrypt(message=message)
print('--- 1-a ---')
print(message)
print(cipher)
print(caesar.decrypt(cipher=cipher))

caesar = CaesarCipher(key=21, symbols=symbols)
message = '1234567890'
cipher = caesar.encrypt(message=message)
print('--- 1-b ---')
print(message)
print(cipher)
print(caesar.decrypt(cipher=cipher))


# 2
caesar = CaesarCipher(key=2, symbols=symbols)
cipher = 'Kv?uqwpfu?rncwukdng?gpqwijB'
print('--- 2-a ---')
plain = caesar.decrypt(cipher)
print(cipher)
print(plain)
print(caesar.encrypt(message=plain))

caesar = CaesarCipher(key=22, symbols=symbols)
cipher = 'XCBSw88S18A1S 2SB41SE .8zSEwAS50D5A5x81V'
print('--- 2-b ---')
plain = caesar.decrypt(cipher)
print(cipher)
print(plain)
print(caesar.encrypt(message=plain))