維吉尼亞密碼（Vigenere Cipher）體制的加密和解密

移位密碼和代換密碼，一旦密鑰被選中，則每個字母對應的數字都會被加密，變換成對應的唯一數字。 這種密碼體制被稱為單表代換密碼。

維吉尼亞密碼是一種多表代換密碼，發明者是16世紀的法國人Blaise de Vigenere。

假設m=6，密鑰字為CIPHER，對應如下的數字串K=(2,8,15,7,4,17)。要加密的明文為：thiscryptosystemisnotsecure，將明文串轉換為對應的數字，每六個為一組，使用密鑰字進行模26下的加密運算，如下所示：

則相應的密文為：VPXZGIAXIVWPUBTTMJPWIZIWZT。

 

代碼實現（Python 3）

def vigenere_cipher_encrypt(message:str, keys=()):
    SYMBOLS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    translated = ''
    n = 0

    keylen = len(keys)
    message = message.upper()
    for symbol in message:
        if symbol in SYMBOLS:
            symbolIndex = SYMBOLS.find(symbol)
            key =  keys[n % keylen]
            translatedIndex= (symbolIndex + key) % 26
            translated = translated + SYMBOLS[translatedIndex]
            n = n + 1
        else:
            translated = translated + symbol
    print(translated)


def vigenere_cipher_encrypt_by_keystring(message:str, keystring:str):
    keys = translate_keystring(keystring)
    vigenere_cipher_encrypt(message, keys)


def vigenere_cipher_decrypt(message:str, keys=()):
    SYMBOLS = 'abcdefghijklmnopqrstuvwxyz'
    translated = ''
    n = 0

    keylen = len(keys)
    message = message.lower()
    for symbol in message:
        if symbol in SYMBOLS:
            symbolIndex = SYMBOLS.find(symbol)
            translatedIndex= (symbolIndex - keys[n % keylen])%26
            translated = translated + SYMBOLS[translatedIndex]
            n = n + 1
        else:
            translated = translated + symbol
    print(translated)


def vigenere_cipher_decrypt_by_keystring(message:str, keystring:str):
    keys = translate_keystring(keystring)
    vigenere_cipher_decrypt(message, keys)


def translate_keystring(keystring:str):
    SYMBOLS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    keys = []

    keystring = keystring.upper()
    for symbol in keystring:
        if symbol in SYMBOLS:
            symbolIndex = SYMBOLS.find(symbol)
            keys.append(symbolIndex)
        else:
            keys.append(-1)

    return tuple(keys)