维吉尼亚密码（Vigenere Cipher）体制的加密和解密

移位密码和代换密码，一旦密钥被选中，则每个字母对应的数字都会被加密，变换成对应的唯一数字。 这种密码体制被称为单表代换密码。

维吉尼亚密码是一种多表代换密码，发明者是16世纪的法国人Blaise de Vigenere。

假设m=6，密钥字为CIPHER，对应如下的数字串K=(2,8,15,7,4,17)。要加密的明文为：thiscryptosystemisnotsecure，将明文串转换为对应的数字，每六个为一组，使用密钥字进行模26下的加密运算，如下所示：

则相应的密文为：VPXZGIAXIVWPUBTTMJPWIZIWZT。

 

代码实现（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)