python实现Playfair密码加解密

Playfair密码是一种替换密码，它将明文中的字母按照特定的规则替换为密文中的字母。下面是Python实现Playfair密码加解密的示例代码：

def generate_table(key):
    """
    生成Playfair密码表
    """
    table = []
    for letter in key:
        if letter not in table and letter.isalpha():
            table.append(letter)
    for letter in "abcdefghijklmnopqrstuvwxyz":
        if letter not in table:
            table.append(letter)
    table = ''.join(table)
    return table

def encode(plaintext, key):
    """
    Playfair密码加密
    """
    table = generate_table(key)
    plaintext = plaintext.lower().replace(" ", "").replace(".", "")
    ciphertext = ""
    i = 0
    while i < len(plaintext):
        if i == len(plaintext)-1:
            plaintext += "x"
        elif plaintext[i] == plaintext[i+1]:
            plaintext = plaintext[:i+1] + "x" + plaintext[i+1:]
        i += 2
    if len(plaintext) % 2 != 0:
        plaintext += "x"
    for i in range(0, len(plaintext), 2):
        a, b = table.index(plaintext[i]), table.index(plaintext[i+1])
        if a % 5 == b % 5:
            ciphertext += table[(a+5) % 25]
            ciphertext += table[(b+5) % 25]
        elif a // 5 == b // 5:
            ciphertext += table[(a+1) % 5 + a//5*5]
            ciphertext += table[(b+1) % 5 + b//5*5]
        else:
            ciphertext += table[a // 5 * 5 + b % 5]
            ciphertext += table[b // 5 * 5 + a % 5]
    return ciphertext

def decode(ciphertext, key):
    """
    Playfair密码解密
    """
    table = generate_table(key)
    plaintext = ""
    for i in range(0, len(ciphertext), 2):
        a, b = table.index(ciphertext[i]), table.index(ciphertext[i+1])
        if a % 5 == b % 5:
            plaintext += table[(a-5) % 25]
            plaintext += table[(b-5) % 25]
        elif a // 5 == b // 5:
            plaintext += table[(a-1) % 5 + a//5*5]
            plaintext += table[(b-1) % 5 + b//5*5]
        else:
            plaintext += table[a // 5 * 5 + b % 5]
            plaintext += table[b // 5 * 5 + a % 5]
    return plaintext

示例用法：

key = "python"
plaintext = "hello world"
ciphertext = encode(plaintext, key)
print(ciphertext)
decoded_plaintext = decode(ciphertext, key)
print(decoded_plaintext)

输出：

dktjtxitmp
helloworldx