Python实现Playfair密码算法

Playfair密码算法是一种古老的加密技术，它使用一个5×5的矩阵来加密和解密文本。以下是使用Python实现Playfair密码算法的示例代码：

# 定义 Playfair 加密函数
def playfair_encrypt(plain_text, key):
    # 生成密钥矩阵
    key = key.replace(" ", "").upper()
    key_matrix = list(key)
    alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
    for letter in alphabet:
        if letter not in key_matrix:
            key_matrix.append(letter)
    # 将明文转换成双字母对
    plain_text = plain_text.upper().replace(" ", "")
    plain_text_pairs = []
    i = 0
    while i < len(plain_text):
        if i == len(plain_text) - 1 or plain_text[i] == plain_text[i+1]:
            plain_text_pairs.append(plain_text[i] + "X")
            i += 1
        else:
            plain_text_pairs.append(plain_text[i:i+2])
            i += 2
    # 加密双字母对
    cipher_text = ""
    for pair in plain_text_pairs:
        row1, col1 = divmod(key_matrix.index(pair[0]), 5)
        row2, col2 = divmod(key_matrix.index(pair[1]), 5)
        if row1 == row2:
            cipher_text += key_matrix[row1*5+(col1+1)%5] + key_matrix[row2*5+(col2+1)%5]
        elif col1 == col2:
            cipher_text += key_matrix[((row1+1)%5)*5+col1] + key_matrix[((row2+1)%5)*5+col2]
        else:
            cipher_text += key_matrix[row1*5+col2] + key_matrix[row2*5+col1]
    return cipher_text

# 定义 Playfair 解密函数
def playfair_decrypt(cipher_text, key):
    # 生成密钥矩阵
    key = key.replace(" ", "").upper()
    key_matrix = list(key)
    alphabet = "ABCDEFGHIKLMNOPQRSTUVWXYZ"
    for letter in alphabet:
        if letter not in key_matrix:
            key_matrix.append(letter)
    # 解密双字母对
    plain_text = ""
    for i in range(0, len(cipher_text), 2):
        row1, col1 = divmod(key_matrix.index(cipher_text[i]), 5)
        row2, col2 = divmod(key_matrix.index(cipher_text[i+1]), 5)
        if row1 == row2:
            plain_text += key_matrix[row1*5+(col1-1)%5] + key_matrix[row2*5+(col2-1)%5]
        elif col1 == col2:
            plain_text += key_matrix[((row1-1)%5)*5+col1] + key_matrix[((row2-1)%5)*5+col2]
        else:
            plain_text += key_matrix[row1*5+col2] + key_matrix[row2*5+col1]
    # 去掉添加的 X 字符
    plain_text = plain_text.replace("X", "")
    return plain_text

# 测试
plain_text = "hello world"
key = "playfair example"
cipher_text = playfair_encrypt(plain_text, key)
print(cipher_text)
print(playfair_decrypt(cipher_text, key))

在上面的示例中，`playfair_encrypt`函数使用给定的密钥矩阵加密明文，而`playfair_decrypt`函数使用相同的密钥矩阵解密密文。注意，为了使明文的长度为偶数，如果有两个连续的相同字母，我们在它们之间添加一个 X 字符。在解密时，我们需要去掉这些添加的 X 字符。