仿射密码python

以下是一个简单的 Python 代码实现仿射密码：

def egcd(a, b):
    if a == 0:
        return (b, 0, 1)
    else:
        g, y, x = egcd(b % a, a)
        return (g, x - (b // a) * y, y)

def modinv(a, m):
    g, x, y = egcd(a, m)
    if g != 1:
        raise Exception('No modular inverse')
    else:
        return x % m

def affine_encrypt(text, key):
    # 将文本转换为大写并去除空格
    text = text.upper().replace(" ", "")
    # 确保a和m互质
    a = key[0]
    m = 26
    if egcd(a, m)[0] != 1:
        raise ValueError('a and m must be coprime.')
    # 加密文本
    cipher = ""
    for char in text:
        if char.isalpha():
            # 将字符转换为数字并进行加密
            cipher += chr(((a * (ord(char) - 65)) % m) + 65)
        else:
            cipher += char
    return cipher

def affine_decrypt(cipher, key):
    # 确保a和m互质
    a = key[0]
    m = 26
    if egcd(a, m)[0] != 1:
        raise ValueError('a and m must be coprime.')
    # 计算a的模逆
    a_inv = modinv(a, m)
    # 解密密文
    text = ""
    for char in cipher:
        if char.isalpha():
            # 将字符转换为数字并进行解密
            text += chr(((a_inv * (ord(char) - 65)) % m) + 65)
        else:
            text += char
    return text

# 例子
text = "Hello, World!"
key = (5, 8) # a=5, b=8
cipher = affine_encrypt(text, key)
print("Encrypted text:", cipher)
decrypted_text = affine_decrypt(cipher, key)
print("Decrypted text:", decrypted_text)

这个代码实现了仿射密码的加密和解密功能。输入一个文本和一个密钥（由a和b组成），输出加密后的文本或解密后的文本。