Autokey加密是一种基于密钥自动重复的加密技术，在加密和解密时需要使用到同一个密钥。以下是Python中实现Autokey加密的示例代码：

def autokey_encrypt(plaintext, key):
    key = key.upper()
    key_chars = [i for i in key if i.isalpha()]
    
    ciphertext = ""
    key_index = 0
    for char in plaintext.upper():
        if char.isalpha():
            shift = ord(key_chars[key_index]) - ord('A')
            cipher_char = chr(((ord(char) - ord('A') + shift) % 26) + ord('A'))
            ciphertext += cipher_char
            key_index = (key_index + 1) % len(key_chars)
        else:
            ciphertext += char
    
    return ciphertext

def autokey_decrypt(ciphertext, key):
    key = key.upper()
    key_chars = [i for i in key if i.isalpha()]
    
    plaintext = ""
    key_index = 0
    for char in ciphertext.upper():
        if char.isalpha():
            shift = ord(key_chars[key_index]) - ord('A')
            plain_char = chr(((ord(char) - ord('A') - shift) % 26) + ord('A'))
            plaintext += plain_char
            key_chars.append(plain_char)
            key_index = (key_index + 1) % len(key_chars)
        else:
            plaintext += char
            
    return plaintext

使用方法如下：

plaintext = "ATTACKATDAWN"
key = "LEMON"

ciphertext = autokey_encrypt(plaintext, key)
print(ciphertext) # LXFOPVEFRNHR

decrypted_text = autokey_decrypt(ciphertext, key)
print(decrypted_text) # ATTACKATDAWN