写一个完成栅栏密码的加密与解密

栅栏密码是一种简单的替换密码，它通过将明文中的字符按照规定的数量和顺序排列，然后再读取得到密文。以下是一个 Python 实现的栅栏密码加密和解密程序：

def fence_cipher_encrypt(plain_text, rails):
    fence = [[] for _ in range(rails)]
    rail = 0
    direction = 1
    
    for char in plain_text:
        fence[rail].append(char)
        rail += direction
        
        if rail == 0 or rail == rails - 1:
            direction = -direction
    
    cipher_text = ''.join([char for rail in fence for char in rail])
    return cipher_text

def fence_cipher_decrypt(cipher_text, rails):
    fence = [[] for _ in range(rails)]
    rail = 0
    direction = 1
    
    for char in cipher_text:
        fence[rail].append(None)
        rail += direction
        
        if rail == 0 or rail == rails - 1:
            direction = -direction
    
    index = 0
    for rail in fence:
        for i in range(len(rail)):
            rail[i] = cipher_text[index]
            index += 1
            
    plain_text = ''.join([char for rail in fence for char in rail])
    return plain_text

使用方式示例：

cipher_text = fence_cipher_encrypt('HELLO WORLD', 3)
print(cipher_text) # HOLELWRDLO

plain_text = fence_cipher_decrypt(cipher_text, 3)
print(plain_text) # HELLO WORLD

其中 `fence_cipher_encrypt` 函数接受两个参数：明文 `plain_text` 和栅栏数量 `rails`，返回加密后的密文。`fence_cipher_decrypt` 函数接受两个参数：密文 `cipher_text` 和栅栏数量 `rails`，返回解密后的明文。