python栅栏密码解密

栅栏密码是一种简单的加密方式，它将明文按照一定规律排列，然后读取特定的位置上的字符，形成密文。解密时需要按照相同的规律排列密文，然后读取特定位置上的字符，即可恢复明文。

下面是一个简单的 Python 实现：

def decrypt_fence(ciphertext, rails):
    # 根据密文长度和栅栏数计算每个栅栏的长度
    rail_len = [0] * rails
    full_rails = len(ciphertext) // (rails*2-2)
    for i in range(rails):
        if i == 0 or i == rails - 1:
            rail_len[i] = full_rails
        else:
            rail_len[i] = full_rails * 2
    remainder = len(ciphertext) % (rails*2-2)
    for i in range(remainder):
        if i < rails - 1:
            rail_len[i] += 1
        else:
            rail_len[2*rails-i-2] += 1

    # 根据每个栅栏的长度将密文拆分成多个子串
    rails_text = []
    start = 0
    for i in range(rails):
        end = start + rail_len[i]
        rails_text.append(ciphertext[start:end])
        start = end

    # 根据栅栏规律将子串合并成明文
    plaintext = ''
    for i in range(full_rails+1):
        for j in range(rails):
            if i == 0 or i == full_rails:
                if j < len(rails_text) and len(rails_text[j]) > i:
                    plaintext += rails_text[j][i]
            else:
                if j < len(rails_text) and len(rails_text[j]) > i:
                    plaintext += rails_text[j][i]
                if j < len(rails_text) and len(rails_text[j]) > (2*full_rails-i):
                    plaintext += rails_text[j][2*full_rails-i]

    return plaintext

使用方法：

ciphertext = 'sae_ecnroirnptmsrtof_'
rails = 3
plaintext = decrypt_fence(ciphertext, rails)
print(plaintext)  # 输出："secret_message_to_fbi"

其中，`ciphertext` 是密文，`rails` 是栅栏数。运行结果为明文。