转置解密对某些密钥不起作用？

Question

为什么这个换位解密代码不能处理某些密钥？你知道吗

def transencrypt(word,key):
    '''Traspositon encryption function. This function is used to encrypt a line
using the transposition encryption method. To know how transpositon encryption
works you can visit here https://en.wikipedia.org/wiki/Transposition_cipher.'''
    count1=0
    count2=0
    encrypted=''
    encryptbox=['']*key
    while count1<key:
        count2=count1
        while count2<len(word):
            encryptbox[count1]+=word[count2]
            count2+=key
        encrypted+=encryptbox[count1]
        count1+=1
    return encrypted

def transdecrypt(word,key):
    '''This Function is for the decrypting the encrypted strings encrypted by
transencrypt().This function only requires the encrypted string and the key
with which it has been decrypted.'''
    import math
    count1=0
    count2=0
    decrypted=''
    col=int(math.ceil(len(word)/key))
    decryptbox=['']*col
    while count1<col:
        count2=count1
        while count2<len(word):
            decryptbox[count1]+=word[count2]
            count2+=col
        decrypted+=decryptbox[count1]
        count1+=1
    return decrypted

print(transencrypt('hello world',5))
print(transdecrypt('h dewlolrol',5))

OP's original code source

我试过用密钥5加密“hello world”，但在解密时我得到了错误的结果。使用其他键也可以。你知道吗

Answer 1

问题是字符串长度（11）没有平均地划分到键（5）中，因此字符串"hello world"编码成组h d-ew-lo-lr-ol，即"h dewlolrol"。这很好，但是解密例程将"h dewlolrol"分割成h d-ewl-olr-ol，并生成错误的结果"heoo wlldlr"。你知道吗

有几种可能的解决方法：

1）用数组替换encryptbox字符串，并将加密单元填充到偶数宽度段：h d-ew -lo -lr -ol即"h dew lo lr ol "

这将允许您的解密例程工作，但您将在解密结束时得到空格，加密字符串的大小将不同于原始字符串。你知道吗

或

2）根据要解码的剩余字符串的长度和预期段的剩余数量，动态调整解密逻辑以确定段必须收缩多少。这意味着您的解密例程不能像现在这样类似于加密例程。但是它允许您处理当前加密例程的输出，并且加密的字符串可以保持与原始字符串相同的长度。你知道吗

下面是一个粗略的修改，按照上面的方法#2，您可以看到它允许加密例程保持简单，但解密例程必须更复杂才能弥补它：

import math

def transencrypt(string, key):
    '''
    Transpositon encryption function. This function is used to encrypt a line
    using the transposition encryption method. To learn how transpositon encryption
    works you can visit here https://en.wikipedia.org/wiki/Transposition_cipher.
    '''

    encrypted = ''
    length = len(string)

    for start in range(key):
        for offset in range(start, length, key):
            encrypted += string[offset]

    return encrypted

def transdecrypt(string, key):
    '''
    This function is for the decrypting the strings encrypted by
    transencrypt(). This function only requires the encrypted
    string and the key with which it was decrypted.
    '''

    decrypted = ''
    length = len(string)
    width = int(math.ceil(length / key))

    for start in range(width):
        offset = start
        remaining_key = key
        remaining_length = length
        remaining_width = width

        while offset < length:
            decrypted += string[offset]
            offset += remaining_width

            remaining_key -= 1

            if remaining_key > 0:
                remaining_length -= remaining_width
                remaining_width = int(math.ceil(remaining_length / remaining_key))

    return decrypted[:length]

if __name__ == '__main__':
    import sys

    string = sys.argv[1]
    key = int(sys.argv[2])

    print(transencrypt(string, key))
    print(transdecrypt(transencrypt(string, key), key))

**输出*

> python3 test.py "hello world" 5
h dewlolrol
hello world
>