python - 我该如何解决这种密码学问题？

Question

我正在尝试使用 interactivepython.org 网站自学编程。我遇到了一个似乎无法解决的问题。我已经连续工作了 3 个小时，并且正在绞尽脑汁。完全不知道如何分解。

问题:

Decoding a secret message:

The description may seem daunting, but the solution is not that hard. You can use the built-in string datatype with the associated built-in functions and while loop (with ‘len’ function) or a for loop (with ‘in’ operator) to traverse the string. Also, use the ’chr’ and ’ord’ functions (which are based on ASCII code) discussed in course material. Make sure to look at the examples in the course material and do #18 and #19 in Exercises 2. Answer for #19 is provided and it can give valuable hints for solving this problem.

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Your country is at war and your enemies are using a secret code to communicate with each other. You have managed to intercept a message that read as follows:

:mmZ\dxZmx]Zpgy

The message is obviously encrypted using the enemy’s secret code. You have just learned that their encryption method is based upon the ASCII code (you can find this set easily by searching online). Individual characters in a string are encoded using this system. For example, the letter ‘A’ is encoded using the number 65 and ‘B’ is encoded using the number 66.

Your enemy’s secret code takes each letter of the message and encrypts it as follows (using a secret key):

If (OriginalChar + Key > 126) then
    EncryptedChar = ((OriginalChar + Key) - 127) + 32
Else 
    EncryptedChar = (OriginalChar + Key)

For example, if the enemy uses Key = 10 then the message ”Hey” would be encrypted as:

Character   ASCII
H         72
e         101
y         121

Encrypted H = (72 + 10) = 82 = R in ASCII
Encrypted e = (101 + 10) = 111 = o in ASCII
Encrypted y = 32 + ((121 + 10) - 127) = 36 = $ in ASCII

Consequently, “Hey” would be transmitted as “Ro$”.

Write a program that decrypts the intercepted message. You only know that the key used is a number between 1 and 100. Your program should try to decode the message using all possible keys between 1 and 100. When you try the valid key, the message will make sense. For all other keys, the message will appear as gibberish.

HINT: You will need to implement a decrypt function that takes in an encrypted message as string and a key as integer and returns the decrypted message as string. You can decrypt each letter of the message as follows:

If (EncryptedChar - Key < 32) then
    DecryptedChar = ((EncryptedChar - Key) + 127) - 32
Else
    DecryptedChar = (EncryptedChar - Key)

NOTE: You should also implement an encrypt function that takes in a regular message as string and a key as integer and returns the corresponding encrypted message as string (the algorithm to encrypt a message is mentioned above in the problem description). This function would help you in encrypting any regular message, which then can be passed to your decrypt function to be decrypted.

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For Encryption: You should ask the user for any regular message and a key and output the corresponding encrypted message.

Sample run:

Enter a regular message to encode:
Attack at dawn!
Enter a key value (between 0 and 100) for encoding:
88
The encoded message is: 
:mmZ\dxZmx]Zpgy

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For Decryption: You should ask the user for an encrypted message and output 100 well-formatted, decrypted messages (using keys between 1 and 100) along with the corresponding key value.

Sample run (the gibberish messages below are not accurate):

Enter an encrypted message to decode:
:mmZ\dxZmx]Zpgy 
The following are the decoded messages for keys 1 to 100:
Key: 1 –> Decoded Message: whfuihwuiidh89
Key: 2 –> Decoded Message: 9ehkaOY3ewine
...
Key: 87 –> Decoded Message: Buubdl!bu!ebxo”
Key: 88 –> Decoded Message: Attack at dawn!
...
Key: 100 –> Decoded Message: on3dwp389/wi8

这是我目前拥有的代码:

def encrypt(message, key):
    result = ""
    for char in message:
        result += encryptedChar
    return result

Answer 1

这是 Joran Beasley 的一个更简单(但更长)的答案。

理解之后，您可以使用 ord() 获取字符的“编号”并使用 chr()“恢复”字符，这非常简单将您获得的代码“翻译”为正确的 Python 代码。

从以下部分开始:

If (OriginalChar + Key > 126) then
    EncryptedChar = ((OriginalChar + Key) - 127) + 32
Else 
    EncryptedChar = (OriginalChar + Key)

如果您从已经编写的代码开始，您可以将上面的代码翻译成:

def encrypt(message, key):
    result = ""
    for char in message:
        if (ord(char) + key > 126):
            result += chr(ord(char) + key - 127 + 32)
        else:
            result += chr(ord(char) + key)
    return result

你可以对解密部分做同样的事情，然后写一个简单的菜单。
这是剩余的代码(您必须在顶部添加 encrypt 函数:

def decrypt(message):
    for key in range(1, 101):
        result = ""
        for char in message:
            if (ord(char) - key < 32):
                result += chr(ord(char) - key + 127 - 32)
            else:
                result += chr(ord(char) - key)
        print('key: {} -'.format(key), result)

if __name__ == '__main__':
    print('1 - Encrypt')
    print('2 - Decrypt')
    inp = input('select 1 or 2: ')
    if inp == '1':
        msg = input('Enter message: ')
        key = int(input('Enter key (1-100): '))
        print('Encrypted message:')
        print(encrypt(msg, key))
    else:
        msg = input('Enter message: ')
        decrypt(msg)