My python encryption software isn't working

Question

So I wrote a little script in python that brings up a gui with 2 buttons and a text field. you type into the text area and click encrypt and the text all of a sudden looks crazy. coooooool.

but then when you hit decrypt, it doesn't go back to the original. here is the code

from Tkinter import *
import ttk

root = Tk()
textArea = Text(root, state = "normal")

def encrypt():
    message = textArea.get('1.0', 'end')
    newMessage = ''
    lastChar = ''

    for c in message:
        if lastChar != '':
            newMessage += chr((ord(c) + ord(lastChar)) % 256)
            lastChar = chr((ord(c) + ord(lastChar)) % 256)
        else:
            newMessage += chr((ord(c) + 5) % 256)
            lastChar = chr((ord(c) + 5) % 256)

    textArea.delete('1.0', 'end')
    textArea.insert('end', newMessage)

def decrypt():
    message = textArea.get('1.0', 'end')
    newMessage = ''
    lastChar = ''

    for c in message:
        if lastChar != '':
            newMessage += chr((ord(c) - ord(lastChar)) % 256)
            lastChar = chr((ord(c) - ord(lastChar)) % 256)
        else:
            newMessage += chr((ord(c) - 5) % 256)
            lastChar = chr((ord(c) - 5) % 256)

    textArea.delete('1.0', 'end')
    textArea.insert('end', newMessage)

encrypt = ttk.Button(root, text = "Encrypt", command = encrypt)

encrypt.pack({"side": "top"})

decrypt = ttk.Button(root, text = "Decrypt", command = decrypt)

decrypt.pack({"side": "top"})

textArea.pack({"side": "bottom"});

mainloop()

the problem is that it doesn't show the original text. It just seems to make it more cryptic. what is wrong here? Please help.

update: a changed it so it just adds 5. and It works. So that tells me that it's the part where I add last characters code value. There is still one problem: it adds a new line and this strange line character (not the pipe ---> |).

now the code is fixed. thanks. here is the result: WW1ueCVueCV1d2p5eX4laHR0cTMlWW1mc3AlfnR6JXh5ZmhwJXR7andrcXR8JWt0dyV5bWolZnN4fGp3Mw8=

after I made the decrypt so It doesn't take away five, it worked. Thanks again.

Borealid · Accepted Answer · 2012-02-19 04:25:41Z

5

You can't put arbitrary bytes into a text area.

Your encryption algorithm is purely bytewise - it works on the numeric values of the characters. This isn't going to work, even for ASCII, because byte 0 is an ASCII NUL - treated specially, and byte 127 (what 'Z', character 122, becomes when shifted up by five) is a DEL character: not what you want! Moving beyond ASCII, the real world's UTF-8 has thousands of invalid byte sequences.

If you're going to be using bytewise encryption like this, you can't put the results into a text field - it's binary data. If you want it to be able to go into a text field, you must make sure it's valid text without control characters, NUL bytes, or invalid byte sequences.

One easy way of making the binary sanitary is to Base64-encode it on encryption, and have the first step of decryption be Base64 decoding.

Final note: you're also encrypting the newline at the end of the text area. Probably not something you wish to do.

edited Feb 19, 2012 at 4:25

answered Feb 19, 2012 at 4:19

Borealid

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7 Comments

Lysol Over a year ago

well as long as the new line gets decrypted, what does it matter? I just wanted this to be a little thing where you put a message, encrypt it, send it to a friend perhaps, and have him decrypt it. thanks for the help

Borealid Over a year ago

@AidanMueller That's fine, just wanted you to be aware. Try Base64-encoding the data and not storing it as a string type and you should be all right.

Lysol Over a year ago

I hope this isn't a dumb question. but how would I go about that

Lysol Over a year ago

thanks. I'm not really a python programmer. I just know a little. I want to learn more. I'm going to add that to my list of stuff to do.

Borealid Over a year ago

@AidanMueller One other thing: the modulus operator (%) doesn't behave how you'd expect with negative numbers, so you need to stop giving it negatives (try adding 256 first).

|

Roland Smith · Accepted Answer · 2012-02-19 14:12:43Z

2

This seems a rather trivial "encryption" to break. Everyone who has the source can read your messages. A secure cryptosystem should not depend on the algorithm being secure.

You could use the Python Cryptography Toolkit (www.dlitz.net/software/pycrypto/). It has a host of different encryption algorithms available. You can find some examples of how to use it at: http://www.laurentluce.com/posts/python-and-cryptography-with-pycrypto/

If you don't care about security, why not use e.g. one of the standard encodings (see §7.8 'codec' of the standard library reference);

The Ceasar cipher:

>>> res = 'this is a test'.encode('rot_13')
>>> print res
guvf vf n grfg
>>> print res.decode('rot_13')
this is a test

Base64 encoding:

>>> res = 'this is a test'.encode('base64')
>>> print res
dGhpcyBpcyBhIHRlc3Q=

>>> print res.decode('base64')
this is a test

answered Feb 19, 2012 at 14:12

Roland Smith

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1 Comment

Lysol Over a year ago

thanks for the effort. but I just made this for the fun of it. I got the idea from YouTube. gigafide has a video about encryption. and he makes a simple encryption in vbs. So I made one in python. and I know there are preexisting ways. but what's the fun in that. also, I voted up your answer cause you only have 31 rep points and your answer was helpful.

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