Convert PHP FOR loop to Python FOR LOOP to XOR 2 strings

Question

I'm using this php loop to do a basic xor of 2 different length strings $text and $key which works very well, I need to do the same xor in Python to unscramble the string, I'm struggling to convert this FOR loop into a Python FOR loop, can someone please help:-

    for($i=0; $i<strlen($text); )
    {
        for($j=0; ($j<strlen($key) && $i<strlen($text)); $j++,$i++)
        {
            $outText .= $text{$i} ^ $key{$j};
        }
    }

Why do you use two variables $i and $j here. They have all the time same value. And you probably initialize your $i somewhere outside of this code snippet. — ex4
– ex4, Commented Apr 24, 2020 at 13:44
Sorry I missed a line of the php, the top line should be for($i=0; $i<strlen($text); ) I'll edit the code — Andrew Smith
– Andrew Smith, Commented Apr 24, 2020 at 13:48
And you probably know it, but in cryptography point of view using same key over and over again makes your algorithm very easy to break. So using proper encryption algorithm would be better if coding something for production use. Or it would be good training project to improve this algorithm a bit and use feedback from text to modify key on each round. — ex4
– ex4, Commented Apr 25, 2020 at 2:16

ex4 · Accepted Answer · 2020-04-25 02:10:52Z

1

I'd use while in this case..

i = 0
j = 0
out = ""
while i < len(text):
 if j == len(key): 
   j=0
 out += chr(ord(text[i])^ord(key[j]))
 i+=1
 j+=1

Or with foreach you can make code a bit shorter. Foreach in string loops through every char in it.

j = 0
out = ""
for letter in text:
 if j == len(key): j = 0
 out += chr(ord(letter)^ord(key[j]))
 j += 1

edited Apr 25, 2020 at 2:10

answered Apr 24, 2020 at 13:40

ex4

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6 Comments

Andrew Smith Over a year ago

Sorry I missed the top line from the php, I have edited the code in the question

Andrew Smith Over a year ago

Thanks for the update, trying your first loop, when I xor in php and then try to xor back in Python the output is still scrambled, the loop seems to work but its not descrambling the php scramble, I'll do some more testing, any ideas?

ex4 Over a year ago

Could you give a short example of scrambled data with key. I'm not sure what PHP ^ operator does with two chars.

Andrew Smith Over a year ago

Actually I think its me, still experimenting, I'm putting the scrambled data in a sqlite db then reading it back out and applying the descramble, I think it might be that the scrambled data is being altered as it goes in and out of the db

Andrew Smith Over a year ago

It works! I made a rather stupid error where the old db was not being overwritten with the new one with the correct key, lol Just one thing, in your second loop it threw a syntax error in Python 3, should it be for rather than foreach? Thanks for your help!

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