XOR encryption is null-terminating my strings!

This is a discussion on XOR encryption is null-terminating my strings! within the C Programming forums, part of the General Programming Boards category; I'm dealing with two strings, one being the string that I want to encrypt using XOR encryption and one being ...

  1. #1
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    XOR encryption is null-terminating my strings!

    I'm dealing with two strings, one being the string that I want to encrypt using XOR encryption and one being my "key" that I XOR the original string against.

    for (a simplified) example:
    Code:
    while ( szOriginal[i] != '\0' && szKey[i] != '\0' )
    {
    	szOriginal[i] = szOriginal[i] ^ szKey[i];
    	i++;
    }
    HOWEVER, the problem is that if by chance szOriginal[i] == szKey[i], the XOR operation will result in 0! This will replace what was a valid character in the string with a null character.

    Then, when I go to print the encrypted string (or do other things with it), the printing ends prematurely because the new null character terminates it.

    How do most programmers deal with this?

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    Write encrypted using length. don't use functions that expect null terminated string.
    Eg use something like
    Code:
    fwrite(encrypted_string,1,len,stdout);

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    Quote Originally Posted by Bayint Naung View Post
    Write encrypted using length. don't use functions that expect null terminated string.
    Eg use something like
    Code:
    fwrite(encrypted_string,1,len,stdout);
    Ah, ok. I don't know why I didn't think to do this... I had been using fread() and such and yet for some reason used fputs() instead of just fwrite().

    Thank you very much!

  4. #4
    Algorithm Dissector iMalc's Avatar
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    If you use printf to display it then yes that will stop at the first zero byte. But really, once you start XORing it's no longer text, it's binary data, so it doesn't make sense to use functions designed for printing text. It would be more appropriate to display each character in hexadecimal one at a time.
    Of course "most programmers" wouldn't be caught dead using such a weak form of "encryption".
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    C++ Witch laserlight's Avatar
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    Quote Originally Posted by iMalc
    Of course "most programmers" wouldn't be caught dead using such a weak form of "encryption".
    It depends on how the key actually is generated, but that could well be part of a one time pad implementation since each byte of the plaintext is matched with each byte of the key.

    ... oh, but not really. steez, your loop is problematic in that if the key is shorter than the plaintext, you're going to end up with a bunch of bytes that are left as plaintext.
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    Algorithm Dissector iMalc's Avatar
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    Point taken.
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    Quote Originally Posted by iMalc View Post
    If you use printf to display it then yes that will stop at the first zero byte. But really, once you start XORing it's no longer text, it's binary data, so it doesn't make sense to use functions designed for printing text. It would be more appropriate to display each character in hexadecimal one at a time.
    Of course "most programmers" wouldn't be caught dead using such a weak form of "encryption".
    I'm not printing it to view as output, rather I'm writing to a file to encrypt the file (e.g. a .txt file or a .jpg). For this reason it makes much more sense that I should have just used fwrite() to begin with, but I made a mistake.

    The key and the source data (to be encrypted) are read from separate files (whose filenames are passed as args to the program).

    And thank you for your condescension, I'm only a week into writing my own programs and this is my first exposure to any sort of encryption. Have to start somewhere.


    Quote Originally Posted by laserlight View Post
    ... oh, but not really. steez, your loop is problematic in that if the key is shorter than the plaintext, you're going to end up with a bunch of bytes that are left as plaintext.
    I actually use different variables for each counter, and I check within my loop if it has reached the end of the key. Something like if k > keySize (which I find earlier with fseek). If it has, it goes back to the beginning of the key (k = 0).
    Last edited by steez; 05-31-2011 at 04:31 PM.

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