Thread: Exclusice-OR

I have to write a stream cipher using the exclusive-OR (XOR), I have to read text from a file and XOR it to rand. How do I compare the two, I just want to know which one I need to convert, and convert to what, so that they both have the same number of bits.

I think the best way is to open the file in binary using fread(). Of course you'll need to use rand() to generate a random number (or an array if you want to make a longer *password*). Make sure you put the settings for fread() right, reading a specific number of blocks, each with sizeof(int) bytes. Then compare each two corresponding integers using the ^ operator and obviously save the result (probably in another file).

I know two ways to compare the right two integers. One obvious way is to loop through the clear text and cypher, and test whether you reached the end of the password to reset it. Something like this:

Code:

for (i=0; i < cleartext_length; i++)
{
	if (j == password_length) j=0;
	cyphertext[i] = cleartext[i] ^ password[j++];
}

I wrote this fast so don't depend on it

Another way is to divide the clear text length by the password length, then loop between them like this (you'll need the remainder too):

Code:

div = cleartext_length / cypher_length;

for (i=0; i < div; i++)
	for (j=0; j < cypher_length; j++)
		cyphertext[i*cypher_length+j] = cleartext[i*cypher_length+j] ^ cypher[j];

mod = cleartext_length &#37; cypher_length;
if (mod)
	for (j=0; j < mod; j++)
		cyphertext[cleartext_length-mod+j] = cleartext[cleartext_length-mod+j] ^ cypher[j];

Again, I wrote these in a rush so don't depend on them.

Code:

for (i; (strncmp(stream[i], EOF) != 0); ++i)

This looks highly suspicious. In fact, i don't even understand why you can compile it since strncmp takes 3 arguments and you only have 2. Maybe i'm missing something somewhere else. Or maybe you forgot to include string.h.

Your code is a bit hard to understand since we don't know how your variables are declared.

Anyway.