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Using XOR on an Example - Security

This is the second part of a series covering cryptography algorithms. If by any chance you have missed its first part, I urge you to check it out right now. It is called "An Introduction to Cryptography." In order to understand this article, it is crucial to grasp the concepts explained in that part.

TABLE OF CONTENTS:
  1. A Sequel to Cryptography
  2. Using XOR on an Example
  3. Breaking XOR Encryption
  4. Conclusions
By: Barzan "Tony" Antal
Rating: starstarstarstarstar / 7
July 24, 2007

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Let's XOR our example text file with the password "longpassword123."

%'pl$=#-m:f49ne%!+#*"<#8(=w%#)#<8<s#>+pbml<(nz#8nf"'!zm@DF +u)!!s)?nP$(*-l+p8anw)>:/l9+p8cnp(+=g*>*ebm1y{5{uw

Check out the full C program:

#include <fstream>

#include <stdio.h>

#include <string.h>

using namespace std;

char srcfile[30], dstfile[30], key[255];

void main()

{

   printf("Source file: "); scanf("%s",srcfile);

   printf("nDestination file: "); scanf("%s",dstfile);

   printf("nPassword (up to 255 chars): ");

   //the start of the key preparation process

   for (int i=0; i<255; i++)

        key[i]='§';

   scanf("%s",key); int temp=0; char key_backup[255];

   for (i=0;i<255;i++)

   {

        if (key[i]=='')

           break;

        temp=i;

   }

   strcpy(key_backup, key);

   for (i=temp; i>=0; i--)

       key[temp-i]=key[temp-i] & (~key_backup[i]);

   //key preparation process is done; we're ready to XOR!

   ifstream s(srcfile, ios::binary);

   ofstream d(dstfile, ios::binary);

   if (!s || !d)

       printf("nERROR: Couldn't open one of the files!");

   i=0; char c;

   while ((s.read((char*)&c, sizeof(c))))

   {

       if (key[i]=='')

          i=0;

       c=key[i] ^ c;

       d.write((char*)&c, sizeof(c));

       i++;

   }

   s.close();  d.close();

   printf("nAll done. ");

}

Once again, the download button below leads to an archive that contains the ready-to-run executable and source code of the above algorithm. Enjoy!



 
 
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