Thread: Decrytion

Wanted to know if there is any way in which the following output ./a.out will generate a file for me 255 Seperate times, I know this may sound weird but basically what i am doing is decrypting a file using brute force.

student:~/encoder1$ ./a.out text.bin attempt (1 - 255)

for every time i attempt to try a KEY which works (1-255) a new file should be generated in my folder, the reason being so that i can see which is the correct file with the written text in plain English rather than the Unencypted file.

I just want to know if this is even possible, or is there an alternate way around this for examples by creating a for loop within my code so that only the correct file which is in UPPER or LOWER case in English without and special charectors is detected indicating to me the correct key.

Maybe just append to one file but depends how much text you are talking about, or name your file by counter index and create new in loop, but are you sure of the potential permutations of your approach, brute force usually huge perms

Also consider a rudimentary AI to filter candidates, set a number of strings as common words like 'the' or there', 'and' etc, compare content against strings and incr a counter wit each match, set a threshold to define if count high enough to qualify

Code:

/*
  Encrypts a file using a permutation cipher.

  The permutation is simply ^ with a key [0-255]. 

*/

#include <stdio.h>
#include <stdlib.h>

void usage(char *);


main(int argc, char *argv[])
{
	FILE *encryptedout;
	FILE *plainin;
	int key;
	char *plainfilename, *encryptedfilename;


	if (argc != 4) usage(argv[0]);
	plainfilename = argv[1];
	encryptedfilename = argv[2];
	key = atoi(argv[3]);  
	  
	plainin = fopen(plainfilename,"r");
	encryptedout = fopen(encryptedfilename,"w");
     	  
	int c;
	while((c = getc(plainin)) != EOF)
	{
	  // The following line is the encryption algorithm
	  char ec = (char) (c^key);
	  putc(ec,encryptedout);
	}

	close(plainin);
	close(encryptedout);
}





void usage(char * commandname)
{
  printf("USAGE: %s plaintextfile encryptedfile key\n",commandname);        
  printf(" Where plaintextfile is the file you wish to encrypt\n");
  printf(" and the encrypted version is placed in encryptedfile.\n");
  printf(" key takes a value in the range (0-255).\n");
  exit(0);
}

As you can see from the code the problem is that i cant come up with some sort of loop which allows me to try 255 keys without the manual brute force method . Ive had a look into the Bash manual but cant seem to find anything specific to what im looking for and unfortunately pipping is too complicated for me to fully understand given my current knowledge of C.

I need to come up with some way to do do 255 keys without having to manually try it , or even if i have to manually type it the there should be some kind of indication showing me that the file contains the unencypted file .. I can cheat around this as I already know what the key for my file is, so if there is some way i can get a detection of a sequence of words within the file then it will know it is unencrypted.

Is there anyway to overcome this problem ?