One Time

brazilnut · 11-02-2007, 12:30 PM

First off, sorry for the size (but it's complete(ish)!). The problem is that sometimes it cut's out decrypting, presumably on an EOF (either -1 or 255), I thought it could only be coming from the encrypted msg (out.txt) file so I took the catch off, but to no avail, therefore how to handle?

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>

void rand_init()
{
	long ltime;
	unsigned int itime;
    ltime = time(NULL);
    itime = (unsigned) ltime/2;		//	? mod max of int?
    srand(itime);
}

unsigned int rand_gen()
{
	while(1)
	{
		unsigned int n = abs(rand() % 255);
		if(n != EOF)	{	return n;	}
	}
}

int gen_random_file(char * fname, int size)
{
	FILE *fo;
	int i;
	rand_init();
	if ((fo = fopen(fname, "wb")) != NULL)
	{
		for(i=0;i<size;i++)
		{
			unsigned int ic = rand_gen();
			char cc = (char)ic;
			putc(cc, fo);
		}
		fclose(fo);
	}
}

int filesize(char * fname)
{
	struct stat stbuf;
	if(stat(fname, &stbuf) == -1)
	{
		return -1;
	}
	return stbuf.st_size;
}

int encrypt(char * fname_key, char * fname_msg, char * fname_out,  int mode)
{
	FILE *fim, *fik, *fo;
	char cm, ck;
	int n;
	int i = 0;
	if ((fim = fopen(fname_msg, "rb")) != NULL)
	{
		if ((fik = fopen(fname_key, "rb")) != NULL)
		{
			if ((fo = fopen(fname_out, "wb")) != NULL)
			{
				//while ( ((cm = getc(fim)) != EOF) && ((ck = getc(fik)) != EOF) )
				while ( ((cm = getc(fim))) && ((ck = getc(fik)) != EOF) )
				{
					if(mode)
					{
						n = ( ((unsigned int)((unsigned char)cm)) + ((unsigned int)((unsigned char)ck)) ) % 256;
					}
					else
					{
						n = ( ((unsigned int)((unsigned char)cm)) - ((unsigned int)((unsigned char)ck)) ) % 256;
					}
					putc((char)n, fo);
					i++;
				}
				fclose(fo);
			}
			fclose(fik);
		}
		fclose(fim);
	}
	printf("i: %d \n", i);
	return 0;
}

int main(int argc, char *argv[])
{
	char *fname_msg = "msg.txt";
	char *fname_key = "key.txt";
	char *fname_out = "out.txt";
	
	int len = filesize(fname_msg);
	printf("len: %d \n", len);
	
	// GENERATE A NEW KEY
	int iret = gen_random_file(fname_key, len);	//	char * fname, int size
	
	//	ENCRYPT
	iret = encrypt(fname_key, fname_msg, fname_out, 1);	//	char * fname_key, char * fname_msg, char * fname_out
	
	//	DECRYPT
	iret = encrypt(fname_key, fname_out, "dec.txt", 0);	//	char * fname_key, char * fname_msg, char * fname_out
	
	return 0;
}

Other suggestions / (c)crit welcome...

brazilnut · 11-02-2007, 01:04 PM

It seems to err out (or eof) when 'cm == 0', in this line

Code:

while ( ((cm = getc(fim))) && ((ck = getc(fik)) != EOF) )

rsashok · 11-02-2007, 02:01 PM

I'd recommend not to use getc() for reading binary streams. Use something else, for example read(). If you want to use getc(), then test for the end of file with feof(), don't compare with EOF. There is a logical problem in the line:

Code:

while ( ((cm = getc(fim))) && ((ck = getc(fik)) != EOF) )

You probably meant:

Code:

while( ((cm = getc(fim) != EOF) && (ck = getc(fik) != EOF))

These two lines are not equivalent.

brazilnut · 11-02-2007, 02:46 PM

Yes, if you see first code that same line is commented out.
I've now changed char's to ints and use the 'f' variations, and now am trying feof(), however it always seems to go over by 1? This is how the loop looks now:

Code:

while( (feof(fim)==0) && (feof(fik)==0) )
{
	cm = fgetc(fim);
	ck = fgetc(fik);
	if(mode)
		n = (cm + ck ) % 256;
	else
		n = (cm - ck ) % 256;
	putc(n, fo);
}

However, I don't seem to be getting the 0 error?

rsashok · 11-02-2007, 03:45 PM

feof():
Return Value

The feof function returns a nonzero value after the first read operation tht attempts to read past the end of the file. It returns 0 if the current position is not end of file. There is no error return.

Do something like this:

Code:

int buf_im[1024], buf_ik[1024], i,im_count, ik_count, lim;

while( !feof(fim) && !feof(fik) )
{
	im_count = read(buf_im, sizeof(char), 1024, fim);
        ik_count = read(buf_ik, sizeof(char), 1024, fik);
        lim = min(im_count, ik_count);
        for(i = 0; i < lim; i++)
        {
            cm = buf_im[i];
	    ck = buf_ik[i];
            cm = (mode) ? (cm + ck) : (cm - ck);
            n = cm & 0x0ff;  /* this is the same as n % 256, but faster */
	    putc(n, fo);
        }
}

I didn't try it, but you might give it a run.

brazilnut · 11-02-2007, 04:08 PM

I tried that first, it also overruns... I've gone back to:

Code:

while ( ((cm = fgetc(fim)) != EOF) && ((ck = fgetc(fik)) != EOF) )
{
	if(mode)
		n = (cm + ck ) % 256;
	else
		n = (cm - ck ) % 256;
	putc(n, fo);
}

I'm not sure if it was and int / char thing, however from over 100 runs i've had no errors. Later i'll devise a way of setting the key and conjuring the error and see if it still invokes it?
Cheers!