feof() trouble

Lewk of Serthic · #1 14-Sep-2005, 16:32

I'm having some trouble with my little encryption program. It compiles alright but gives me a "Illegal Operation" when I run it, right after I input a key.

CPP / C++ / C Code:

#include <stdio.h>

encrypt(unsigned long* v, unsigned long* k) 
{
     unsigned long v0=v[0], v1=v[1], sum=0, i;           /* set up */
     unsigned long delta=0x9e3779b9;                     /* a key schedule constant */
     unsigned long k0=k[0], k1=k[1], k2=k[2], k3=k[3];   /* cache key */
     for (i=0; i < 32; i++) 
     {                            /* basic cycle start */
         sum += delta;
         v0 += (v1<<4)+k0 ^ v1+sum ^ (v1>>5)+k1;
         v1 += (v0<<4)+k2 ^ v0+sum ^ (v0>>5)+k3;         /* end cycle */
     }
     v[0]=v0; v[1]=v1;
}

main()
{
    char infile[20];
    char outfile[20];

    printf("Enter the name of the file you wish to encrypt.\n");
    scanf("%s",infile);
    printf("Enter the name of the file you wish to create.\n");
    scanf("%s",outfile);

    FILE *fin, *fout;

    if ((fin = fopen(infile, "r")) == NULL)
    {
            printf("Cannot open %s.",infile);
    }

    else if ((fout = fopen(outfile, "w")) == NULL)
    {
            printf("Cannot create %s.",outfile);
    }
    else
    {
        unsigned long c;
        unsigned long key;
            
        printf("Please enter key.\n");
        scanf("%lu",key);
        while (!feof(fin))
        {
            fscanf(fin,"%lu",&c);
            encrypt(&c,&key);
            fprintf(fout,"%lu",c);
        }
        fclose(fin);
        fclose(fout);
    }
}

You may reconize the function encrypt() as the TEA encryption alogarithm, I didn't write it and doubt it's part of the problem.

I'm using the Dev-C++ compiler on a windows pc, if that matters.

Any help will be appreciated.

Lewk of Serthic · #2 14-Sep-2005, 16:39

I was using

CPP / C++ / C Code:

scanf("%lu",key);
while (!feof(fin))

and although it ran through a couple cycles, I beleive it was failing to stop on EOF.

WaltP · #3 14-Sep-2005, 17:58

Check out this link for information on feof().

Lewk of Serthic · #4 14-Sep-2005, 20:06

Quote:

Originally Posted by WaltP

Check out http://www.gidforums.com/showpost.php?p=20674&postcount=2 for information on feof().

Can I use...

CPP / C++ / C Code:

if (fgets(buf, 20, st) == NULL)
    {
        // exit the loop when done
        break;
    }

but with fscanf() instead of fgets()?

davekw7x · #5 14-Sep-2005, 20:32

Quote:

Originally Posted by Lewk of Serthic

I'm having some trouble with my little encryption program. It compiles alright but gives me a "Illegal Operation" when I run it, right after I input a key.

In addition to the problems with eof testing that Walt pointed out:

1.
Your scanf is wrong:

CPP / C++ / C Code:

scanf("%lu",key);

Should be something like the following to keep things legal:

CPP / C++ / C Code:

scanf("%lu",&key);

However, it won't give you a very useful encryption/decryption function for general files. What if the file does not consist of lines of ascii representations of unsigned long ints?

2.
The TEA algorithm works on 64-bit blocks of data (Can be two 32-bit ints or eight 8-bit chars). The key is a 128-bits (four 32-bit ints or 16 8-bit chars).

The function that you are using only works for the following:

v is an array of two 32-bit ints
k is an array of four 32-bit ints

3. You need to get two 32-bit ints into the key. You need to read the file a byte at a time (actually 8 bytes at a time, since the algorithm works on 64-bit blocks of data). You need to open the file in binary mode in order to be able to encrypt and decrypt and get the exact same file back again. You can pack the chars of the file into two 32-bit ints if you want to use your function (and take care of the case that the file length is not a multiple of eight).

Regards,

Dave

WaltP · #6 14-Sep-2005, 23:43

Quote:

Originally Posted by Lewk of Serthic

Can I use...

CPP / C++ / C Code:

if (fgets(buf, 20, st) == NULL)
    {
        // exit the loop when done
        break;
    }

but with fscanf() instead of fgets()?

You should be able to figure that out on your own. What does fscanf() return? What does fgets() return? Are they the same? If so, yes. If not, how would you have to change the if?

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feof() trouble

Elaboration

fscanf()