Question

For a lab I’ve got to do, I need to create a program that will take a simple string from a text file and encrypt it using a key – a number between 0 and 255. It will read the file into an array and encrypt (or decrypt) this array into another array by XOR-ing each byte with the key. In the end, it writes the modified array into a second file.

I’ve mostly got it – what I have below compiles just fine. It doesn’t, however, copy anything to the second file. Help!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CRYPT(a, b) (a ^ b)

int main(int argc, char *argv[])
{
    FILE *fp1, *fp2;
    int a[100], b, key;
    int i = 0;

    // opens file containing string to be encrypted
    if((fp1 = fopen(argv[2], "rb")) == NULL)
    {
            printf("Error - could not open file or file does not exist\n");
            return;
    }

    // opens file encrypted string will be saved to
    fp2 = fopen(argv[3], "wb");

    // converts string to integer
    key = atoi(argv[1]);

    while(fread(a, sizeof(a), 100, fp1))
    {
            while (i != '\0');
            {
                    b = CRYPT(a[i], key);
                    fwrite(&b, sizeof(a), 1, fp2);
                    i++;
            }
    }


    return 0;

}

Answer 1

I think the problem lies here –

while (i != '\0');

You are initializing i to 0 and in the while loop you are checking whether or not i is equal to NULL. The integer value of NULL, or \0 is 0. As a result, the expression is false and your loop is never being executed.

Also remove the extra semi-colon at the end of this while loop.

From the reference –

size_t fread ( void * ptr, size_t size, size_t count, FILE * stream );

Reads an array of count elements, each one with a size of size bytes, from the stream and stores them in the block of memory specified by ptr.
The postion indicator of the stream is advanced by the total amount of bytes read.
The total amount of bytes read if successful is (size * count).

So you also need to change your fread function to this –

fread(a, sizeof(int), 100, fp1)

Similarly, you also need to change your fwrite –

fwrite(&b, sizeof(int), 1, fp2);

The edited code should look something like this –

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CRYPT(a, b) (a ^ b)

int main(int argc, char *argv[])
{
    FILE *fp1, *fp2;
    int a[100], b, key;
    int i = 0;
    int data_read = 0;

    // opens file containing string to be encrypted
    if((fp1 = fopen(argv[2], "rb")) == NULL)
    {
            printf("Error - could not open file or file does not exist\n");
            return;
    }

    // opens file encrypted string will be saved to
    fp2 = fopen(argv[3], "wb");

    // converts string to integer
    key = atoi(argv[1]);

    while( (data_read = fread(a, sizeof(int), 100, fp1)) > 0 )
    {
            while(i < data_read)
            {
                    b = CRYPT(a[i], key);
                    fwrite(&b, sizeof(int), 1, fp2);
                    i++;
            }

            i=0;
    }

    return 0;
}