Thread: How to apply XOR encryption to a file

Originally Posted by nohemon

I tried to encrypt an initialized char array and it worked

What is the code that you used for this? Perhaps you did something different, e.g., you used XOR character by character rather than trying to use it with the entire array at once.

Also, printing the ciphertext as normal text might be problematic since some characters could become unprintable. Some approaches to this would be to print the bytes in a hex representation, or to base64 encode the ciphertext.

Yeah, that's exactly what I guessed. Why don't you do the same thing in your code in post #1 as you did in your code in post #3?

Let's see your code with the "one-by-one (by iterating to it)" version.

EDIT:
Well, I decided to adapt and combine your code in post #3 into your code in post #1:

Code:

#include <stdio.h>

#define XOR_KEY 0x6F

int main() {

    FILE *infp;
    char buf[1024];
    infp = fopen("test.txt", "r");
    while ((fgets(buf, sizeof(buf), infp)) != NULL)
    {
        for (int i = 0; buf[i] != '\0'; i++)
        {
            buf[i] ^= XOR_KEY;
        }

        printf("Contents: %s", buf);
    }

    return 0;

}

You're still going to have potential problems with the output as I mentioned earlier if you don't encode the ciphertext in some way to make it text. You also need to check that fopen did not return a null pointer before you use infp, and then you should fclose when done.

Originally Posted by nohemon

It compiles but it outpus nothing. Maybe it's what you said. But why output doesn't show now but it was visible when I iterated through the initialized char array?

If I write the output to a new file? Would it become visible?

*EDIT: I had to use %s and not %c. Thank you for your useful help

Maybe just print it within the XOR loop?

Code:

#include <stdio.h>
 
#define XOR_KEY 0x6F
 
int main() {
 
    FILE *infp;
    char buf[1024];
    infp = fopen("test.txt", "r");
    while ((fgets(buf, sizeof(buf), infp)) != NULL)
    {
        for (int i = 0; buf[i] != '\0'; i++)
        {
            buf[i] ^= XOR_KEY;
            printf("%x", (int)buf[i]);
        }
 
        puts("");
    }
 
    return 0; 
}

I understand this is probably just an exercise, but just so you know, this doesn't even come close to being real encryption.