How to apply XOR encryption to a file

I'm reading a file and I want to apply XOR encryption to it and print it.
This is my code

Code:

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

#define XOR_KEY0x6F

int main() {
   
    FILE *infp;
    char buf[1024];
    infp = fopen("test.txt", "r");
    while ((fgets(buf, sizeof(buf), infp)) != NULL)
    {
        printf("Contents: %s", buf ^ XOR_KEY);
    }
   
    return 0;

}

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I get this error:
error: invalid operands to binary ^ (have 'char *' and 'int')
17 | printf("Contents: %s", buf ^ XOR_KEY);
| ~~~ ^
| |
| char *

I tried to encrypt an initialized char array and it worked, but why am I not able to do this to char buf[1024] array?

Thanks

Quote:

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Originally Posted by nohemon

I tried to encrypt an initialized char array and it worked

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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.

Quote:

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What is the code that you used for this?

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Something like this:

Code:

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#include

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Code:

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<stdio.h>
int main(int argc, char *argv[])
 {
    char xorString[] = "This is a test";
    int i = 0;
    for (int i = 0; xorString[i] != '\0'; i++)
    {
        xorString[i] ^= 0xAA;
    }

    printf("%s", xorString);
   
    return 0;
 }

---

It outputs this: ■┬├┘Λ├┘Λ╦Λ▐╧┘▐ (I'm not sure if you can see it)
I tried applying it to buf, but I get the error that I described on my first post.

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?

Quote:

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Originally Posted by laserlight

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?

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I did. I tried doing it on buf array. I tried one-by-one (by iterating to it), and as a whole, but I got the same error in both situations.

error: invalid operands to binary ^ (have 'char *' and 'int')

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;

}

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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.

Quote:

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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.

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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

Quote:

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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

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Maybe just print it within the XOR loop?

Code:

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#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;
}

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