Read from text file and XOR in C!

Title says it all!

hello.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "enc.h"
 
int main() {
    // Allocate memory in the stack to keep the filename
    char fileName[20];
 
    // Ask for input file name to user
    puts("Enter file name:");
    scanf("%s", fileName);
 
    // File pointer to entered filename, open mode: read
    FILE *original = fopen(fileName, "r");
 
    // Encrypt the filename, and use this name to write the contents to
    enc(fileName);
    FILE *encFile = fopen(fileName, "w");
 
    char readLine[80];
    // Read through the lines in the opened file
    while (fscanf(original, "%79s", readLine) != EOF) {
        // Encrypt the read line
        enc(readLine);
        //Write it to the new file
        fprintf(encFile, "%s\n", readLine);
    }
}

enc.h
void enc(char *string);

enc.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
void enc(char *string) {
    size_t x = strlen(string);
    size_t counter;
    for (counter = 0; counter < x; counter++) {
        string[counter] = string[counter] ^ 31;
    }
}

In Action