lcrypt.lua ported to c, simple no nonsense super fast file encryption

lcrypt.c

/* INFO:
 * made by Felix
 * original source lcrypt.lua ->
 * `https://gist.github.com/Felix-Kyun/5c9cb2eb5a40289825cb2705bd1f1654`
 * @arg -> 0:@mode{e, d} 1:@file_name[i] 2:@file_name[o] 3:@pass
 * example:",
 * `$ lcrypt e file.txt encrypted_file.txt felix@crypt`",
 * `$ lcrypt d encrypted_file.txt file.txt felix@crypt`",
 * @mode <- only 'e' or 'd' meaning encrypt and decrypt respectivly",
 * @pass has to be supplied with no space"};
 */

// #include "misc.h"
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX_ARG_LEN 5

void show_help(char *err_msg); // just shows a help message and exits
int ascii_push(int i, int j);  // used in encrypt() for offsetting bytes
void encrypt(FILE *input_file, FILE *output_file, char *pass, char mode);

FILE *input_file, *output_file;

int main(int argc, char *argv[]) {
  printf("argc: %d", argc);
  if (argc != MAX_ARG_LEN)
    show_help("Takes Exactly 4 Arguments");

  input_file = fopen(argv[2], "r");
  output_file = fopen(argv[3], "w");
  rewind(input_file);

  if (input_file && output_file) {
    encrypt(input_file, output_file, argv[4], (char)argv[1][0]);
  } else
    show_help("Unable To Open Files");

  fclose(input_file);
  fclose(output_file);
}

void show_help(char *err_msg) {
  char *help_txt[] = {
      "LCRYPT",
      "Made By Felix",
      "@arg -> 0:@mode{e, d} 1:@file_name[i] 2:@file_name[o] 3:@pass",
      "example:",
      "`$ lcrypt e file.txt encrypted_file.txt felix@crypt`",
      "`$ lcrypt d encrypted_file.txt file.txt felix@crypt`",
      "@mode <- only 'e' or 'd' meaning encrypt and decrypt respectivly",
      "@pass has to be supplied with no space"};

  size_t len = sizeof(help_txt) / sizeof(help_txt[0]);
  for (int i = 0; i < len; i++) {
    puts(help_txt[i]);
  }
  puts(err_msg);
  exit(69);
  if (input_file)
    fclose(input_file);
  if (output_file)
    fclose(output_file);
}

int ascii_push(int i, int j) {
  int k = i + j;
  if (k >= 0 && k <= 255)
    return k;
  else if (k < 0)
    return 256 + k;
  else
    return k - 256;
}

void encrypt(FILE *input_file, FILE *output_file, char *pass, char mode) {

  char fch, ch = fgetc(input_file);
  size_t pass_len = strlen(pass);
  unsigned long int pos = 0;
  int modifier, tmp = 0;

  if (mode == 'e')
    modifier = 1;
  else if (mode == 'd')
    modifier = -1;
  else
    show_help("invalid call to encrypt func in lcrpy.c");

  while (!feof(input_file)) {

    tmp = pos % pass_len;
    fch = ascii_push(ch, modifier * pass[tmp]);

    putc(fch, output_file);
    ch = fgetc(input_file);
    modifier *= -1;
    pos++;
  }
}