JustForFun

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>

#define PERMISSIONS 0666
#define MASK 0b1111000011110000
#define MODE_ENCRYPT 1
#define MODE_DECRYPT 0

void encryptData(char* data, size_t size);
void decryptData(char* data, size_t size);

int main(int argc, char** argv) {
  if(argc < 3) {
    perror("Need more than 1 args");
    return EXIT_FAILURE;
  }
  int mode = MODE_ENCRYPT;
  if(argc >= 4) {
    if(strcmp(argv[3], "-d")) {
      mode = MODE_DECRYPT;
    }
  }

  char buffer[4096];
  ssize_t cread, cwrite;
  int fd_input, fd_output;
  char* filename_output;
  char* output_ptr;
  filename_output = argv[2];
  if(filename_output == NULL) {
    filename_output = argv[1];
  }
  if((fd_input = open(argv[1], O_RDONLY)) < 0) {
    perror("Error open input file!");
    return EXIT_FAILURE;
  }
  if((fd_output = open(filename_output, O_CREAT | O_WRONLY | O_TRUNC, PERMISSIONS)) < 0) {
    perror("Error open output file!");
    return EXIT_FAILURE;
  }
  output_ptr = buffer;
  while((cread = read(fd_input, buffer, sizeof buffer)) > 0) {
    mode ? encryptData(buffer, sizeof buffer) : decryptData(buffer, sizeof buffer);
    if((cwrite = write(fd_output, buffer, cread)) < 0) {
      perror("IO error!");
      return EXIT_FAILURE;
    }
  }
    close(fd_input);
    close(fd_output);
}

void encryptData(char* data, size_t size) {
  for(register int i = 0; i < size; i++) {
    data[i] ^= MASK;
  }
}

void decryptData(char* data, size_t size) {
  for(register int i = 0; i < size; i++) {
    data[i] ^= MASK;
  }
}