no-reply@allegro.cc (Oregano) — Sun, 08 Apr 2012 03:55:42 +0000

Theres an old topic http://www.allegro.cc/forums/thread/604453 about this where Matthew Leverton post routines for for encrypt and decrypt PNG images, the encrypt
works like a charm!!!, but the decrypt does not compile and my knowledge of file streaming is not nearly enough to deal with the errors, is there anybody that can take a look at it? Thanks.

no-reply@allegro.cc (jmasterx) — Sun, 08 Apr 2012 04:22:58 +0000

Try this:

#SelectExpand
  1
  2#include 
  3#include 
  4#include 
  5#include 
  6
  7const char *password = "allegro";
  8const int password_len = 7;
  9
 10ALLEGRO_FILE_INTERFACE encrypted_io;
 11
 12/* need to remember these two functions */
 13ALLEGRO_FILE *(*stdio_fopen)(const char *path, const char *mode);
 14size_t (*stdio_fread)(ALLEGRO_FILE *f, void *ptr, size_t size);
 15
 16/* open a file using stdio and set the vtable to our interface */
 17static ALLEGRO_FILE *eio_fopen(const char *path, const char *mode)
 18{
 19  ALLEGRO_FILE *f = stdio_fopen(path, mode);
 20  f->vtable = &encrypted_io;   
 21  return f;
 22}
 23
 24/* read the buffer via stdio, and then decrypt it */
 25static size_t eio_fread(ALLEGRO_FILE *f, void *ptr, size_t size)
 26{
 27  int64_t pos = al_ftell(f);
 28  size_t bytes_read = stdio_fread(f, ptr, size);
 29
 30  unsigned char *s = ptr;
 31  const unsigned char *e = s + bytes_read;
 32
 33  while (s < e)
 34  {
 35    *s++ ^= password[pos++ % password_len];
 36  }
 37
 38  return bytes_read;
 39}
 40
 41int main()
 42{
 43  ALLEGRO_DISPLAY *display;
 44  ALLEGRO_BITMAP *bmp;
 45
 46  al_init();
 47  al_init_image_addon();
 48
 49  /* set up our super advanced encryption system */
 50  memcpy(&encrypted_io, al_get_new_file_interface(), sizeof(ALLEGRO_FILE_INTERFACE));
 51  stdio_fopen = encrypted_io.fi_fopen;  
 52  stdio_fread = encrypted_io.fi_fread;    
 53  encrypted_io.fi_fopen = eio_fopen;
 54  encrypted_io.fi_fread = eio_fread;  
 55  al_set_new_file_interface(&encrypted_io);
 56
 57  display = al_create_display(457, 266);
 58
 59  bmp = al_load_bitmap("encrypted.png");
 60  if (bmp)
 61  {  
 62    al_clear_to_color(al_map_rgb(255,255,255));
 63    al_draw_bitmap(bmp, 0, 0, 0);
 64    al_flip_display();
 65    al_rest(5);  
 66  }
 67
 68  return 0;
 69}

This should compile for a C compiler, not C++.

no-reply@allegro.cc (Oregano) — Sun, 08 Apr 2012 04:41:30 +0000

A couple of errors are gone, but i'm still in trouble because this should be part of a c++ project, cannot ask you for more, thank you jmasterx for your quick answer, can this code be ported to work on c++?, somebody guide me in the right direction?

Encrypt / Decrypt PNG