unexpected termination

Hi everybody,

I have a program that encrypts a file ,by reading it 8-bit at a time, manipulating them and puting them into a temp file, when this is done, it copies the temp to the original file.

Now I've got three problems, at least...

First is how can I read a char, manipulate it and putting it back instead of the old char (that would surely save time)? I didn't figure out a way using-
" fstream File (File, ios::out | ios::in | ios::binary) " (thought the then in+out would solve this).

Second is how can I detect an unexpected termination of the program (as in THE PROGRAM IS NOT RESPONDING, crtl+alt+del and so on....)? how can I roll back the changes that were made on the file if he was not fully encrypted / decrypted?

Third is what could I do for my program not to get stuck somewhere on the way when it's handeling "big files" (200mb and up)?


Thanks

Igeru,

1. Just for reference, a byte level encryption like that is not very secure. Unless you are just trying to keep non-hackers out.

2. For that type of I/O, don't use C++ streaming, open with standard file io using open, fopen, or CreateFile. Then you can use seek, fseek, or SetFilePointer to back up a byte and write the new value. If you want to support larger files, use the CreateFile().

3. You may want to take a look at the Windows CryptAPI.

-Erik

1. Yaeh I know of block cipher, just gave an example, but thanks.

2. why shouldn't I use c++ streaming?
by the way, my program opens and deals with the file perfectly, It's just get stuck if I open another program or I move the main window while crypting.. why is that?

3. what is Windows CryptAPI? Can I manipulat it as I want? I use a uniqe cryption system


THANKs

Quote:

---

Originally Posted by Igeru

2. why shouldn't I use c++ streaming?
by the way, my program opens and deals with the file perfectly, It's just get stuck if I open another program or I move the main window while crypting.. why is that?

---

You probably have a tight loop reading the file without a message loop. It probably doesn't stop, it just looks like it stops.

Try to find the c++ streaming equivalant of fseek(). Then you can read, back up, write your new byte, then read the next.

Quote:

---

Originally Posted by Igeru

3. what is Windows CryptAPI? Can I manipulat it as I want? I use a uniqe cryption system

---

The Windows CryptAPI is the interface into SSL. If you are using your own algorithm, don't worry about it.

I see, the loop reading and manipulating the file is in a function and wile it is running the messege loop doesn't run.. how can I resolve this? inserting the whole encryption loop somehow to the messege loop?? threading? is there another way?

For long functions, there are two ways: put it on its own thread, or add a message loop pump. The first way is the best, but it is more complicated programming. The latter is easy, but slows down your algorithm.

Add this in your loop:

Code:

---

{
    MSG msg;
    if( PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) )
    {
        if( msg.message == WM_QUIT ) break;  // or return if necessary
        PeekMessage(&msg, NULL, 0, 0, PM_REMOVE);
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
}

---

or if you are using MFC:

Code:

---

AfxGetApp()->PumpMessage()

---

{
MSG msg;
if( PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) )
{
if( msg.message == WM_QUIT ) break; // or return if necessary
PeekMessage(&msg, NULL, 0, 0, PM_REMOVE);
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}

this does nothing. or I didnt really understand what to do with that..
help?

Quote:

---

Originally Posted by Igeru

{
MSG msg;
if( PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE) )
{
if( msg.message == WM_QUIT ) break; // or return if necessary
PeekMessage(&msg, NULL, 0, 0, PM_REMOVE);
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}

this does nothing. or I didnt really understand what to do with that..
help?

---

If you put that in your loop, it should allow the display to update. If the display is still freezing, then you definately have a bug in your algorithm.

Here is a simple way to do what you are saying:

Code:

---


HANDLE hFile = CreateFile(_T("MyFile.txt"), GENERIC_READ|GENERIC_WRITE, FILE_SHARE_NONE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if( !hFile || (hFile==INVALID_HANDLE_VALUE) )
{
  DWORD dwError = GetLastError();
  TCHAR tszBuf[256];
  _stprintf_s(tszBuf, _T("Error: %d"), dwError);
  MessageBox(NULL, tszBuf, NULL, MB_OK|MB_ICONEXCLAMATION);
}
else
{
  DWORD dwRead, dwWrite;
  BYTE byteBuffer[256];
  while( ReadFile(hFile, byteBuffer, sizeof(byteBuffer), &dwRead, NULL) )
  {
      if( dwRead == 0 ) break;
      for(DWORD i = 0; i < dwRead; ++i)
          byteBuffer[i] = DoMyEncrypt(byteBuffer[i]);
      SetFilePointer(hFile, -dwRead, NULL, FILE_CURRENT);
      WriteFile(hFile, byteBuffer, dwRead, &dwWrite, NULL);
  }
  CloseHandle(hFile);
}

---

If you want to keep the screen updating, add the message dispatch just before the for() loop.