Re: [Selfext-user] Protection of SFX

[Mike W. <mik...@nt...>]

On Friday 12 September 2003 10:38 am, Ho Kei Lau wrote:
> Hello, I would like to protect my SFX from being able to be opened by other
> archivers like WinZip. I want to do it simply by removing the first 4 bytes
> ("MSCF") from the cabinet, then in file_read(), add the 4 bytes to the
> buffer temporarily. I have tried to do this .. but seems it only retrieves
> the header information, but NOT extracting the files
> inside. FNREAD(file_read) {
>  DWORD rslt;
>  if(SetFilePointer(GetHandle(hf), 0, 0, FILE_CURRENT)==g_iExeSize+8)
>  {
>   char* cTmp = (char*) pv;
>         lstrcpy(cTmp, "MSCF");
>         ReadFile(GetHandle(hf), &cTmp[4], cb-4, &rslt, NULL);
>   return (rslt+4);
>  } else
>  {
>   ReadFile(GetHandle(hf), pv, cb, &rslt, NULL);
>   return rslt;
>  }
> }
> What's wrong? Can somebody please help me? Kei 

Hi Kei,

That's an interesting idea.

If you remove the 4 signature bytes (rather than just changing them), you
also need to subtract 4 from nOffset in Extract() so that all the seeking is
offset by 4 bytes.

Currently CheckValid() doesn't open the file using the 'filename|<nnnnnn>'
syntax to fake the start of the file, so a slight mod is needed there (see 
below).

And then finally a change like you have there is needed to file_read().
You can get the offset to the start of the cab data within the executable
using GetPtr(hf) (see below).

Below is a patch for you that does this. I hope this helps, let me know if
you come across any difficulties applying it.

Mike

Index: ExtFdi.cpp
===================================================================
RCS file: /cvsroot/selfext/selfext/ExtFdi.cpp,v
retrieving revision 1.2
diff -u -2 -r1.2 ExtFdi.cpp
--- ExtFdi.cpp	18 Sep 2002 09:25:52 -0000	1.2
+++ ExtFdi.cpp	12 Sep 2003 18:48:22 -0000
@@ -204,5 +204,17 @@
 { 

     DWORD rslt;

-    ReadFile(GetHandle(hf), pv, cb, &rslt, NULL); 

+

+    const file_struct *pfs = GetPtr(hf);

+

+    ReadFile(pfs->m_hFile, pv, cb, &rslt, NULL);

+

+    if (SetFilePointer(pfs->m_hFile, 0, 0, FILE_CURRENT) == pfs->m_nOffset + rslt)

+    {

+        if (cb > 4) cb = 4;

+        char *p = (char *)pv;

+        while (cb--)

+            p[cb] = "MSCF"[cb];

+    }

+

     return rslt; 

 }

@@ -290,5 +302,8 @@
         return LoadOsError(IDS_ERR_EXTRACTING, ERROR_CANCELLED);

 

-    int hf = file_open((LPSTR)pszFilename, _O_BINARY|_O_RDONLY, 0);

+    CHAR szFile[MAX_PATH+11];

+    wsprintf(szFile, "%s|%d", pszFilename, nOffset);

+

+    int hf = file_open(szFile, _O_BINARY|_O_RDONLY, 0);

     if (hf == -1) 

         return LoadOsError(IDS_ERR_EXTRACTING);

@@ -296,10 +311,4 @@
     FDICABINETINFO fdici;

 

-    if (file_seek(hf, nOffset, SEEK_SET) != nOffset)

-    {

-        LoadOsError(IDS_ERR_EXTRACTING);

-        goto handle_error;

-    }

-

     if (!FDIIsCabinet(hfdi, hf, &fdici))

     {

@@ -376,4 +385,6 @@
     if (hfdi == NULL)

         return LoadError(IDS_ERR_CABINIT, IDS_FDIERROR_NONE + erf.erfOper);

+

+    nOffset -= 4;

 

     // check the file is a valid cabinet