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 


I'm resending this since the first has disappeared, so appologies for the
duplicate if the original surfaces.


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