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combining an EXE with a ZIP archive

How To

In this section we walk you through the steps of combining an EXE with a ZIP archive. The basic scheme goes like this: the final file will have an EXE starting at offset null, followed by the data entries of a ZIP archive. The last part of the ZIP archive is the ZIP central-directory which ends at the end of the file.

The basic problem lies in the fact that the zip central-directory entries reference their data section with an offset from the start-of-file so that you can not just append a zip archive after an exe stub. The trick goes like adding the EXE as the first data part of the ZIP archive - so that the offsets for each entry will be correct when we are finished with it.

Again, one can not just use a zip tool to put the EXE as the first part - since each data part is preceded with an infoblock of a few bytes. The data of the first data part will therefore not start at offset zero. We solve this problem with moving the data a few bytes later - so that the final file will not start with a "PK" magic (from the zip info header) but with an "MZ" or "ELF" magic (from the exe info header).

Step 1: Creating The Zip Combination

Choose your exe file (example.exe) and wrap that file into a zip container - ensure that the zip tool does not use any compression algorithm on the data. This is usually done with saying "zero compression level" as an option to the zip tool. Also note that no other file is wrapped as some zip tools reorder the entries from the order on the command line to alphabetic order. Here is an example with infozip's `zip` (e.g. on linux):

    zip -0 -j example.exe    

There is no zip tool that would reorder the data entries in an existing zip archive. This mode is used here - the real compressed data entries can now be added to the existing zip archive that currently just wraps the exe part. With specifying maximum compression ("-9" = compression level 9) and throwing away any subdirectory part ("-j" = junk path) it might look like

   zip -9 -j data/*   

Now we need to move the exe part by a few bytes to the real start of the file. This can be done as easily as writing the exe file again on to the start of the file. However, one can not just use a shell-direction or copy-operation since that would truncate (!!) the zip file to the length of the exe part. The overwrite-operation must be done without truncation. For maximum OS independence the zziplib ships with a little tool in "test/zzipsetstub.c" that you can reuse for this task:

   zzipsetstub example.exe  

This is it - the `unzip` tool can still access all data entries but the first EXE - the first EXE will be listed in the central-directory of the ZIP archive but one can not extract the data since the "PK" magic at offset null has been overwritten with the EXE magic. The data of all the other entries can still be extracted with a normal `unzip` tool - or any tool from the zziplib be used for the same task.

Step 2: Accessing The Data From The Program

There is an example in test/zzipself.c that show how to do it. The OS will provide each program with its own name in argv[0] of the main() routine. This program file (!!) is also the zip archive that carries the compressed data entries along. Therefore, we can just issue a zzip_opendir on argv[0] to access the zip central-directory.

Likewise one can open a file within it by just prepending the string argv[0] to the filename stem, i.e. you could do like

 ZZIP_FILE* f = zzip_fopen ("example.exe/start.gif", "rbi")

however you are advised to use the _ext_io cousin to be platform independet - different Operating Systems use different file extensions for executables, it's not always an ".exe".

Once the file is opened, the data can be zzip_fread or passed through an SDL_rwops structure into the inner parts of your program.

Step 3: Using Obfuscation Along

The next level uses obfuscatation on the data part of the application. That way there is no visible data to be seen from outside, it looks like it had been compiled right into the C source part. One can furthermore confuse a possible attacker with staticlinking the zziplib into the executable (this is possible in a limited set of conditions).

The first pass is again in creating the zip - here we must ensure that only the ZIP archive part is obfuscated but the EXE part must be plain data so that the operationg system can read and relocate it into main memory. Using xor-obfuscation this is easy - applying xor twice will yield the original data. The steps look like this now:

    zzipxorcopy example.exe example.xor
    zip -0 example.xor
    zip -9 data/*
    zzipsetstub example.xor
    zzipxorcopy application.exe

In the second step the open-routine in your application needs to be modified - there are quite some examples in the zziplib that show you how to add an xor-read routine and passing it in the "io"-part of an zzip_open_ext_io routine (see zzipxorcat.c).

   static int xor_value = 0x55;

   static zzip_ssize_t xor_read (int f, void* p, zzip_size_t l)
       zzip_ssize_t r = read(f, p, l);
       zzip_ssize_t x; char* q; for (x=0, q=p; x < r; x++) q[x] ^= xor_value;
       return r;

   static struct zzip_plugin_io xor_handlers;
   static zzip_strings_t xor_fileext[] = { ".exe", ".EXE", "", 0 };
       zzip_init_io (&xor_handlers, 0); = &xor_read;
       ZZIP_FILE*  fp = zzip_open_ext_io (filename, 
                             O_RDONLY|O_BINARY, ZZIP_CASELESS|ZZIP_ONLYZIP,
                             xor_fileext, &xor_handlers);

You may want to pick your own xor-value instead of the default 0x55, the zziplib-shipped tool `zzipxorcopy` does know an option to just set the xor-value with which to obfuscate the data. February 2003