Some people feel that a simple bytewise xor is not strong enough as an obfuscation for the data. There we have the question how to implant a stronger obfuscation routine to protect that data of an application from artwork theft. Perhaps there is even the idea to use an obfuscation in the range of a real crypt routine - in which case I want to recommend strongly to read the reasoning page why it can not be real encryption and that the resulting obfuscation has an upper limit being lower than the crypt routine complexity.
After reminding you of this fact we can go at evaluationg how to implant a stronger obfusction routine to protect your data. The ext/io feature uses a callback routine "read" that must read a block of the given size - for the obfuscation case it will call the "read()" function of the underlying operation system, and the obfuscated block will be deobfuscated before returning it to the caller.
In this mechanism there is not asseration at which file-offset the ext/io-read() callback is triggered. That is the reason we have shown obfuscation with bytewise xor-key example - formally this is using obfuscation blocks of 8bit width being aligned on 8bit boundaries in the data file, and our decryption stream is stateless being the same for each obfuscation block (of 8bit width).
In order for a stronger obfuscation we have to break those
limitations which are directly derived from the natural way
of the handling of files by a contemporary operating system.
This is triggered as the call synopsis of the ext/io read()
callback matches exactly the one of posix, so that
one can use the posix read() function reference as the default
for ensuring the most minimal overhead in accessing non-obfuscated
zip files.
And btw, the abbreviation "posix" stands for
"Portable Open System in Unix".
The trick we show here: the first argument of the ext/io read callback is the file descriptor of the underlying operationg system. While we can not add another argument to the ext/io read call we can pick up additional information with the help of that file descriptor id being globally unique even across multiple threads. One solution would make the application map that descriptor id to a special argument but this is often too much overhead: the current file position is enough.
The current file position is managed by the operation system
via the file descriptor table. There is a function call to
map a file descriptor to the current read position offset
usually named "tell(fd)". Since this call is not mandated by
posix, you can emulate it with the posix lseek() call which
returns the resulting offset after the operation was performed,
so we just seek by a zero offset:
#define tell(fd) lseek(fd,0,SEEK_CUR)
That file offset is measured from the start of the zip archive, not per each zipped file. Remind yourself of that fact when creating your own "zzobfuscate.exe" which should work on the zip archive and not per file before zipping. That is a difference over normal zip archives where the user can atleast recognized the dat file as a zip archive and see a list of files contained in the archive, atleast their names and data start offset.
Now, let's use the file read offset to break the blocking limitations of 8bit/8bit to a larger xor-key. In our example we expand to a 32bit/32bit xor-key giving a search space of 4 billion keys instead of the just 256 keys in 8bit blocking. That is simply done by a static 4 byte xor-key sequence and using modulo operations for alignment. For the 2^X cases any modulo operations shrink to a set of ultra-fast bitwise-and operations.
static char xor_value[4] = { 0x55, 0x63, 0x27, 0x31 };
static zzip_ssize_t xor_read (int f, void* p, zzip_size_t l)
{
zzip_off_t y = tell(f);
zzip_size_t r = read(f, p, l);
zzip_size_t x; char* q = p;
for (x=0; x < r; x++) q[x] ^= xor_value[(y+x)&3];
return r;
}