I know that asking here is a long shot but what the hell.
We are reverse engineering dwg for autocad 2004. Autodesk has changed the format of the dwg somewhat. The information that I've been able to gather from different sources is contradictory. Evan Yares of OpenDWG.org claims a simple Xor / Magic number encryption scheme on file headers and section headers. Autodesk says there is no encryption but that opendwg just hasn't figured out the format yet. That sounds good to me but I still can't figure out the headers. Here's what I do know.
-Using a utility called FileMon.exe I was able to "watch" the file access that autocad was doing. It reads the first 64 bytes, then seeks to 128 and reads 108 bytes. That much happens every time. Then it seeks to some variable position. From save to save, the only bytes that change are within the 108 bytes. This tells me that the variable position (offset) is stored in the 108 bytes somewhere.
Well rather than rambling on and on, I'll wait to see if one of you guys has even looked at this sort of thing. Consider it a hacking challenge like that website someone posted.