DWG 2004 - Tougher Than Thought
reprinted by permission of Ralph Grabowski, editor
April 8, 2003
During the beta phase of AutoCAD 2004, Autodesk explained
that the revised DWG format uses compression to make file sizes
smaller. Last week, the OpenDWG Alliance revealed the DWG format
is also encrypted.
Encryption by Autodesk is not new, having been used for ACIS
objects in DXF files since Release 13. An industry insider says
encryption has been in DWG since R14, but wasn't turned on.
Before OpenDWG's detailed analysis became available, I guessed
that encryption might be used for ShapeManager objects (Autodesk's
home-grown replacement for ACIS solid models) and for password
protection of drawing files. Besides, if PTC could lead in
encrypting their drawing files, why couldn't Autodesk follow?
What's in DWG 2004
The OpenDWG Alliance was busy figuring out what's inside DWG
2004, and last week they summarized their findings. It's worse
than I guessed:
On compression: "Rather than having a single
compression type, each object type appears to have its own
individual algorithm, with a large number of special cases.
Object compression is controlled by a 32-bit flag, which
provides for billions of possible permutations. OpenDWG has
reverse-engineered the compression algorithms for some objects,
but substantial work remains to be done."
On encryption: "Both the file and section headers are
encrypted, but in different manners from each other. While
OpenDWG has been able to determine the algorithm used for both,
it has not been able to determine if the encryption keys used to
scramble the data will remain static, will change in each point
release of AutoCAD, or will ultimately be changed dynamically
under program control.
The alliance has not announced when their DWGdirect libraries
will be updated.
An analyst tells me he is pleased with Autodesk's action in
encrypting objects. For the first time in AutoCAD's twenty-year
history, he says, the DWG format is final stable. Customers no
longer need to fear corrupt drawings, assuming they use AutoCAD
2004.
What's This Mean for You?
Monday morning, a reader asked, "If you get a chance in
upFront could you examine/debate the issue of AutoCAD 2004
security keying the new file format so that the OpenDWG can't
seem to crack it?"
The majority of Autodesk's customers, I suspect, don't care.
It's a problem only for theorists [such as myself] and
third-parties whose livelihood depends on accessing data
directly from DWG files. The list includes vendors of
stand-alone translators, file viewers, and competing CAD
packages, as well as corporate in-house developers.
OpenDWG will "crack" DWG, just as Theorem, Delcam, TTF, and
others eventually cracked Pro/E's drawing encryption. (Martyn
Day reports that it took PTC a year to admit that its drawing
files were encrypted. That and a number of other DWG
2004-related comments can be read at
www.cadwire.net/commentary/?24971.)
The reverse-engineering of DWG 2004 will, however, take
longer than originally anticipated. OpenDWG's delay in releasing
its libraries means that every CAD vendor depending on DWGdirect
will be unable to read and write DWG files compatible with
AutoCAD 2004 for months. There are two parts to the delay: (1)
the alliance needs to update DWGdirect for 2004; and then (2)
the CAD vendors need to implement and test the DWGdirect
libraries with their software.
(The last time Autodesk did a major change to DWG was with
Release 13. I recall it took about six months for DWG viewers
and other products to catch up.)
In the meantime, the work-around is to use DWG 2002, which
AutoCAD 2004 both reads and writes. I've performed tests showing
that features unique to AutoCAD 2004, such as gradient fills,
are retained (but not displayed) by AutoCAD 2002.
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