AutoCAD Features

DWG 2004 - Tougher Than Thought

reprinted by permission of Ralph Grabowski, editor

April 8, 2003

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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

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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