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Securing Drawings by Reducing Intelligence


VG-Wort Access-ZählmarkeCADLock, Inc.

White Paper

Securing Drawings by Reducing Intelligence

2 February 1999

Preface

CAD-generated drawings are no longer comparable to paper drawings. A CAD drawing file contains much more than the simple lines and symbols on a plot may suggest. Most CAD drawings include a lot of manufacturing know-how and design intelligence. A simple CAD drawing made with Autodesk Mechanical Desktop may include all the rules to generate complex families of parts; an architectural floor plan may include costs and manufacturers for the doors and windows used; and any CAD drawing contains building blocks which can be used to generate bills of material, area or mass calculations and more.

Distributing drawings

CAD-generated drawings are no longer only used at a single location. Because of the added intelligence, more and more companies exchange drawings as CAD files rather than paper drawings. That way, other departments can make intelligent use of the design department’s results. Typical uses are production planning, shop drawings, technical documentation, sales support, etc.

However, since many projects today involve the cooperation of multiple companies, electronic drawings including the embedded know-how more often leave the controlled environment of a company. Subcontractors are supposed to receive or deliver electronic drawings of the design parts on which they work. Clients request CAD drawing files as part of the final delivery. Some companies even request electronic drawings as part of your bid. Other companies distribute their complete know-how on the Internet, available to anyone.

Securing your know-how

In many cases the exchange of CAD drawings across company borders makes sense and does no harm, provided there is a complete contractual agreement on responsibility, ownership and handling of these computer files.

However, giving away CAD drawing files normally means losing oversight and control of the drawing and all its embedded intelligence. Any owner of a copy of the file can alter the drawing contents at will.

For years CAD users have tried to protect themselves by preventing modifications to their drawing files. Several strategies worked for a specific CAD system’s current release, but were easily defeated with the next release of that software. A typical example is the use of unequally scaled blocks in AutoCAD, which could not be exploded (reduced to their individual components) in AutoCAD Release 12. Users who relied on that as an anti-modification strategy later found that their drawings had become easily modifiable by anybody using Release 13, because the AutoCAD limitation had been removed.

Today there are two strategies left to protect embedded know-how when distributing CAD drawing files. Strategy One works by reducing access to a drawing’s contents. This strategy is the basis for the CADLock range of products, which allow the originator of a drawing to define and restrict the actions any possible recipient of the drawing may take.

Strategy Two works by removing intelligence from the drawing, thus reducing the potential for loss. This paper describes the pros and cons of using strategy Two.

Reducing drawing intelligence

Eliminating intelligence from a drawing manually is difficult. A drawing consists of many different objects of different intelligence, some visible, some invisible. To reduce drawing intelligence, users may explode building blocks into their basic components, replace solid models with 2 dimensional outlines, and delete everything they do not want others to see. Hopefully, they do not overlook block definitions, invisible layers, non-graphical dictionaries and whatever else may be hidden in an intelligent drawing.

It’s easier to let a computer do these things. Prior to CAD, the paper drawing was the only thing transferred. It contained the minimum amount of information others were supposed to receive. The equivalent to a paper drawing in CAD is the plot file. Users may simply have the CAD program plot the drawing but redirect the plot commands to a file instead of a plotter. Depending on the model of the plotter, the resulting file just contains a series of dots or lines, which define the output on paper.

Using a plot file

The generated plot file can be sent to an output device of appropriate type and creates the correct printout. It contains nothing more than the commands needed to produce this output.

However, if it is necessary to supply a CAD drawing, a file other parties can work on, then a plot file may not meet the requirements.

Many programs are available that take a plot file and convert it back to a CAD drawing file. Typical input formats are HP-GL, HP-GL/2, DXB, or WMF, typical output formats are DXF and DWG. The CAD file generated by such a conversion program can then be distributed and used, but it no longer contains much of the real intelligence from the original CAD drawing.

What is in a plot file?

The value of a plot file (or the CAD drawing file generated from a plot file) depends on the contents of the file. Therefore, it is necessary to have a closer look at the contents of a plot file. There are four areas of interest: object visibility, 3D versus 2D, coordinate systems and object disintegration.

Basically the plot file contains visible information only. This means all invisible information from the original CAD drawing is lost during the plot file generation. This includes any logical relationship of objects, symbol definition, settings and more.

Since a plot is a 2D output format, all 3D objects are projected (flattened) according to the intended view direction. Hidden lines may be eliminated during this projection. (Note that there are also 3D ‘plot files’ such as STL or VRML, which do not reduce the output to 2D or eliminate hidden lines.) Multiple viewports containing the same object are squashed onto a single page, which eventually results in multiple copies of the same lines in the output.

The target for a plot file is a sheet of paper, which means a 2D coordinate system with finite extents and a resolution limited according to the target output device (for example, 200 steps per inch). Therefore all coordinates from the original drawing are translated to paper coordinates and rounded to the nearest value that fits to the device resolution.

Depending on the capabilities of the plot device, complex drawing objects are automatically broken down into simpler objects the device can handle. This includes curves like splines, ellipses or even circles, which are often approximated by a series of straight lines. The same holds true for symbols and characters. While some output devices can draw text as text, the CAD system generating the output usually plots text as simple lines to guarantee visual matching of drawing and output. Filled areas and non-continuous lines within the CAD system also often result in the creation of multiple simpler objects within the plot file.

What can you get from a plot file?

Given the above list of topics that are lost during the translation of a CAD drawing file to a plot file, it should be clear that the drawing you can get from a plot file will only consist of objects otherwise printed on paper. Invisible information cannot be restored, so all object relations, block definitions, etc. are lost. It’s also clear that 3D information cannot be restored from a flattened drawing.

As the plot file uses its own, discrete coordinate system, the original drawing’s coordinates are also lost. However, you can move and re-scale the drawing to restore the original coordinates. You will be left with the loss in precision due to the limited resolution used in the plot file, however.

The disintegration of complex objects finally gives you a drawing full of short line segments. That way you will get curves that look like a circle but will not allow you to snap to its center. However, be aware that some programs to restore a CAD drawing from a plot file even recreate intelligence from the limited information in the file, e.g. by recreating a circle from a series of lines that resemble the geometric shape of a circle, or text from a series of dots which visually resemble characters.

Intelligent plot files

A new trend in distributing drawings is the use of ‘intelligent’ plot files. One example is the Autodesk DWF file format. Primarily intended to view drawing files through the Web, DWF is a flat 2D projection of the original CAD drawing just like a plot file.

However, a DWF maintains a drawing’s intelligence in various places where typical plot files lose it. This includes coordinates. A DWF maintains the original drawing coordinates for all objects, or to be exact, it contains its own coordinate system and a transformation matrix, which allows you to re-calculate the original coordinates. Since the internal coordinates are 32-bit integers, there is little loss in precision.

The disintegration of objects is found in a DWF file as well, although to a lesser extent. Several complex objects like circles or text have their exact definition retained in the DWF file. The DWF format also allows the use of solid fills, although this is limited to polygons. Non-continuous lines are still broken into segments.

The main difference between DWF as an intelligent plot file and a typical non-intelligent plot file of type HP-GL or DXB is the preservation of invisible information. A DWF may contain several forms of invisible information which is lost during normal plot file generation. This includes symbols hidden behind a solid hatch, object-to-layer relationships, URL tags, and named views, for instance.

What can you get from an intelligent plot file?

Since DWF is a documented file format, it is possible to write a computer program that translates the embedded information back into a CAD drawing. CADLock, Inc. have published a prototype of such a program, and other programs that provide some kind of DWF translation facilities may be available commercially in the future. .

By reading the intelligent plot file you can restore a 2D drawing with its true coordinates and many objects unchanged. In the restored drawing you can again snap to an endpoint or to a circle’s center. You can even restore some of the logical relationships inside the drawing by putting objects back onto their original layers.

Of course you cannot restore a 3D drawing from the flattened view, and you will lose most of the inner structure of the drawing such as symbols and dimension objects.

How secure is the distribution of plot files?

Can you protect your technical know-how by distributing drawings as (intelligent) plot files? It depends.

It depends mostly on two things: what do you want the recipient do with the file and what contents are to be protected. And of course it depends on what level of security is acceptable to you.

If you want to distribute a floor plan and you want the recipient to be able to use software to extract the number of doors or windows used (because he is going to deliver them), sending a plot file, even an intelligent one, is not good enough. They will need a file with at least enough intelligence left to create a bill of materials. This is the kind of scenario for which the CADLock family of products is designed.

If you have to send a drawing and you want the recipient to be able to do absolutely nothing with it except plot, you may use a non-intelligent plot file. This way you delete almost all intelligence and know-how from your drawing. But for certain types of drawings even this is not secure enough. If you think of a map or site plan, a properly re-scaled plot file imported into a CAD system is very similar to the original drawing and discloses all the details you wanted to hide.

When using an intelligent plot file, be aware that a large amount of drawing intelligence is preserved. The recipient can still snap exactly to endpoints and other nodes, he can calculate areas or measurements and he can extract exact outlines or views. Since some invisible properties are also available to the recipient, an intelligent plot file of a typical shop drawing, when imported into a CAD system, is almost identical to a version of the original drawing that has been reduced to its individual components. In AutoCAD terms, the drawing recipient is in much the same position as if he had been provided with a drawing in which all blocks had been exploded and purged. While this makes some editing operations more difficult than in the original drawing, it is a simple matter for the recipient to move a building's column, change a dimension or replace your title block with his own, before plotting the drawing.

Conclusion

Protecting drawings by reducing their intelligence content is a valid strategy, especially for drawings that contain a large amount of invisible information. Creating a 2D or 3D plot file from the drawing automatically deletes most of the design know-how embedded in a CAD drawing file. Depending on the file format chosen and the needs of the recipient, this process may either keep or delete too much valuable information, however. Any attempt at security by reduction of intelligence requires a careful analysis of the needs of users of the CAD data, weighed against the need to keep certain information unavailable and therefore inaccessible.

 

Dietmar Rudolph

Vice President (Europe), CADLock, Inc.

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Last modification: 01.04.2011