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Date: Sun, 18 Jan 1998 21:37:28 -0500 From: John Young <jya@pipeline.com> Subject: Re: CAD Stego (fwd)

Why the focus on color, the easiest thing to avoid using, or layers, ditto? Superfluous for serious CAD, these dainty decorations from Martha Stewartville.

The reason I addressed them is that they were what you brought up. Those dainty decorations are critical to every industrial use of CAD that I have ever been exposed to or used in an industrial setting. I'll have to disagree with your opinion on their utility.

Most messages could be concealed in "hatched" objects, bit-bloated as they are, indistinguishable from the elements used to hatch. All black and white; maybe a dot of red for cashmere dupe.

Ok, let's address hatching from the perspective of use as a means to encrypt data. Let's further use AutoCAD since it is by far the most used in industrial settings. I think you will find that most other CAD programs use one or more of the methods in AutoCAD. AutoCAD offers a standard set of hatching objects via the HATCH and BHATCH commands. In both cases the actual hatch patterns are 1-bit patterns meant to be tiled. The HATCH command itself is normaly used for gross backgrounds and such because it doesn't respect object boundaries. The BHATCH command does respect object boundaries and is used much more in industrial and machine drafting. In both cases the color of both the background and line color must be specified. The hatching pattern is then simply tiled across the appropriate area. It is possible to add other non-standard hatch patterns but again, they are 1-bit deep and tend to be regular so there isn't a lot of room to put data. It is possible to include AutoLisp routines to do such hashing but I fail to see how one would stego encrypted data in source code describing how to draw the pattern. There are two means to store reference to those patterns in a file. The first is simply to include a pointer to a standard table of hash patterns that every copy of the program will understand. I suspect that there isn't much room there for stego. Now if we extend the standard hash table we are required to include those hash patterns in our file and suitable coding to tell the foreign program to include them appropriately. I don't see a lot of room there for stego. The alternate is to include the actual hash pattern in the file specificaly not linked to the hash table, in other words an indpendant object we will manipulate through normal mechanisms. While I will agree that if you could guarantee that Mallet didn't actualy view the file as a graphic but rather as a file via some sort of character editor (eg od -c or od -h as the simplest) there isn't much room for getting caught. However, if you were to bring it up as a graphic and view it you find the actual hash pattern is limited because it can be of only a particular size for the hashing display and handling routines to handle. CAD programs at their basest level understand a limited set of objects; coordinate pairs, sets of such pairs for drawing lines, and color. Other geometric objects (eg circles or hatches) are usualy built up through some sort of algorithmic application of these basic objects. In the better CAD programs those objects are routines that are shared as standard functions in the program or through a cookie-cutter mechanism. The way new functions are added is to add some form of PDL (eg AutoLisp) function. The cookie-cutter mechanism usualy is limited by the 1-bit depth and the limited size of the patterns. Let's look at the mechanism that most CAD and graphics programs use for color manipulation. In very few programs that I have ever come across are the colors of each pixel actualy represented by some depth that allows the full range of colors for every pixel. In general some color map mechanism that takes a limited set of colors (usualy arrayed in a table) and allows the program to select a given color from that table. Such special effects as color cycling are created by writing a program that dynamicaly changes the color stored in each member of that table. This is very fast and allows every pixel for a given color to change in concert instead of the delay of having to go to every pixel and test its color and then change it; very slow. The way that most programs allow more colors than allowed by a given color table is they create some sort of interrupt for each line of the dispaly when the display gets a retrace pulse. When this pulse happens (on a 1024 x 768 it occurs every 1/768 seconds - a long time by computer standards) a routine changes the color map to the one appropriate for that line of the display. Some higher performance displays use a d/a mechanism so that each pixel on the display is addressible. These sorts of displays allow the color map to be changed for each pixel but they are very expensive in hardware and list price, both tend to limit the applicability to commen stego exchange outside a limited group. ____________________________________________________________________ | | | The most powerful passion in life is not love or hate, | | but the desire to edit somebody elses words. | | | | Sign in Ed Barsis' office | | | | _____ The Armadillo Group | | ,::////;::-. Austin, Tx. USA | | /:'///// ``::>/|/ http://www.ssz.com/ | | .', |||| `/( e\ | | -====~~mm-'`-```-mm --'- Jim Choate | | ravage@ssz.com | | 512-451-7087 | |____________________________________________________________________|