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Re: CAD Stego (fwd)
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> Date: Sun, 18 Jan 1998 21:37:28 -0500
> From: John Young <[email protected]>
> 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.
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