HIDE: embedding msgs into snd & graphics

[Hal <74076.1041@CompuServe.COM>]

Tim mentions that Adobe Photoshop can be used to overlay messages into 
the low-order bits of a graphics image.  Photoshop is expensive, so I'm 
wondering whether it would be worthwhile for me to write a simple, free 
utility just for this purpose.  I would have it take a GIF file and a 
binary file to be embedded, and produce an output GIF with the low order 
bit of each byte changed to be the next bit of the embedded binary file.  
For output, it would do the opposite - produce a binary file determined 
solely by the low-order bits of the GIF file.
 
I played with GIF a few years ago and wrote a viewer, so I have some 
familiarity with that format.  It doesn't sound too hard to write a 
program like this.  One concern is whether such a program would be 
redundant, whether widely available tools already exist to perform the 
same function.  Perhaps there are PD image-processing tools that could 
be adapted.  If anybody knows of any please let me know.
 
For this kind of program to be useful, you'd want to use PGP in its 
long-discussed "stealth mode".  This would be a mode in which PGP would 
produce output that was basically indistinguishable from random data.  
Presently PGP puts out some header fields which can be used to recognize 
that a file is a PGP file.  Stealth mode would suppress this 
information.  PGP would not be able to automatically choose which key to 
use to decrypt such a file, but since most people have only one secret 
key this would not be a big problem.
 
The PGP developer's group has been talking about this for a long time 
(over a year) but nobody has cared enough to do anything about it.  
Maybe it should be done.
 
Hal Finney
74076.1041@compuserve.com