MISC: steganography & pics

[Karl Barrus]

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Cypherpunks,

I too have played around with graphics formats, specifically the TIFF
format.  It's a large and complicated format, but it does leave room
for customization.  I just mention this because I'm more familiar with
TIFF than the GIF format.

Skipping over 99% of the details: the TIFF format has a header (magic
numbers and stuff), tags and corresponding information fields, and
picture data - real terminology exists, like image file directories
and so forth, but I'll skip that.  The TIFF format allows you to
specify quite a bit of info: information fields such as time of
creation, author name, comments, host system; picture information such
as rows, columns, whether the minimum value corresponds to black or
white; color models: RGB, CMY, CMYK, greyscale; compression model:
none, LZW, JPEG; alpha values, and many other fields.  Finally, the
picture data is stored, non-sequentially if desired (that is, row 1,
then row 137, the row 54, etc.)  TIFF is extendible by simply defining
new tags.

Because the TIFF format is so all encompassing, few software packages
can read and write the entire thing - that is, valid TIFFs may be
created that may not be recognized by other software.  Sam Lieffler
(sp?) at sgi has written a comprehensive TIFF manipulation package
available via anonymous ftp which helps to overcome this problem.  

Anyway, for steganography purposes, we wouldn't want to define a
"crypto" tag obviously!  Maybe we could adapt one of the other fields,
like host computer or whatever, so that the signal that a TIFFs
contains hidden information is that one of the fields has a certain
value, to be arranged in advance or whatever.  

A 256x256 image contains 65536 pixels, each of which could contribute
1 bit to a message, which comes to 8192 bytes (say each pixel is 8 bit
and not 24 or something fancy like that), more than enough for a
message.  

This sounds like fun - I imagine in a few weeks there will be a
message to the list to retrieve the recently posted image from
alt.binaries.pictures.misc and read the hidden message!

/-----------------------------------\
| Karl L. Barrus                    |
| elee9sf at menudo.uh.edu             | <- preferred address
| barrus at tree.egr.uh.edu (NeXTMail) |
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