From: Jef Poskanzer <jef@ee.lbl.gov>
Date: Tue, 1 Mar 94 09:10:33 PST
To: cypherpunks@toad.com
Subject: Re: standard for stegonography?
Message-ID: <9403011710.AA08621@hot.ee.lbl.gov>
MIME-Version: 1.0
Content-Type: text/plain


>Now that I think about it, one wouldn't have to sacrifice any bandwidth 
>whatsoever!  As, the stego program could be made to do wrap-around 
>encoding.

Indeed, that's what I implemented.  The current man page is appended.
---
Jef

pnmstego(1)               USER COMMANDS               pnmstego(1)

NAME
     pnmstego - insert a stegonographic message into  a  portable
     anymap

SYNOPSIS
     pnmstego [-offset n] [-components rgb] [-bitsched  10010...]
     file [pnmfile]

DESCRIPTION
     Reads a portable anymap as input.  Hides another file within
     the anymap by changing the low-order bits of each pixel, and
     writes the modified anymap to standard output.

     Stegonography means "hidden writing" - it's  for  storing  a
     message  in  such  a way that an opponent won't even be sure
     that the message is there, let alone  be  able  to  extract,
     decrypt, and read it.  Pnmstego lets you store a file in the
     low-order bits of a PPM or PGM image.  For a typical  maxval
     of  256, this means the pixel values will be changed by only
     1/256, which should not be noticable to someone viewing  the
     image.

     Only PGM and PPM images may be used - trying to hide a  file
     in a PBM image would be easily visible, so it's not allowed.

OPTIONS
     The program has a bunch of flags that let you specify varia-
     tions  on  exactly how the bits are hidden in the image.  If
     you don't specify any flags, the default options give you  a
     reasonable storage method, to wit:
         -offset 0 -components b -bitsched 1
     Letting the options default like this has the advantage that
     you  can  send  a  stegoed file to someone without having to
     specify what flags you used.  You can also put the file on a
     public  FTP  server  for people to retrieve, without needing
     any other communication.  However, there is a  disadvantage:
     people  who  the  file  is  not intended for will be able to
     extract it too.  If the resulting file  is  recognizable  in
     some  way,  for instance if it's encrypted by PGP, then some
     of the purpose of the stegonographic storage will have  been
     compromised.   (A PGP file has a standard header and stores,
     in cleartext, the keyid needed to decrypt a file.)

     So, if you want the file to be absolutely positively hidden,
     you  can  use  non-default  settings  for the options.  Just
     don't forget what settings you used, and  remember  that  if
     you  want  to send the file to someone else you will have to
     communicate the necessary settings somehow.

     Now, as for what the options do:

     -offset     Specifies how far into the image to  start.   If
                 the file runs off the end of the image, it wraps
                 around back to the  beginning,  so  don't  worry
                 about  specifying too large an offset.  In fact,
                 you can even specify an offset larger  than  the
                 image  size  -  the  initial  value gets wrapped
                 around too  (modulus).   Basically,  any  32-bit
                 number can be used.

     -components This lets you specify which of the  red,  green,
                 or  blue  color  components  is used for storing
                 bits.  You give it a string made up from one  or
                 more of the letters r, g, and b.  The default is
                 b, use only the blue component.  See the section
                 below  on quantization for why the default isn't
                 rgb.  If the input image is a PGM file,  instead
                 of PPM, this option is silently ignored.

     -bitsched   This lets you skip using some possible bit posi-
                 tions  in  the image.  If the file you're hiding
                 is not so big that you need the whole  image  to
                 store  it,  you  can  use this option to hide it
                 better.  The  argument  is  just  an  arbitrary-
                 length string of 1's and 0's - the 1's say which
                 bit positions should be used, and the 0's  which
                 should  be  skipped.   The default is to use all
                 bit positions.

QUANTIZATION
     Normally, to store an image in  a  colormapped  file  format
     such as GIF, you first run it through ppmquant.  However, if
     you try to quantize, then stego, and then  convert  to  GIF,
     you will find that the ppmquant step will fail with an error
     about too many colors.   This  is  because  the  stego  step
     increased  the number of colors.  You might think, ok, stego
     first, then quantize; nope, the quantization  would  destroy
     the  stegonographic  message.   Here's  what you do instead:
     quantize down to half the number of colors you want  in  the
     final  file,  and  then use only one of the three color com-
     ponents to store the hidden file.  Using only  one  bit  per
     pixel, the number of colors can at most double.  This is why
     the default for the -components flag is to  use  only  blue.
     So, the pipeline to use is:
         ppmquant 128 image.ppm | pnmstego message.txt |
           ppmtogif > hidden.ppm

SEE ALSO
     pnmdestego(1), pnm(5)

AUTHOR
     Copyright (C) 1994 by Jef Poskanzer.