From: Jef Poskanzer <jef@ee.lbl.gov>
Date: Thu, 3 Mar 94 17:54:14 PST
To: Eli Brandt <ebrandt@jarthur.cs.hmc.edu>
Subject: Re: Standard for Stenography?
Message-ID: <9403040154.AA16047@hot.ee.lbl.gov>
MIME-Version: 1.0
Content-Type: text/plain


Eli makes a reasonable case for leaving out the length field
altogether.  The desteg program would produce a file of width*height
bits, and it would be up to the next layer to produce text from that.
However, I'm not sure it's a *compelling* case.  If adding the length
doesn't actually hurt security, I'm inclined to keep it.

>Tangentially, why choose bit permutation for your second-level
>encryption?  There are plenty of schemes that will be a lot faster
>than doing all that bitmangling.

Slowness is not necessarily bad - it also makes it harder for
attackers to search through large numbers of images for ones
with hidden data.  But the main thing that the permutation
gives you is that it spreads out the data bits among unmodified
bits, making statistical tests harder.  For a 1000 byte message
in a 640x480 image, only 2% of the bits will be changed.  If that
2% was all jammed into the first 80000 pixels of the file, it
might be detectable; if it's spread evenly throughout the file,
it's probably safer.
---
Jef