Re: Standard for Stenography?

[Hal <hfinney@shell.portal.com>]

From: Jef Poskanzer <jef@ee.lbl.gov>
> 
> >  Firstly, congratulations for Sergey Goldgaber's stubborn pushing of
> >this topic, for Bill Stewart's observation: "simple stego-programs,
> >stealthy encryption programs"
> 
> I disagree with pretty much everything in your message, and since I'm the
> one who opened the topic and who is writing the code, my opinion would seem
> to count for quite a bit more than yours.  I'm not going to repeat the
> reasons why the kind of standard you propose is a bad idea, you can fetch
> the messages as easily as I can.
> 
> Cc:ed to the list only so that no one thinks Gary's proposal was accepted.
> The permutation idea remains the best.

I share Jef's disagreement with the spectacularly bad "neon sign"
steganography header, but I don't think Sergey's approach was correct
and I hope he does not feel the issue is closed yet.  Bill Stewart is
IMO far more experienced and has far better understanding of the issue
than Sergey, who has been a list member for only a few weeks and again
IMO suggests a very naive security-through-obscurity approach.

Bill Stewart, Norm Hardy, and other list members who have more experience
and who have discussed these issues in the past will I think agree that the
correct approach is to separate the function of the stegonography program
to be a simple and clean insertion, and to have other components be
responsible for assuring that what is inserted is statistically indistin-
guishable from what is replaced.

This notion that a "secret offset" will prevent the stego from being
discovered is highly naive IMO.  The correct approach is to make it so
that the stego cannot be recognized even if the opponent knows where it is.

Adding offsets is like attempting to "improve" regular RSA by putting a
secret amount of noise padding at the front (not of a stego file, but of
an openly encrypted file).  This is unnecessary if you trust your encryption,
and if you don't trust it then this approach should not make you trust it.

Similarly, if your stego is so weak that knowing where it is in the file will
allow the opponent to detect it, adding a random offset should not make you
feel secure.  The correct approach is to have statistical identity between
what you are inserting and what you are removing.  The stego program itself
should then be as simple as possible.

Now I will add my own little moral lesson, in the spirit of Tim and Jef.
Sometimes when these discussions are re-hashed, old-timers are too busy or
bored to join in.  New list members express naive views that are not vigor-
ously refuted.  This is OK, but then some other new member takes these views
to represent list consensus.

I think it is great that Jef is working on a steganography implementation,
but IMO the notion of "random offsets" is so fundamentally misguided that I
hope he will reconsider.

Hal Finney
hfinney@shell.portal.com