Has this photo been de-stegoed? (and Anonymity)

[Major Variola (ret)]

At 06:22 PM 12/10/03 +0200, Anatoly Vorobey wrote:
>On Tue, Dec 09, 2003 at 04:20:20PM -0600, Declan McCullagh wrote:
>> We have anonymity in Web browsing (more or less, thanks to Lance &
>> co). It's not NSA-proof, but it's probably subpoena-proof.
>>
>> We have anonymity in email thanks to remailers (to the extent they're

>> still around).
>>
...
>
>alt.anonymous.messages has a healthy amount of traffic.

One could count some fraction of all the *.binaries.* on usenet
as anonymous communications (via stego), but then you'd have to know
how many are stego'd, and that is the game after all.


At 02:24 PM 12/8/03 -0500, Tyler Durden wrote:
>Is it possible to determine that the photo 'originally' (ie, when it
was
>sent to me) contained stegoed information, but that it was intercepted
in
>transit and the real message overwritten with noise or whatever?

Yes.  Trivially, If your correspondent told you, but that's out of
band.  Otherwise,
If there *remains* info which was not washed out "in transit", then that

would be an inband way.  Maybe all the pictures with a red flower
in them are carriers, and this content isn't washed out.  Maybe its a
more subtle crypto-watermark, independent of the stego'd message.

>Now I know pretty much nothing about this subject, but I would suppose
that
>de-stegoing a photo must like some kind of spatial spectral fingerprint
that
>should be visible after the photo is FFT'd (is there freeware software
out
>there?).

1. How do you know the signature of the unaltered carrier-medium?
E.g., have you measured the LSBit noise from my camera recently?
Under which lighting conditions?

2. Don't you think I can measure the properties of my carrier and shape
the stego'd info to match?   (This does get into an arms race over what
properties to measure.)

>Now I IMAGINE that a sophisticated interceptor could substitute
'believable'
>de-stego-ing noise so that it would look like the photo never had any
stego
>in the first place. OR...is this actually 'impossible' to do perfectly?

You don't just put your message in the LSBits or whatever.  You
compress,
encrypt, and possibly redundantly code them.  Then you shape the noise
to match the bits you're replacing.


>And then, what if the interceptor tried to put an alternate message in
there
>instead? Is there a way to tell that there was originallya different
message
>there?

Depends on the coding.

>My assumption first of all is that nothing was done to prepare the
photo
>against these possibilities.

Just make sure you did the original analog recording and destroy the
original after you stego it.  Best also if you never post unstego'd
messages
so the Adversary can't measure your raw carrier.

A simple stego message was placed without real
>thought about whether it might be intercepted and altered.

You shouldn't stego life-critical messages without proper training in
the use of your tools.
(That training may vary with personality, see _Silk and Cyanide_.  Some
like "why",
some like "do this".)

-----
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