Re: ‘Secretbook’ Lets You Encode Hidden Messages in Your Facebook Pics
ben at liddicott.com
Fri Apr 12 00:01:34 BST 2013
That isn't possible, up to a limit. Proof is that any such transformation
can carry only a limited number of bits of data. Therefore any
steganographic message can be destroyed by a transformation using the same
stego technique to embed a different message of sufficient length.
The limit is that the technique cannot destroy parts of the image that
humans care about, which is by definition limited to things which are
noticeable - at which point it is arguably no longer steganography.
If Facebook were to do such a thing as a matter of policy, secret messages
would be limited to such things as gang signs and T-shirt slogans.
On 11 Apr 2013 22:58, "Ian Batten" <igb at batten.eu.org> wrote:
> On 11 Apr 2013, at 11:56, Richard Clayton <richard at highwayman.com> wrote:
> > The particular proposal here seems to have been specifically designed to
> > survive Facebook's transform rather than to survive more general changes
> > to the image.
> Which is an endless arms race, of course. If Facebook, or people who
> might lean on Facebook, decide to perturb pictures in such a way that
> steganography is corrupted, then there are a limitless number of ways that
> might be done. Especially if the photographs are assumed to only be
> displayed on screen, rather than used for critical editing and enlarged
> printing. You can propose a method of steganography which passes today's
> transformation, and as soon as the method is out there, the transformation
> can be changed to break it.
> What would be interesting, but a rather more substantial piece of work (to
> put it mildly), would be a technique which is demonstrably robust in the
> face of any transformation which preserves specific properties of the
> image, while also being undetectable in the same use. Rather than being
> proof against particular transformations, it would be proof against all
> transformations other than those which visibly break the images. That
> would break the cycle of the arms race.
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