‘Secretbook’ Lets You Encode Hidden Messages in Your Facebook Pics

[Peter Fairbrother]

On 12/04/13 00:01, Ben Liddicott wrote:
> 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.


That proof doesn't work - the choice of which message/image to send can 
be significant.

There is, in real life, no such thing as the entropy of a message - it 
depends on context, and like physical entropy, only the entropy of the 
entire system has meaning.

Consider one bit - it may be an error, it may be "one sugar please", it 
may be the press of the button which launches a nuclear attack. It 
depends on what the bit is connected to.

As we don't know the whole system - parts of it, eg a codebook to 
translate message choice to hidden message, are hidden from us - we 
can't talk about the entropy of the image in a meaningful way.

> 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.


-- Peter Fairbrother