Re: A revision of my text stego scheme
 From: jbriggs444 <jbriggs444@xxxxxxxxx>
 Date: Mon, 23 May 2011 04:54:02 0700 (PDT)
On May 21, 10:30 am, MokKong Shen <mokkong.s...@xxxxxxxxxxx> wrote:
I have a presumably very dumb question: Isn't it a general
property of hash functions that it is difficult to predict
what a change of input has on the hash output? If that's
indeed the case, then one might under circumstances have
to do a couple of tries to get the right stego bit in
your scheme IMHO, while in my scheme it is deterministic
in the sense that one knows exactly that a change is right.
Yes, the easiest way to predict what effect a change
in the input will have on the output is typically by
computing the hash.
The expected value of the number of tries to find a
random modification that affects a particular hash bit
is 2 (assuming a good hash function). The expected
value of the number of tries to find a random
modification that affects a particular pair of hash bits
in a particular way is 4.
In general, the expected value of the number of tries
to find a one in n match is n.
That's 0.5 bits per try for either a one or a two
bit match.
You could improve the bits per try by shifting to
base 3, thus getting log(3)/log(2) bits per 3 tries
That's about 0.528 bits per try.
The optimal criterion is for a one in 2.718
match, yielding just under 0.531 bits per try.
You may recognize Euler's number.
[This computation is my excuse for feeding the troll]
your scheme IMHO, while in my scheme it is deterministic
in the sense that one knows exactly that a change is right.
Yes. You said that already. And I agreed already.
.
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