# 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, Mok-Kong Shen <mok-kong.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.

.

**Follow-Ups**:**Re: A revision of my text stego scheme***From:*Mok-Kong Shen

**References**:**A revision of my text stego scheme***From:*Mok-Kong Shen

**Re: A revision of my text stego scheme***From:*Mok-Kong Shen

**Re: A revision of my text stego scheme***From:*Globemaker

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**Re: A revision of my text stego scheme***From:*Mok-Kong Shen

**Re: A revision of my text stego scheme***From:*rossum

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**Re: A revision of my text stego scheme***From:*Mok-Kong Shen

**Re: A revision of my text stego scheme***From:*jbriggs444

**Re: A revision of my text stego scheme***From:*Mok-Kong Shen

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