Re: Compression and crypto



David A. Scott wrote:
It would be very hard to
if not impossible to check all keys and see how many make a nice jpeg.

Why? I can't imagine what reason you might have in mind.

Because of the computational cost of exhaustively examining all
keys? But you already said you want informatin-theoretic security
(security against adversaries who can perform an unlimited amount
of computation), so that can't be it.

Because of the difficulty of recognizing a valid JPEG? But that's
not very hard. If you think recognizing valid JPEGs (as opposed to
random white noise) is infeasible, then you have underestimated the
advances in vision and graphics. It's certainly not a good foundation
to rest the security of your scheme on. So that can't be it, either.

So if it's not one of those reasons, what reason do you have for
stating that it is "very hard if not impossible" to check all keys?

It sounds to me like tomstdenis is correct. I have yet to see any
valid counterargument that I could understand.

P.S. I don't need to look at the code of BICOM to know that there are
some things that no bijective compression algorithm can achieve.

Let me attempt to clarify it. I original stated with by bijective
programs and BICOM I could take a one byte output file and get more
than one possible valid english text message that compress and encrypt
to it. Dave is that wrong is this is one of Tommy's many times makeing his
point.

I don't understand what you are trying to say, so I don't know how to
answer. I do understand Tom's claims and they look accurate to me.

I then went on to state why that even if I don't use my routines
but only Matt's BICOM that the one byte would map into many millions
of possible files depending on the key and that it was trival to
produce more than 256 such files.

That might depend on whether encryption is length-preserving or not.
If encryption is length-preserving and the ciphertext is only one
byte long, I guess there are only 256 possible plaintexts (and the
number of possible keys seems irrelevant), unless I'm missing something.
.



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