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RE: Redundancy in XOR encryption
in any practical or semi-practical application, you'll have to have a way to decompress the
perfectly compressed data. A dictionary? A Huffman-tree-ish sort of thing? Are you going
to transfer it out-of-band? **It** becomes the target of interest.
From: [email protected][SMTP:[email protected]]
Sent: Tuesday, September 17, 1996 12:33 PM
To: [email protected]
Subject: Re: Redundancy in XOR encryption
> > Compress P to get perfect compression (ie. 0 redundancy)
> > Encrypt F (the compressed text) using a repeated key XOR
> > of course this is all rather theoretical as there is no such thing as
> > perfect compression, but I just thought it might be interesting to
> > see if this is indeed strong, superficially it appears so to me...
> I think that if the cryptanalyst knows that F has zero redundancy
> that he can run searches from 0 to n bits for the key and have
> the computer flag solutions that have zero redundancy.
I never though of that.
> I also think that a perfectly compressed file would have a relative
> entropy value close to one also, hence the computer could flag possibles
> that have both characteristics.
yeah, these two are reasonably unlikely to occur together (only a
reasoned guess, anyone got any comments on this?)
so we really have a weakish system.
> Hence, instead of searching for plaintext by counting coincidences,
> we are searching the decrypts for solutions that have zero redundancy
> and a relative entropy value close to one. How many solutions will
> have both these qualities? I don't know. But if the compression method
> is known, brute force will be tried, and only having to try to
> decompress (read) data that has the resultant characteristics
> of compressed information will speed things up by quite a bit.
Yeah, this is still a form of brute force but I was thinking of this
in terms of a smallish (sub 200 bit) key, so brute force against
solutions with 0 entropy is a realistic possibility.