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*To*: "'[email protected]'" <[email protected]>*Subject*: RE: Redundancy in XOR encryption*From*: "[email protected]" <[email protected]>*Date*: Wed, 18 Sep 1996 08:18:56 -0700*Cc*: "'[email protected]'" <[email protected]>*Encoding*: 46 TEXT*Sender*: [email protected]

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... > > > > Paul: > 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.

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