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*To*: [email protected]*Subject*: Re: Redundancy in XOR encryption*From*: [email protected]*Date*: Tue, 17 Sep 1996 19:33:09 +0000*Comments*: Authenticated sender is <[email protected]>*Priority*: normal*Sender*: [email protected]

> > > > 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. anyone else got a faster way to attack this highly theoretical, will-never-be-implemented, type system?? I`d imagine there is some sort of way to measure the entropy "mixed in" by the XOR thus giving a foothold in the key, but I can`t think of anything right now, anyone got any ideas? Datacomms Technologies web authoring and data security Paul Bradley, [email protected] Http://www.fatmans.demon.co.uk/crypt/ "Don`t forget to mount a scratch monkey" -----BEGIN PGP PUBLIC KEY BLOCK----- Version: 2.6.3ia mQCNAjH9j+cAAAEEAMBvREiQR0ot9dFCO0TiSCSunAYLv2g1Bc6I3bz8FzKXNH53 6mieJf/W4rD+CxJpT0q9RQaaoRtkHJLwbjfK2il3D7mEahMAyqvF/xRJNqkXfhM3 sRJM0Jh43l+W0M5vwokbEbk25/bxWWGspTsLD3YHbzKnG6pOcL5OPIRbv66xAAUR tCdQYXVsIEJyYWRsZXkgPHBhdWxAZmF0bWFucy5kZW1vbi5jby51az6JAJUDBRAy NwfvNkCBjDT0xHEBATQPA/9TORmN/UjNecj03q4anpvdyCLiez5sKuNbnYK50RiP Jj4QpWWvST3smyQ0A86DrZY/re056MXwQmARESx0rFZxdnD0oORICl5r8dJLIy3b j8rbA5olXwZwKz73/X5s13v/pvHYX4cIsbVK8NHXqh5llSKt6TBAuGgkIGF29z5k C4kAlQMFEDI3B9mdtf/umVkv7QEBcRYD/1FBteLqsUmr81euxqqnnrpLlyHb58B/ 9sdATuua4uSjX46hXDZ264YozspNrzSB4NEdrmXOWVX3fiE0ga6XkSSkIeF23V90 En37Z0BdbFzgF00FRYTFyTq8eezQrdg/+rBPUsZUmG5wpq3e12FKHQsX01i+1mB2 YmqqwCV5e95eiQCVAgUQMh8uSb5OPIRbv66xAQEqJwP/fxQyiCasjFcbDpsFfsYp put5cCC/9pOx6X3DlbKShPMpUOS+A9HsTEmJQN8Iawv1nSwPdtc2cR/GhW6ilVjW LSloGdMVLabm9pGpZZMkRaZlXFUkOv7VhfgsUiL+vIDryBCAwUZCzQiWycjt/cPi mUqFH41Z7NkyO8ZFdi5GGX0= =CMZA -----END PGP PUBLIC KEY BLOCK-----

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