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, Paul@fatmans.demon.co.uk 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-----