paul@fatmans.demon.co.uk wrote:
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I have a question I hope someone here might be able to answer:
As the method of cryptanalysis of XOR (Ie. index of coincidence) relies on redundancy in the plaintext, would the following be strong:
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 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. 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. Others may disagree with my thought-experiment and my approach, but I think this is quite possible ... even to persons with limited computing resources. Brian Durham bdurham@metronet.com