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This is simply saying that if a cryptosystem has a functional composition that is $$ h_{k}(x) = f_{k_1}(g_{k_2}(x)) $$ then you can find a key for single encryption that works as the double encryption. For example: consider the permutation cipher where a permutation is a key. The permutations are forming a group, named permutation group, under the ...


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The combination of different schemes may be somewhat advantageous with regard to certain custom hardware (ASICs), but I think that the disadvantages outweigh them. I assume that there is some kind of fixed time that is acceptable to the user. Nobody will use a procedure where she or he has to wait several minutes. Modern password hashing schemes are ...


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