# Properties of Ideal Straight P-Boxes

What properties should an ideal straight P-Box exhibit?

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Could you add a definition of "straight P-Box", for those of us who don't know it? – Paŭlo Ebermann Jan 7 at 17:20

Straight P-Boxes should have only 1 input for each output and the same number of inputs as outputs.

Straight P-Boxes will also be invert-able.

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I love the default dictionary! – ponsfonze Jan 5 at 17:49
OK, removed that, but where is "ideal", shouldn't it be "Ideal P-Boxes will also be invert-able"? – owlstead Jan 5 at 18:13
OK, looked it up, and it should be Straight P-Boxes will also be invert-able. Still missing the word "ideal " in the answer though. – owlstead Jan 5 at 20:13
You probably also want to add that the P-Boxes need to maximize diffusion across inputs and outputs, to make linear and differential cryptanalysis more difficult. Optimal diffusion = ideal P-Box, I suppose. – Thomas Jan 6 at 2:46
I will try and revise my answer to have the complete information if it is missing anything. – ponsfonze Jan 7 at 18:41
 The question is about "straight P-boxes". I have no idea what these are, but it doesn't sound same as "S-boxes". – Paŭlo Ebermann♦ Jan 7 at 17:19 @PaŭloEbermann, I see that I mis-read the question; I thought it said S-box. A search on "straight P-box" finds that Wikipedia says a straight P-box is a permutation (reordering) of $n$ bits. It's just a transposition (re-ordering) of the bits. Still, "ideal" cannot be defined without reference to the block cipher in which it be used. – D.W. Jan 7 at 20:05