Question

24. Determine whether each of the P-boxes with the following permutation table is a straight P-box, a compression P-box, or an expansion P-box. Detail your reason.    

a). P-box:   

1 1 2 3 4 4

b). P-box:

1 3 5 6 7

Answer 1

Solution:-

The first P box permutation is a compression P box.

And the second P box is expansion P box.

Explanation -

As given in question the first P box takes 6 input digits which are 1 1 2 3 4 4 and produces five output digits which are 1 3 5 6 7. So first P box is a compression P box.

The second P box if we consider in opposite direction. Then it is an expansion P box. As it take five input digits from second P box which are 1 3 5 6 7 and produces six output digits which are 1 1 2 3 4 4. So it is expansion P box, if we consider it working in opposite direction.

permutation box (or P-box) is a method of bit-shuffling used to permute or transpose bits. P-boxes are typically classified as compression, expansion, and straight, according as the number of output bits is less than, greater than, or equal to the number of input bits.

1) Straight P box are that takes n bit as input and produces m bits as output and n = m.

2) Compression P box are that takes n bit as input and produces m bits as output and n > m.

3) Expansion P box are that takes n bit as input and produces m bits as output and n < m.