Question

What are the steps in the Feistel function?

(a) expansion, mixing, substitution, permutation

(b) extract, transform, load

(c) extract, load, transform

(d) expansion, divide, mixing, permutation

I got this question in unit test.

This intriguing question originated from Cryptography in division Number Theory and Cryptography of Discrete Mathematics

Answer 1

Right choice is (a) expansion, mixing, substitution, permutation

Easy explanation: The Feistel function which occurs in every block has 3 steps

i) Expansion:- The incoming 32-bit block has half of its bits duplicated, making it a 48-bit block.

ii) Mixing:- The new, 48-bit input block is put through an XOR gate with this round’s unique subkey.

iii) Substitution:- The mixed, 48-bit block is divided into 8 6-bit pieces. Each of these 8 pieces is put through an S-block which will output only 4-bits using non-linear-transformation. Permutation: The 32 output bits are then arranged in a specific permutation that ensures that they will be distributed among different S-blocks in the next round. This is the most important part of security in DES and it helps to avoid simple, algebra-based attacks.