CIS 5371 Cryptography. 5a. Pseudorandom Objects in Practice Block Ciphers B ased on: Jonathan Katz and Yehuda Lindell Introduction to Modern Cryptography. Block ciphers as encryption schemes or pseudorandom permutations.
5a. Pseudorandom Objects in Practice
Based on: Jonathan Katz and Yehuda LindellIntroduction to Modern Cryptography
A substitution-permutationnetwork is a direct implementation of this paradigm.
The basic idea is to break the input up into small parts and then feed these parts through different S-boxes (random permutations).
It is easy to demonstrate that the avalanche effect holds in a substitution-permutation network, when the following hold:
The S-boxes are designed so that any change of at least a single bit to the input to an S-box results in a change of at least two bits in the output.
The mixing permutations are designed so that the output bits of any given S-box are spread into different S-boxes in the next round.