Substitution–Permutation (SP) Network

Related Concepts

Block Ciphers

Definition

Shannon [1] suggested to use several mixing layers interleaving substitutions and permutations to build strong block ciphers. Such design is called a substitution–permutation sandwich or a substitution–permutation network (SPN). Although weak on its own, a line of substitutions followed by a permutation has good “mixing” properties: Substitutions add to local confusion and permutation “glues” them together and spreads (diffuses) the local confusion to the more distant subblocks (Substitutions and Permutations).

Theory

If one considers flipping a single bit at the input of such a network, it effects the m output bits of particular S-box which in turn are sent to different S-boxes by a permutation. Thus, inputs/outputs of up to m S-boxes would be effected by the avalancheof change. These are again permuted into different S-boxes, covering almost all the S-boxes of the network. On the output of such network, about half of the bits are effected...