Substitution Box (S-Box)
A substitution box (S-box) is a lookup table in block ciphers that maps an m-bit input to an n-bit output, the core component providing nonlinearity.
Purpose
It injects nonlinear confusion into block ciphers, making them indistinguishable from linear transformations and resisting linear cryptanalysis.
Mechanism
An S-box is an arbitrary m→n bit mapping implemented as a table lookup. Design criteria include low differential uniformity, high nonlinearity, and the strict avalanche criterion. The AES S-box is based on GF inversion; the DES S-boxes were designed by the NSA.
Use cases
- AES/DES round functions
- The nonlinear layer of block ciphers
- Targets of differential/linear analysis