Sbox

An S-box, short for substitution box, is a fundamental component in many symmetric-key algorithms. It takes a fixed number of input bits and outputs a fixed number of bits, performing a nonlinear substitution that creates confusion in the cipher. S-boxes are usually implemented as lookup tables or compact logic circuits.

In block ciphers, S-boxes are used within the rounds to transform small bit-strings. DES employs eight 6-to-4 bit S-boxes that operate on half-byte inputs, feeding a Feistel network. In the Advanced Encryption Standard, the SubBytes step uses a single 8-by-8 S-box applied to every byte; this S-box is constructed from the multiplicative inverse in GF(2^8) followed by an affine transformation and has a defined inverse for decryption.

Qualities and design considerations include nonlinearity, balance, and differential and linear resistance. A well-designed S-box provides high nonlinearity and uniform output distribution. S-boxes can be static, fixed for a cipher, or, in some designs, derived dynamically from the key. Hardware implementations often use ROM lookups, while software uses arrays and inlined code.

The term is widely used as S-box or s-box; related topics include S-box design and cryptanalytic methods that exploit linearity and differential properties.