SubBytes

SubBytes is a nonlinear byte substitution step used in the Advanced Encryption Standard (AES), also known as Rijndael. In each round, the state, a 4x4 array of bytes, has every byte replaced independently by a value from a fixed 8-bit S-box. This substitution is applied to all 16 bytes of the state in parallel.

The AES S-box is a fixed table derived from a specific mathematical construction. It is created by taking the multiplicative inverse in the finite field GF(2^8) (with 0 mapped to 0) and then applying an affine transformation over GF(2). The resulting 256-entry permutation provides a strong nonlinearity, which is essential for AES's resistance to linear and differential cryptanalysis.

In operation, SubBytes is followed by ShiftRows and MixColumns in encryption rounds, with a final round that omits MixColumns. For decryption, the inverse SubBytes (InvSubBytes) uses the corresponding inverse S-box to revert the byte substitutions introduced during encryption. The transformation is central to the diffusion and confusion properties of AES.

Implementation considerations typically favor a table-driven approach, using a 256-entry lookup table for the S-box in software. Hardware implementations may store the S-box in read-only memory or generate it through compact logic. The inverse S-box is used during decryption. The SubBytes step, together with the other AES round transformations, is defined in the AES standard (FIPS 197) and is a cornerstone of AES's security and efficiency.