SubstitutionPermutation

Substitution-permutation refers to a design framework for symmetric key block ciphers in which the cipher state is repeatedly transformed by alternating nonlinear substitution steps with linear permutation steps. The aim is to achieve confusion and diffusion so that the relationship between plaintext, ciphertext, and key becomes complex and difficult to exploit.

In an SPN, rounds typically begin with a key addition (often an XOR with a round key), followed by a substitution layer and a permutation layer. The substitution layer uses S-boxes to provide nonlinearity (confusion). The permutation layer permutes the outputs of the substitution stage to spread the influence of each input bit across the state, yielding diffusion. The exact arrangement can vary: S-boxes may be applied to discrete units (bits or bytes), and the permutation may be a fixed reordering or a linear mixing operation across multiple units.

Notable SPN-based ciphers include the Advanced Encryption Standard (AES), which uses SubBytes, ShiftRows, and MixColumns as its nonlinear and diffusion steps, along with AddRoundKey. Other SPN designs include lightweight ciphers such as PRESENT, which uses a small S-box and a compact permutation, suitable for constrained hardware.

The SPN framework contrasts with Feistel networks; SPNs generally offer straightforward parallelization of rounds and can achieve strong diffusion with relatively few rounds when well-designed S-boxes and permutations are used. Evaluations focus on the choice of S-box, the diffusion properties of the permutation, and resistance to differential and linear cryptanalysis.