Substitution-Permutation Network

A substitution-permutation network (SPN) is a design paradigm for block ciphers that alternates between substitution layers (S-boxes) and permutation layers to achieve the cryptographic goals of confusion and diffusion. The S-boxes provide local nonlinear transformations that obscure the relationship between key and ciphertext, while the permutation layers spread the influence of each bit across the entire block. By repeating these two operations for multiple rounds, an SPN transforms a weak local operation into a strong global transformation.

The substitution-permutation structure was formalized by Claude Shannon in his 1949 paper Communication