The AES Encryption Algorithm: Unpacking the Substitution-Permutation Network (SPN)

Introduction

The Advanced Encryption Standard (AES) is a widely used block cipher that has been the de facto standard for encrypting data at rest and in transit for over two decades. Its security and efficiency have made it a cornerstone of modern cryptography. At the heart of AES lies its innovative Substitution-Permutation Network (SPN) structure, which provides the foundation for its remarkable security properties. In this post, we'll delve into the anatomy of the SPN, exploring its components, operation, and implications for the security of AES.

The SPN Structure

The SPN is a series of alternating layers of substitution and permutation operations. The substitution operations, implemented using S-boxes, introduce non-linearity and confusion, while the permutation operations, implemented using mixing layers, introduce diffusion. This combination of substitution and permutation is the key to AES's remarkable security properties.

S-Boxes

The S-boxes in AES are 8-bit to 8-bit substitution tables. Each S-box takes an 8-bit input and produces an 8-bit output. The S-boxes are designed to be non-linear, meaning that the output depends on the input in a complex