Optimized implementation of Rijndael256 on modern x86-64 platforms

Abstract

The Advanced Encryption Standard (AES) was standardized in 2001 by NIST and has become the de facto block cipher used today. AES is a block cipher with a block size of 128 bits and is based on the proposal by Rijmen and Daemen, named “Rijndael”. The Rijndael proposal includes a definition for a block cipher with 256 bits block size (and a 256-bits key), which we call here Rijndael256. This variant has not been standardized. This paper describes software optimization methods for fast computations of Rijndael256 on modern x86-64 platforms equipped with AES-NI and with vector AES-NI instructions. We explore several implementation methods and report a speed record for Rijndael256 at 0.27 cycles per byte.