Paper 2026/897

SEFA: A Secure, Efficient, and Flexible Algorithm Design Strategy for Block Ciphers and Sponge Permutations

Abstract

Substitution-permutation networks (SPNs) are the most popular construction method for block ciphers and sponge permutations. Basically, we can divide SPNs into two groups by considering their diffusion layer profiles: bit-wise versus byte-wise (or nibble-wise) diffusion. In this paper, we suitably combine both approaches and present a more efficient and flexible construction that achieves a combination of small S-boxes, linear layers consisting only of XOR operations and bit permutations, and a small number of rounds. As concrete results, we present the family SEFA of several lightweight and flexible block ciphers, AEADs, and cryptographic hash functions, including: * SEFA-128/256: A TFHE-friendly block cipher with 128-bit block, 256-bit key, using 4-bit S-boxes, through 16 rounds. * SEFA-512/256: A wide-block AEAD suitable for encrypting 256-bit blocks with 256-bit keys, using 8-bit S-boxes, through 16 and 10 rounds. * SEFA-1536: A hash function using sponge construction with a 1536-bit state permutation including 8-bit S-boxes and 12 rounds. We analyze the security from several attack perspectives. Also, we present hardware implementation results, along with comprehensive performance benchmarks, to demonstrate performance and flexibility.