Paper 2026/229

ANIMAGUS: A Provably Secure Accordion Mode of Operation

Abstract

Block ciphers are designed to operate on fixed length blocks of bits. A block cipher mode of operation is used in order to encrypt variable-length input. These modes process multiple data blocks and ensure information security through the application of block cipher algorithms. While there are several NIST-approved block cipher modes, they exhibit certain inherent limitations and security vulnerabilities. Hence, the necessity for a novel mode has emerged. NIST aims to design an accordion mode, characterized as a tweakable variable-input-length strong pseudorandom permutation, offering enhanced security and performance attributes compared to existing modes. It lastly proposed to develop accordion mode types from different variants of HCTR2 algorithm. Thus, we propose a new design based on HCTR2 structure for an accordion mode in this article. The proposed design accommodates variable-length inputs while ensuring strong pseudorandom permutation security. It also exhibits adaptability with any underlying block cipher and facilitates variable-length tweaks. This ultimately provides a more versatile framework compared to numerous existing schemes in the literature.