Paper 2026/229

ANIMAGUS: A Provably Secure Tweakable Wide-Block Mode

Abstract

Motivated by the accordion mode approach, we propose a length preserving tweakable wide block mode based on the hash-encrypt-hash paradigm. The construction uses AES and HMAC-SHA3-256 with five purpose separated working keys derived from a single master key, replacing algebraic universal hashing with a cryptographic keyed hash. An encrypted internal seed couples two separately keyed head branches and determines the counter mode keystream. The head layers permit parallel evaluation, while inter-branch dependence is retained even when the tail is empty. The mode supports every bit length within its specified range, starting from $2n$ bits without padding, where $n$ is the block size. Variable length external information is incorporated into a fixed length core tweak through application specific preprocessing. We establish birthday bound tweakable variable-input-length strong pseudorandom permutation security under the stated block cipher, keyed hash, and key derivation assumptions. The analysis covers adaptive encryption and decryption queries, varying message lengths, repeated tweaks, and reused internal values. We also describe derived interfaces for authenticated encryption with associated data, tweakable encryption, and deterministic authenticated encryption, distinguishing their additional security requirements from the core guarantee.