Paper 2026/2430 Original: eprint.iacr.org/2026/2430 ↗

Distinguishers for the Squeezing of the Alternating Sponge and Duet

Abstract

Recently, the Institute of Commercial Cryptography Standards (ICCS) launched the Next Generation Commercial Cryptographic Algorithm Program (NGCC), which includes a standardization process for cryptographic hash functions. In this note, we analyze the mode of operation of Duet, one of the hash-function candidates. Duet is a permutation-based hash function directly inspired by the Alternating Sponge (ASP). Both constructions use two permutations on $b=r+c$ bits, split into an outer part of $r$ bits and an inner part of $c$ bits. We first present a distinguisher against ASP requiring $\mathcal{O}(2^r)$ permutation queries, which contradicts the claimed indifferentiability bound in a non-trivial parameter regime. We then introduce a distinguisher and a state-recovery attack against Duet. Both attacks require only a constant number of $h$-bit output blocks, with $h\leq c$, and $\mathcal{O}(2^{r+c-h})$ permutation queries. These attacks do not affect the claimed collision or preimage resistance, but they distinguish Duet's multi-block output from random, with consequences if Duet is used as a XOF or a KDF; state recovery also enables length extension.