International Association
for Cryptologic Research

Sponge paradigm, used in the design of SHA-3, is an alternative hashing technique to the popular Merkle-Damg\r ard paradigm. We revisit the problem of finding $B$-block-long collisions in sponge hash functions in the auxiliary-input random permutation model, in which an attacker gets a piece of $S$-bit advice about the random permutation and makes $T$ (forward or inverse) oracle queries to the random permutation. Recently, significant progress has been made in the Merkle-Damg\r ard setting and optimal bounds are known for a large range of parameters, including all constant values of $B$. However, the sponge setting is widely open: there exist significant gaps between known attacks and security bounds even for $B=1$. Freitag, Ghoshal and Komargodski (CRYPTO 2022) showed a novel attack for $B=1$ that takes advantage of the inverse queries and achieves advantage $\Omega(\min(S^2T^2/2^{2c}$, $ (S^2T/2^{2c})^{2/3})+T^2/2^r)$, where $r$ is bit-rate and $c$ is the capacity of the random permutation. However, they only showed an $O(ST/2^c+T^2/2^r)$ security bound, leaving open an intriguing quadratic gap. For $B=2$, they beat the general security bound %$O(ST^2/2^c+T^2/2^r)$, by Coretti, Dodis, Guo (CRYPTO 2018) for arbitrary values of $B$. However, their highly non-trivial argument is quite laborious, and no better (than the general) bounds are known for $B\geq 3$. In this work, we study the possibility of proving better security bounds in the sponge setting. To this end, \begin{itemize} \item For $B=1$, we prove an improved $O(S^2T^2/2^{2c}+S/2^c+T/2^c+T^2/2^r)$ bound. Our bound strictly improves the bound by Freitag et al., %Ghoshal and Komargodski, and is optimal for $ST^2\leq 2^c$. %and is optimal up to a factor of $(ST^2/2^c)^{2/3}$ for $ST^2>2^c$. \item For $B=2$, we give a considerably simpler and more modular proof, recovering the bound obtained by Freitag et al. %, Ghoshal and Komargodski. \item We obtain our bounds by adapting the recent multi-instance technique of Akshima, Guo and Liu (CRYPTO 2022) which bypasses limitations of prior techniques in the Merkle-Damg\r ard setting. To complement our results, we provably show that the recent multi-instance technique cannot further improve our bounds for $B=1,2$, and the general %$O(ST^2/2^c+T^2/2^r)$ bound by Correti et al., for $B\geq 3$. \end{itemize} Overall, our results yield the state-of-the-art security bounds for finding short collisions, and fully characterize the power of the multi-instance technique in the sponge setting. \keywords{Collision \and hash functions \and Sponge \and multi-instance \and pre-computation \and auxiliary input}

We revisit the problem of finding B-block-long collisions in Merkle-Damgard Hash Functions in the auxiliary-input random oracle model, in which an attacker gets a piece of S-bit advice about the random oracle and makes T oracle queries. Akshima, Cash, Drucker and Wee (CRYPTO 2020), based on the work of Coretti, Dodis, Guo and Steinberger (EUROCRYPT 2018), showed a simple attack for 2\leq B\leq T (with respect to a random salt). The attack achieves advantage \Tilde{\Omega}(STB/2^n+T^2/2^n) where n is the output length of the random oracle. They conjectured that this attack is optimal. However, this so-called STB conjecture was only proved for B\approx T and B=2. Very recently, Ghoshal and Komargodski (CRYPTO 22) confirmed STB conjecture for all constant values of B, and provided an \Tilde{O}(S^4TB^2/2^n+T^2/2^n) bound for all choices of B. In this work, we prove an \Tilde{O}((STB/2^n)\cdot\max\{1,ST^2/2^n\}+ T^2/2^n) bound for every 2< B < T. Our bound confirms the STB conjecture for ST^2\leq 2^n, and is optimal up to a factor of S for ST^2>2^n (note as T^2 is always at most 2^n, otherwise finding a collision is trivial by the birthday attack). Our result subsumes all previous upper bounds for all ranges of parameters except for B=\Tilde{O}(1) and ST^2>2^n. We obtain our results by adopting and refining the technique of Chung, Guo, Liu, and Qian (FOCS 2020). Our approach yields more modular proofs and sheds light on how to bypass the limitations of prior techniques. Along the way, we obtain a considerably simpler and illuminating proof for B=2, recovering the main result of Akshima, Cash, Drucker and Wee.

We study collision-finding against Merkle-Damgård hashing in the random-oracle model by adversaries with an arbitrary $S$-bit auxiliary advice input about the random oracle and $T$ queries. Recent work showed that such adversaries can find collisions (with respect to a random IV) with advantage $\Omega(ST^2/2^n)$, where $n$ is the output length, beating the birthday bound by a factor of $S$. These attacks were shown to be optimal. We observe that the collisions produced are very long, on the order $T$ blocks, which would limit their practical relevance. We prove several results related to improving these attacks to find short collisions. We first exhibit a simple attack for finding $B$-block-long collisions achieving advantage $\tilde{\Omega}(STB/2^n)$. We then study if this attack is optimal. We show that the prior technique based on the bit-fixing model (used for the $ST^2/2^n$ bound) provably cannot reach this bound, and towards a general result we prove there are qualitative jumps in the optimal attacks for finding length $1$, length $2$, and unbounded-length collisions. Namely, the optimal attacks achieve (up to logarithmic factors) order of $(S+T)/2^n$, $ST/2^n$ and $ST^2/2^n$ advantage. We also give an upper bound on the advantage of a restricted class of short-collision finding attacks via a new analysis on the growth of trees in random functional graphs that may be of independent interest.