Xiaoen Lin, Zhengrong Lu, Yantian Shen, Chongxu Ren, Hongbo Yu, Enming Dong, Wenhao Wu

Practical Preimage Attack on 5-Round Keccak[r=640, c=160]

Introduction

Practical preimage attack on 5-round keccak[r=640, c=160]. Explore a practical preimage attack on 5-round Keccak[r=640, c=160] using a novel differential cryptanalysis technique. Solves a Crunchy Contest challenge with ~2^59.7 complexity.

Abstract

Internal differential cryptanalysis has recently yielded a series of new results in the preimage cryptanalysis of round-reduced Keccak. While these existing attacks share a similar high-level framework, they differ in their technical details. This paper provides a comparative analysis of these prior approaches and introduces a variant framework specifically suitable for 5-round Keccak[r=640, c=160]. This technique integrates certain advantages from prior works, so that the complexity of matching the all-zero initial value and the digest can be separated while the period still remains only one-quarter of the lane size, which avoids the need for a complex Forward-TIDA. Although this technique only provides a constrained application scope, we successfully give the practical preimage attack on 5-round Keccak[r=640, c=160] with the complexity of around 259.7, which firstly solve a 5-round pre-image challenge of the Crunchy Contest.

Review

This paper presents a significant advancement in the cryptanalysis of round-reduced Keccak, specifically focusing on preimage attacks. Building upon the recent successes of internal differential cryptanalysis, the authors provide a valuable comparative analysis of existing methods before introducing their own variant framework. This contextualization is crucial, as it positions the new work within a rapidly evolving research landscape, highlighting how the proposed technique refines and specializes prior approaches for a specific target: 5-round Keccak[r=640, c=160]. The clear focus and methodical approach to analyzing predecessors underscore the depth of understanding brought to this challenging area. The technical contribution lies in a novel framework that judiciously integrates advantages from previous works. A key innovation is the separation of complexity for matching the all-zero initial value and the digest, which streamlines the attack process. Furthermore, the framework maintains a small period (one-quarter of the lane size), which is critical for efficiency as it explicitly avoids the need for a complex Forward-TIDA — a known bottleneck in some prior constructions. While the authors acknowledge that this technique has a "constrained application scope," this specificity is often a hallmark of practical and effective cryptanalysis, tailoring the method precisely to the vulnerabilities of the targeted round function. The culmination of this research is a concrete and practical preimage attack on 5-round Keccak[r=640, c=160], boasting a complexity of approximately 2^59.7. This result is not merely theoretical but is validated by its success in solving a 5-round preimage challenge of the Crunchy Contest, marking a significant milestone as the first such solution. This practical demonstration underscores the efficacy and implementability of their proposed framework. The work represents a valuable addition to the literature on Keccak cryptanalysis, offering both an insightful comparison of existing techniques and a potent, specialized attack method with demonstrable real-world impact.

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