New methods of attacking cryptographic systems have been developed to address vulnerabilities exposed by advances in quantum computing; researchers from Fujian Normal University and Beijing University of Posts and Telecommunications present new quantum attacks against two Feistel-type constructions, New Structure IV and LBlock, using a chosen-plainattack approach. These attacks cover eight rounds of New Structure IV and nine rounds of LBlock, enabling key recovery on both ciphers.

The work details new attacks against the cipher types New Structure IV and LBlock, which are built using a Feistel structure, a design that repeatedly splits data into halves then combines them with keys to create encrypted blocks.

The team constructed methods to differentiate between cipher output and random data after eight rounds for New Structure IV and nine rounds for LBlock under a chosen-plainattack scenario. These attacks use advances in quantum computing to break down encrypted data more efficiently than previously possible using chosen-plaintechniques.

A key component of their work is constructing what’s known as a ‘quantum distinguisher’; think of it like a test designed to tell if an encrypted message genuinely appears random or contains subtle patterns revealing information about the encryption key, similar to spotting a counterfeit coin by how consistently it lands on one side. The team successfully created distinguishers covering eight rounds of New Structure IV, and nine rounds of LBlock enabling them to recover keys from both systems with reduced time complexity compared to previous methods. But can these findings be extended further; potentially undermining even longer encryption sequences?

LBlock attacks achieve complexity reduction and expanded round vulnerability while impacting New Structure IV

A 2¹⁶ reduction in time complexity was achieved when attacking LBlock by researchers and Beijing University of Posts and Telecommunications. This surpasses previous work from Liu et al. presented at ASIACRYPT 2025, which faced a certain computational barrier.

The breakthrough enables attacks on more rounds of LBlock than previously possible, expanding potential vulnerabilities within the cipher’s design. Analysis of LBlock’s internal structure also reduced the required key guess space, from 76 bits to just 52 bits, a substantial decrease lowering overall attack cost from \(O(2^{38})\) operations to \(O(2^{26})\). at INSCRYPT 2024 and reduces computational effort by a factor of \(2^{n/2}\).

While these results demonstrate important progress in understanding vulnerabilities within both ciphers, they currently focus on controlled quantum environments and do not account for the substantial engineering challenges inherent in building sufficiently large and stable quantum computers needed to mount such attacks.

Quantum attacks incrementally advance toward complete cryptographic breaks

Improvements in attacking New Structure IV and LBlock using quantum techniques raise a vital question: how much further can these methods be pushed before encountering fundamental limits. Full key retrieval demands analysis beyond their current nine or eight round distinguishers, as acknowledged by the researchers, this highlights an ongoing tension between incremental gains and truly breaking these ciphers. Despite this, work from the Fujian Normal University and Beijing University of Posts and Telecommunications team is significant because it demonstrates tangible progress in applying quantum computing to cipher evaluation.

The construction of quantum distinguishers for both ciphers represents an advancement in assessing security against future quantum threats; it moves beyond proving attacks are possible toward quantifying their complexity. This approach allows more precise evaluations than previous methods, revealing how reductions in time complexity impact practical feasibility and lower computational costs. These improvements reduce time complexity by a factor of 2^n/2 for New Structure IV and either 2¹⁶ or 2¹² for LBlock, prompting further analysis into alternative cryptographic designs or mitigation strategies.

This research details new quantum attacks on two block cipher constructions, New Structure IV and LBlock, used in symmetric cryptography. The authors developed quantum distinguishers that allow key recovery with reduced computational effort; specifically, the attack on New Structure IV was improved by a factor of 2^n/2, while reductions of 2¹⁶ or 2¹² were achieved for LBlock depending on the number of rounds attacked.

These findings demonstrate progress in evaluating cryptographic systems against potential future threats from quantum computers and provide more precise assessments of their security margins through quantified complexity reduction. Further analysis is needed to extend these results beyond current round limits.

👉 More information
🗞 Quantum Cryptanalysis on Two Feistel-Type Constructions
✍️ Yan-Ying Zhu, Bin-Bin Cai, Fei Gao and Song Lin
🧠 ArXiv: https://arxiv.org/abs/2610.27506

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