The impossible boomerang attack is a powerful cryptanalytic technique, but existing key recovery methods face several limitations that restrict its applicability. Specifically, the key pre-guessing is coarse-grained, S-box details are ignored in the differential propagation, the complexity estimation and the key guessing order determination remain rudimentary. To overcome these issues, we introduce three key improvement measures. First, we propose a flexible partial key and difference pre-guessing technique based on directed graphs, enabling selective identification of required keys and differences for generating partial pairs and quartets. Second, we propose a pre-sieving technique to early eliminate invalid quartets by exploiting cipher-specific details. Third, we introduce an automatic key-guessing strategy based on the same directed graphs to efficiently determine valid guessing orders. We integrate these techniques to develop a unified key recovery framework for impossible boomerang attacks, accompanied by a formal and precise characterization of the overall complexity. This is the first framework to support flexible key and difference pre-guessing while incorporating block cipher details during key recovery for impossible boomerang attacks. Crucially, it enables the automatic generation of detailed recovery steps, a capability missing in prior work. As applications, under the four related-key/tweakey setting, we apply the framework to ARADI, a low-latency cipher proposed by the National Security Agency (NSA), and SKINNYe v2, a threshold-implementation-friendly cipher proposed at EUROCRYPT 2020. For ARADI, we achieve the first full-round attack with 2130data, 2253.78time, and 2235.75memory complexity. For SKINNYe v2, we present the first 34-round impossible boomerang attack with 266data, 2253.75time, and 2239.75memory complexity. These results demonstrate the framework’s significance and its substantial improvement in advancing the impossible boomerang attack.

Fast correlation attack is one of the most powerful attack methods for LFSR-based stream ciphers, and the primary problem of the attack is to construct the linear approximations with great absolute correlations. For some stream ciphers with complex structures of linear approximations, the search for the maximum absolute correlation of linear approximations has always been a difficult problem because of the extremely high amount of masks that need to be searched. In this paper, an analysis method for searching maximum absolute correlation based on the linear mask structure is developed, including the filtering technology based on mask propagation trail, a structural characteristic of linear approximations of linear transformations with fewer active bytes, and linear approximation equivalence theorem of composite function composed of the parallel identical S-boxes and linear transformation. These methods efficiently reduce the exhaustive time complexity of the masks. As applications, this paper proves that the suprema of absolute correlations of all the linear approximations for the five XOR-versions of SNOW family stream ciphers (i.e., SNOW 2.0⊕, SNOW 3G⊕, SNOW-V⊕, SNOWVi⊕, SNOW 5G⊕) are 2−9/2−15:893/2−37:964/2−37:964/2−37:964. The exhaustive time complexity of the masks can be reduced fromO(232)/O(296)/O(2384)/O(2384)/O(2384) toO(224)/O(231.98)/O(239.98)/O(239.98)/O(239.98), respectively. Furthermore, we give the provable security evaluations of the five ciphers against fast correlation attacks under the success probability of 0:99 for the known fast correlation attack method. For SNOW-V⊕/SNOW-Vi⊕/SNOW 5G⊕, the time/data/memory complexity of the optimal fast correlation attacks are allO(2227.54)/O(2227.72)/O(2227.72). The results show that SNOWV⊕/SNOW-Vi⊕/SNOW 5G⊕cannot guarantee the claimed 256- bit key security for the known fast correlation attack methods if we ignore the design constraint that the maximum length of keystream for a single pair of key and IV is 264. For SNOW 2.0⊕and SNOW 3G⊕, the time/data/memory complexity of the optimal fast correlation attacks areO(2151.94)/O(2151.35)/O(2151.35) andO(2165.91)/O(2165.43)/O(2165.43), respectively. The results show that both SNOW 2.0⊕and SNOW 3G⊕can guarantee the claimed 128-bit key security for the known fast correlation attack methods. In addition, this paper also discusses that the existing fast correlation attacks based on multiple linear approximations are invalid for these five ciphers.

As the practical applications of fully homomorphic encryption (FHE), secure multi‐party computation (MPC) and zero‐knowledge (ZK) proof continue to increase, so does the need to design and analyze new symmetric‐key primitives that can adapt to these privacy‐preserving protocols. These designs typically have low multiplicative complexity and depth with the parameter domain adapted to their application protocols, aiming to minimize the cost associated with the number of nonlinear operations or the multiplicative depth of their representation as circuits. In this paper, we propose two differential fault attacks against a one‐way function RAIN used for Rainier (CCS 2022), a signature scheme based on the MPC‐in‐the‐head approach and an FHE‐friendly cipher HERA used for the RtF framework (Eurocrypt 2022), respectively. We show that our attacks can recover the keys for both ciphers by only injecting a fault into the internal state and requiring only one normal and one faulty ciphertext blocks. Thus, we can use only the practical complexity of 2 26.6 /2 28.8 /2 30.4 bit operations to break the full‐round RAIN with 128/192/256‐bit keys. For full‐round HERA with 80/128‐bit key, our attack is practical with complexity the complexity of 2 20 encryptions with about 2 16 memory.