記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Universitatsbibliothek der Ruhr-Universitat Bochum  

Fully homomorphic encryption (FHE) enables computation on encrypted data without decryption, providing strong guarantees for privacy-preserving applications. However, its practicality heavily depends on the efficiency of the underlying cryptographic primitives. To this end, FHE-friendly symmetric ciphers have been designed to achieve a balance between security and homomorphic efficiency. Among them, FASTA (a variant of Rasta) and HERA are two FHE-friendly symmetric-key primitives proposed at CT-RSA 2022 and ASIACRYPT 2021, respectively. Previous cryptanalytic results of FASTA and HERA were achieved by peeling off the last nonlinear layer. In this paper, we present an improved algebraic cryptanalysis of FASTA by exploiting structural properties of its affine layers through chosen-IV algebraic attacks. We demonstrate that both the first and last nonlinear layers of FASTA can be removed, which significantly reduces the algebraic degree of the resulting system. As a result, we achieve the first key-recovery attack on 4-round FASTA, with time complexity about 2118.8, memory complexity 294.2, and data complexity 234.1, improving the best attack by 1 round. Overall, our findings reveal exploitable algebraic weaknesses in the affine layer of FASTA. For HERA, our refined chosen-IV algebraic attack based on the eXtended Linearization (XL) algorithm significantly enlarges the feasible parameter range, enabling attacks on prime moduli that were previously considered out of reach. This is mainly because the integration of the XL algorithm further decreases the number of keystream words required during the online phase of the chosen-IV algebraic attack, thus reducing the cost of its offline phase, i.e., a better tradeoff between the offline and online complexity can be achieved in our improved attack. This highlights the central role of the XL algorithm in enabling efficient algebraic attacks on FHE-friendly symmetric ciphers.

DOI： 10.46586/tosc.v2025.i4.357-380

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.62056/a0iv7ta5v

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1109/ACCESS.2025.3601597

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記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：Springer Nature Switzerland  

DOI： 10.1007/978-3-031-71073-5_12

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/s10623-023-01309-2

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記述言語：英語  

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その他リンク： https://dblp.org/db/journals/iacr/iacr2025.html#HashimotoOS25

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その他リンク： https://dblp.org/db/journals/iacr/iacr2024.html#YoshiokaOH24

開催年月日： 2024年7月

記述言語：日本語  

別の研究会掲載誌名 : 情報処理学会研究報告, セキュリティ心理学とトラスト(SPT), 2024-SPT-56(73) セッション番号 : (73)

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開催年月日： 2015年

記述言語：日本語   会議種別：口頭発表（一般）  

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開催年月日： 2007年

記述言語：日本語   会議種別：口頭発表（一般）  

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開催年月日： 2007年

記述言語：日本語   会議種別：口頭発表（一般）  

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開催年月日： 2007年

記述言語：日本語   会議種別：口頭発表（一般）  

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