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arXiv:1010.1624

Quantum Generic Attacks on Feistel Schemes

Hong Wang, Zhi Ma

Published Oct 8, 2010• 0 citations
arXivHTMLPDF

Abstract

The Feistel scheme is an important structure in the block ciphers. The security of the Feistel scheme is related to distinguishability with a random permutation. In this paper, efficient quantum algorithms for distinguishing classical 3,4-round and unbalanced Feistel scheme with contracting functions from random permutation are proposed. Our algorithms realize an exponential speed-up over classical algorithms for these problems. Furthermore, the method presented in this paper can also be used to consider unbalanced Feistel schemes with expanding functions.

DOI
10.48550/arxiv.1010.1624

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The Feistel scheme is an important structure in the block ciphers. The security of the Feistel scheme is related to distinguishability with a random permutation. In this paper, efficient quantum algorithms for distinguishing classical 3,4-round and unbalanced Feistel scheme with contracting functions from random permutation are proposed. Our algorithms realize an exponential speed-up over classical algorithms for these problems. Furthermore, the method presented in this paper can also be used to consider unbalanced Feistel schemes with expanding functions.
No polynomial classical algorithms can distinguish between the 3-round Feistel cipher with internal permutations and a random permutation. It means that the 3-round Feistel cipher with internal permutations is secure against any chosen plaintext attack on the classical computer. This paper shows that there exists a polynomial quantum algorithm for distinguishing them. Hence, the 3-round Feistel cipher with internal permutations may be insecure against a chosen plaintext attack on a quantum computer. This distinguishing problem is an instance that can be efficiently solved by exploiting the quantum parallelism. The proposed algorithm is the first application of Simon's algorithm to cryptographic analysis.
Abstract Quantum attacks employing superposition queries (Q2 model) have been demonstrated to compromise numerous classically secure block-cipher constructions. These attacks embed the target encryption structure into a period-finding problem, which dedicated quantum algorithms, including the Simon and BV algorithms, can solve. After restoring the secret state, attackers can launch an attack by recovering the key (the secret state contains key information) or distinguishing it from random permutations. In this paper, we investigate the quantum security of Feistel-like ciphers in a related-key setting. For several constructions, we demonstrate how to define a periodic function that can be evaluated using the Simon algorithm with only O(n)O(n)O(n) O ( n ) quantum queries to the encryption oracle, where n represents the block size. This enables secret-state recovery attacks on Type-1/2 generalized Feistel schemes, as well as key-recovery attacks on the Feistel-FK and Misty L-KF constructions. All these attacks achieve an exponential reduction in query complexity compared to the best known classical or quantum methods.
Quantum algorithms are raising concerns in the field of cryptography all over the world. A growing number of symmetric cryptography algorithms have been attacked in the quantum setting. Type-3 generalized Feistel scheme (GFS) and unbalanced Feistel scheme with expanding functions (UFS-E) are common symmetric cryptography schemes, which are often used in cryptographic analysis and design. We propose quantum distinguishing attacks on Type-3 GFS and UFS-E in the quantum chosen plaintext attack setting. The results of key recovery are better than those based on exhaustive search in the quantum setting.
It turned out that in addition to problems with classical asymmetric cryptography in the post-quantum period, there are certain doubts about the strength of symmetric cryptographic schemes.This paper demonstrates that on Type III Generalized Feistel Scheme (GFS), by selectively fixing specific parts of the plaintext at the input to the GFS, it is possible to reduce the problem of distinguishing between random text and encrypted output of the same GFS to the Simon problem through different approaches.Our method enables the cracking of the cipher up to 𝑑 rounds in polynomial time, while a more sophisticated approach based on different formulas from other paths of the cipher can crack 𝑑 + 1 rounds with the same time complexity in quantum adversary model.These distinct approaches yield varying results in terms of scheme security, indicating the potential to break more rounds in the GFS using the same methodology.
By studying the attack technology based on Simon, Grover and other quantum algorithms for the security of block cipher structure, this paper proposes new quantum attack on 6 generalized unbalanced Feistel structures. We improve the attack results of n-cell structure, New Structure I/III/IV structure and FBC-like structure, and give a quantum attack on Skipjack structure for the first time. Based on Simon algorithm, this paper constructs a n2-round, a 12/12/6-round and a 4-round quantum distinguisher about n-cell structure, New structure I/III/IV and FBC-like structure, improving n2-n-1 rounds, 6 rounds/3 rounds/1 round and 1 round separately, comparing to the previous results. In addition, this paper also constructs a 9-round quantum distinguisher about Skipjack structure. Moreover, this paper proposes a quantum key recovery attack on the above six structures in combination with Grover algorithm, and the time complexities of attacks are analyzed.
Feistel schemes are important components of symmetric ciphers, which have been extensively studied in the classical setting. We examine the extension methods of differential distinguishers of Feistel key-function and Feistel function-key schemes. The schemes are subjected to quantum differential collision distinguishing attacks based on the methods. The results show that the complexity is lower than that of differential attacks using only Grover algorithm, and the complexity of differential collision attack based on the Brassard-Høyer-Tapp and Grover algorithms is lower than that of quantization when using only the Grover algorithm. The results also show that different algorithms and methods can be combined to produce a more effective cryptanalysis approach. This provides a research direction for post-quantum cryptographic analysis and design.