Welcome to the resource topic for 2021/1348
Title:
Beyond quadratic speedups in quantum attacks on symmetric schemes
Authors: Xavier Bonnetain, André Schrottenloher, Ferdinand Sibleyras
Abstract:In this paper, we report the first quantum key-recovery attack on a symmetric block cipher design, using classical queries only, with a more than quadratic time speedup compared to the best classical attack. We study the 2XOR-Cascade construction of Ga{\v{z}}i and Tessaro (EUROCRYPT~2012). It is a key length extension technique which provides an n-bit block cipher with 5n/2 bits of security out of an n-bit block cipher with 2n bits of key, with a security proof in the ideal model. We show that the offline-Simon algorithm of Bonnetain et al. (ASIACRYPT~2019) can be extended to, in particular, attack this construction in quantum time Õ(2^n), providing a 2.5 quantum speedup over the best classical attack. Regarding post-quantum security of symmetric ciphers, it is commonly assumed that doubling the key sizes is a sufficient precaution. This is because Grover’s quantum search algorithm, and its derivatives, can only reach a quadratic speedup at most. Our attack shows that the structure of some symmetric constructions can be exploited to overcome this limit. In particular, the 2XOR-Cascade cannot be used to generically strengthen block ciphers against quantum adversaries, as it would offer only the same security as the block cipher itself.
ePrint: https://eprint.iacr.org/2021/1348
Talk: https://www.youtube.com/watch?v=MrkjA1TDvZs
Slides: https://iacr.org/submit/files/slides/2022/eurocrypt/eurocrypt2022/30/slides.pdf
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