## CryptoDB

### Paper: Beyond quadratic speedups in quantum attacks on symmetric schemes

Authors: Xavier Bonnetain , Loria André Schrottenloher , Cryptology Group, CWI Ferdinand Sibleyras , NTT Laboratories Search ePrint Search Google Slides EUROCRYPT 2022 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 $\widetilde{\mathcal{O}}{2^n}$, 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.
##### BibTeX
@inproceedings{eurocrypt-2022-31842,
title={Beyond quadratic speedups in quantum attacks on symmetric schemes},
publisher={Springer-Verlag},
author={Xavier Bonnetain and André Schrottenloher and Ferdinand Sibleyras},
year=2022
}