**Improved Classical and Quantum Algorithms for Subset-Sum**

*Xavier Bonnetain and Rémi Bricout and André Schrottenloher and Yixin Shen*

**Abstract: **We present new classical and quantum algorithms for solving random subset-sum instances. First, we improve over the Becker-Coron-Joux algorithm (EUROCRYPT 2011) from $\widetilde{\mathcal{O}} \left(2^{0.291 n}\right)$ downto $\widetilde{\mathcal{O}} \left(2^{0.283 n}\right)$, using more general representations with values in $\{-1,0,1,2\}$.

Next, we improve the state of the art of quantum algorithms for this problem in several directions. By combining the Howgrave-Graham-Joux algorithm (EUROCRYPT 2010) and quantum search, we devise an algorithm with asymptotic cost $\widetilde{\mathcal{O}} \left(2^{0.236 n}\right)$, lower than the cost of the quantum walk based on the same classical algorithm proposed by Bernstein, Jeffery, Lange and Meurer (PQCRYPTO 2013). This algorithm has the advantage of using classical memory with quantum random access, while the previously known algorithms used the quantum walk framework, and required quantum memory with quantum random access.

We also propose new quantum walks for subset-sum, performing better than the previous best time complexity of $\widetilde{\mathcal{O}} \left(2^{0.226 n}\right)$ given by Helm and May (TQC 2018). We combine our new techniques to reach a time $\widetilde{\mathcal{O}} \left(2^{0.216 n}\right)$. This time is dependent on a heuristic on quantum walk updates, formalized by Helm and May, that is also required by the previous algorithms. We show how to partially overcome this heuristic, and we obtain an algorithm with quantum time $\widetilde{\mathcal{O}} \left(2^{0.218 n}\right)$ requiring only the standard classical subset-sum heuristics.

**Category / Keywords: **foundations / subset-sum, representation technique, quantum search, quantum walk, list merging

**Original Publication**** (with minor differences): **IACR-ASIACRYPT-2020

**Date: **received 12 Feb 2020, last revised 10 Nov 2020

**Contact author: **xbonnetain at uwaterloo ca, remi bricout at inria fr, andre schrottenloher at inria fr, yixin shen at irif fr

**Available format(s): **PDF | BibTeX Citation

**Version: **20201110:153718 (All versions of this report)

**Short URL: **ia.cr/2020/168

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