Cryptology ePrint Archive: Report 2021/407

Improved Quantum Algorithms for the k-XOR Problem

André Schrottenloher

Abstract: The k-XOR problem can be generically formulated as the following: given many n-bit strings generated uniformly at random, find k distinct of them which XOR to zero. This generalizes collision search (two equal elements) to a k-tuple of inputs.

This problem has become ubiquitous in cryptanalytic algorithms, including variants in which the XOR operation is replaced by a modular addition ($k$-SUM) or other non-commutative operations (e.g., the composition of permutations). The case where a single solution exists on average is of special importance.

At EUROCRYPT 2020, Naya-Plasencia and Schrottenloher defined a class of ``quantum merging algorithms'' for the k-XOR problem, obtained by combining quantum search. They represented these algorithms by a set of ``merging trees'' and obtained the best ones through linear optimization of their parameters.

In this paper, we give a simplified representation of merging trees that makes their analysis easier. We give better quantum algorithms for the Single-solution k-XOR problem by relaxing one of the previous constraints, and making use of quantum walks. Our algorithms subsume or improve over all previous quantum algorithms for Single-solution k-XOR. For example, we give an algorithm for 4-XOR (or 4-SUM) in quantum time $\widetilde{\mathcal{O}}(2^{7n/24})$.

Category / Keywords: secret-key cryptography / Quantum algorithms, merging algorithms, k-XOR, k-SUM, bicomposite search.

Original Publication (with major differences): SAC 2021

Date: received 26 Mar 2021, last revised 2 Sep 2021

Contact author: andre schrottenloher at m4x org

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Version: 20210902:140408 (All versions of this report)

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