Easy and hard functions for the Boolean hidden shift problem
This work addresses quantum query complexity for cryptographers and quantum algorithm designers, providing insights into function hardness, but it is incremental as it builds on prior methods like quantum rejection sampling.
The paper tackles the Boolean hidden shift problem by showing that bent functions allow exact one-query solutions, while hardest instances include delta functions, and random functions require only two queries, with an algorithm based on the pretty good measurement and Fourier analysis.
We study the quantum query complexity of the Boolean hidden shift problem. Given oracle access to f(x+s) for a known Boolean function f, the task is to determine the n-bit string s. The quantum query complexity of this problem depends strongly on f. We demonstrate that the easiest instances of this problem correspond to bent functions, in the sense that an exact one-query algorithm exists if and only if the function is bent. We partially characterize the hardest instances, which include delta functions. Moreover, we show that the problem is easy for random functions, since two queries suffice. Our algorithm for random functions is based on performing the pretty good measurement on several copies of a certain state; its analysis relies on the Fourier transform. We also use this approach to improve the quantum rejection sampling approach to the Boolean hidden shift problem.