A $2^{n/2}$-Time Algorithm for $\sqrt{n}$-SVP and $\sqrt{n}$-Hermite SVP, and an Improved Time-Approximation Tradeoff for (H)SVP

We show a $2^{n/2+o(n)}$-time algorithm that finds a (non-zero) vector in a lattice $\mathcal{L} \subset \mathbb{R}^n$ with norm at most $\tilde{O}(\sqrt{n})\cdot \min\{\lambda_1(\mathcal{L}), \det(\mathcal{L})^{1/n}\}$, where $\lambda_1(\mathcal{L})$ is the length of a shortest non-zero lattice vector and $\det(\mathcal{L})$ is the lattice determinant. Minkowski showed that $\lambda_1(\mathcal{L}) \leq \sqrt{n} \det(\mathcal{L})^{1/n}$ and that there exist lattices with $\lambda_1(\mathcal{L}) \geq \Omega(\sqrt{n}) \cdot \det(\mathcal{L})^{1/n}$, so that our algorithm finds vectors that are as short as possible relative to the determinant (up to a polylogarithmic factor). The main technical contribution behind this result is new analysis of (a simpler variant of) an algorithm from arXiv:1412.7994, which was only previously known to solve less useful problems. To achieve this, we rely crucially on the ``reverse Minkowski theorem'' (conjectured by Dadush arXiv:1606.06913 and proven by arXiv:1611.05979), which can be thought of as a partial converse to the fact that $\lambda_1(\mathcal{L}) \leq \sqrt{n} \det(\mathcal{L})^{1/n}$. Previously, the fastest known algorithm for finding such a vector was the $2^{.802n + o(n)}$-time algorithm due to [Liu, Wang, Xu, and Zheng, 2011], which actually found a non-zero lattice vector with length $O(1) \cdot \lambda_1(\mathcal{L})$. Though we do not show how to find lattice vectors with this length in time $2^{n/2+o(n)}$, we do show that our algorithm suffices for the most important application of such algorithms: basis reduction. In particular, we show a modified version of Gama and Nguyen's slide-reduction algorithm [Gama and Nguyen, STOC 2008], which can be combined with the algorithm above to improve the time-length tradeoff for shortest-vector algorithms in nearly all regimes, including the regimes relevant to cryptography.