Paper 2010/189

New generic algorithms for hard knapsacks

Nick Howgrave-Graham and Antoine Joux

Abstract

In this paper, we study the complexity of solving hard knapsack problems, i.e., knapsacks with a density close to $1$ where lattice-based low density attacks are not an option. For such knapsacks, the current state-of-the-art is a 31-year old algorithm by Schroeppel and Shamir which is based on birthday paradox techniques and yields a running time of $\TildeOh(2^{n/2})$ for knapsacks of $n$ elements and uses $\TildeOh(2^{n/4})$ storage. We propose here two new algorithms which improve on this bound, finally lowering the running time down to $\TildeOh (2^{0.3113\, n})$ for almost all knapsacks of density $1$. We also demonstrate the practicality of these algorithms with an implementation.