Provably weak instances of Ring-LWE

TL;DR

This paper identifies and demonstrates efficient attacks on certain instances of the Ring-LWE problem for general number rings, revealing weak cases and extending attack methods beyond cyclotomic fields.

Contribution

It constructs explicit weak instances of Ring-LWE for general number fields and extends existing attacks to a broader class of fields, including power-of-two cyclotomic fields.

Findings

Efficient attack runs in linear time relative to the modulus q.

Certain Ring-LWE instances can be transformed into Poly-LWE instances.

Power-of-2 cyclotomic fields are vulnerable to the proposed attacks.

Abstract

The ring and polynomial learning with errors problems (Ring-LWE and Poly-LWE) have been proposed as hard problems to form the basis for cryptosystems, and various security reductions to hard lattice problems have been presented. So far these problems have been stated for general (number) rings but have only been closely examined for cyclotomic number rings. In this paper, we state and examine the Ring-LWE problem for general number rings and demonstrate provably weak instances of Ring-LWE. We construct an explicit family of number fields for which we have an efficient attack. We demonstrate the attack in both theory and practice, providing code and running times for the attack. The attack runs in time linear in q, where q is the modulus. Our attack is based on the attack on Poly-LWE which was presented in [Eisentr\"ager-Hallgren-Lauter]. We extend the EHL-attack to apply to a larger class of number fields, and show how it applies to attack Ring-LWE for a heuristically large class of fields. Certain Ring-LWE instances can be transformed into Poly-LWE instances without distorting the error too much, and thus provide the first weak instances of the Ring-LWE problem. We also provide additional examples of fields which are vulnerable to our attacks on Poly-LWE, including power-of-$2$ cyclotomic fields, presented using the minimal polynomial of $\zeta_{2^n} \pm 1$.