  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 $\zeta_{2^n} \pm 1$.
