Flush, Gauss, and Reload - A Cache Attack on the BLISS Lattice-Based Signature Scheme

We present the first side-channel attack on a lattice-based signature scheme, using the Flush+Reload cache-attack. The attack is targeted at the discrete Gaussian sampler, an important step in the Bimodal Lattice Signature Schemes (BLISS). After observing only 450 signatures with a perfect side-channel, an attacker is able to extract the secret BLISS-key in less than 2 minutes, with a success probability of 0.96. Similar results are achieved in a proof-of-concept implementation using the Flush+Reload technique with less than 3500 signatures. We show how to attack sampling from a discrete Gaussian using CDT or Bernoulli sampling by showing potential information leakage via cache memory. For both sampling methods, a strategy is given to use this additional information, finalize the attack and extract the secret key. We provide experimental evidence for the idealized perfect side-channel attacks and the Flush+Reload attack on two recent CPUs.