BASALISC: Flexible Asynchronous Hardware Accelerator for Fully Homomorphic Encryption

Fully Homomorphic Encryption (FHE) allows for secure computation on encrypted data. We present BASALISC, an architecture family of FHE hardware accelerators that aims to substantially accelerate FHE computations in the cloud. BASALISC implements the BGV scheme, targets a range of parameter sets, and directly supports and implements BGV bootstrapping. We propose a new generalized version of bootstrapping that can be implemented with optimized Montgomery multipliers that cost 46% less in silicon area and 40% less in power consumption. BASALISC is a RISC architecture with a four-layer memory hierarchy, including a two-dimensional conflict-free inner memory layer that enables 32 Tb/s radix-256 NTT computations without pipeline stalls. Our conflict-resolution data permutation hardware is re-used to compute BGV automorphisms without additional hardware and without throughput penalty. BASALISC additionally includes a custom multiply-accumulate unit familiar in DSP architectures, with which we accelerate tight BGV key switching loops. The BASALISC computation units and inner memory layers are designed in asynchronous logic, allowing them to run at different speeds to optimize each function. BASALISC is designed for ASIC implementation with a 1 GHz operational frequency, and is already underway toward tape-out with a 150mm2 die size in a 12nm Global Foundries process.The BASALISC toolchain comprises both a custom compiler and a joint performance and correctness simulator. We evaluate BASALISC in multiple ways: we study its physical realizability; we emulate and formally verify its core functional units; and we study its performance on a single iteration of logistic regression training over encrypted data. For this application, comprising from up to 900K high-level BASALISC instructions down to 27B low-level instructions, we show a speedup of at least 2,025x over HElib.