NTT Architecture for a Linux-Ready RISC-V Fully-Homomorphic Encryption Accelerator

This paper proposes two architectures for the acceleration of Number Theoretic Transforms (NTTs) using a novel Montgomery-based butterfly. We first design a custom NTT hardware accelerator for Field-Programmable Gate Arrays (FPGAs). The butterfly architecture is expanded to a Modular Arithmetic Logic Unit (MALU) and for greater reuse and easier programmability a six-stage pipeline Linux-ready RISC-V core is extended with custom instructions. The performance of the proposed architectures is assessed on a Xilinx Ultrascale+ FPGA and with an Application-Specific Integrated Circuit (ASIC) on
$28{n}\text{m}$
CMOS technology. In FPGA, the results for custom acceleration show reductions of 30%, 90% and 42% in the number of Lookup tables (LUTs) and registers, Block RAMs (BRAMs) and Digital Signal Processors (DSPs), while providing a speedup of 1.9 times, in comparison with the state of the art. The ASIC results show that at 1 GHz the proposed architecture is in average 45% and 52% less area and power hungry, respectively, compared to the state of the art. Furthermore, the proposed MALU, operating as an additional execution unit, increases the overall area of the extended RISC-V core by only 10%, without significant changes in the frequency of operation.