Energy-Efficient Flexible RNS-CKKS Processor for FHE-Based Privacy-Preserving Computing

This article proposes a total hardware solution for the RNS-CKKS algorithm supporting flexible parameter sets with 2.7-to-13.3- $\mu $ J/boot/slot energy consumption. Targeting fully homomorphic encryption (FHE), we focus on the key-switch operation, which is the most essential, complicated operation for the bootstrapping process to alleviate accumulated error and enable unlimited FHE computations. Introducing novel two-level scheduling schemes for low-latency key-switch operation, the proposed hardware reduces key-switch latency by 47% and increases resource utilization by $1.9\times $ compared with a straightforward process. Moreover, with optimized hardware engines dynamically supporting various ciphertext parameter sets, the proposed hardware fabricated in 28-nm CMOS technology enables the most efficient FHE operations with various bit-security levels than conventional hardware, especially $4.3\times $ better bootstrapping energy efficiency than state-of-the-art hardware.